Title of Invention: SMMR (small molecule metabolite reporters) for use as in vivo glucose biosensors Abstract: Small Molecule Metabolite Reporters (SMMRs) for use as in vivo glucose biosensors, sensor compositions, and methods of use, are described. The SMMRs include boronic acid-containing xanthene, coumarin, carbostyril and phenalene-based small molecules which are used for monitoring glucose in vivo, advantageously on the skin. Detailed Description: Part general_description_paragraphs: Part RELATED APPLICATIONS: This application is the U.S. national phase under 35 U.S.C. §371 of International Application No. PCT/US2004/043087, filed Dec. 23, 2004, which claims priority under 35 U.S.C. §119(e) to Provisional Application Nos. 60/532,667, filed Dec. 24, 2003 and 60/571,170, filed May 14, 2004. Part FIELD OF THE INVENTION: This invention provides compositions and methods for designing small molecule metabolite reporters (SMMRs) for optical reporting of cell metabolism and intracellular or extracellular metabolite or analyte concentrations. The specific application of this work is to design molecules that are able to optically report the concentration of the biologically active molecule D-glucose, including other small molecule analytes or metabolic processes, in or near human keratinocytes located within the viable epidermis of human or mammalian skin. The skin glucose levels are then used to infer blood glucose levels. In particular, this invention provides compositions and methods for several non-invasive techniques to determine in vivo blood glucose levels based upon the direct measurement of glucose levels present in the skin. Part BACKGROUND OF THE INVENTION: Identifying and understanding the risk factors associated with diabetes is invaluable for the development and evaluation of effective intervention strategies. Lacking normal regulatory mechanisms, diabetics are encouraged to strive for optimal control through a modulated life style approach that focuses on dietary control, exercise, and glucose self-testing with the timely administration of insulin or oral hypoglycemic medications. Invasive forms of self-testing are painful and fraught with a multitude of psychosocial hurdles, and are resisted by most diabetics. Alternatives to the currently available invasive blood glucose testing are highly desirable. Conventional approaches seek to reduce or eliminate the skin trauma, pain, and blood waste associated with traditional invasive glucose monitoring technologies. In general, non-invasive optical blood glucose monitoring requires no samples and involves external irradiation with electromagnetic radiation and measurement of the resulting optical flux. Glucose levels are derived from the spectral information following comparison to reference spectra for glucose and background interferants, reference calibrants, and/or application of advanced signal processing mathematical algorithms. Candidate radiation-based technologies include: 1) mid-infrared (MIR) spectroscopy, 2) near-infrared (NIR) spectroscopy, 3) far-infrared (FIR) spectroscopy, 4) radio wave impedance, 5) infrared photoacoustic spectroscopy and 6) Raman spectroscopy. Each of these methods uses optical sensors, and relies on the premise that the absorption pattern of infrared light (700-3000 nm) can be quantitatively related to the glucose concentration. Other substances such as water, protein, and hemoglobin are known to absorb infrared light at these wavelengths and easily obscure the relatively weak glucose signal. Other approaches are based on microvascular changes in the retina; acoustical impedance, NMR spectroscopy and optical hydrogels that quantify glucose levels in tear fluid. While putatively non-invasive, these technologies have yet to be demonstrated as viable in clinical testing. Nearly non-invasive techniques tend to rely on interstitial fluid extraction from skin. This can be accomplished using permeability enhancers, sweat inducers, and/or suction devices with or without the application of electrical current. One device recently approved by the FDA relies on reverse iontophoresis, utilizing an electrical current applied to the skin. The current pulls out salt, which carries water, which in turn carries glucose. The glucose concentration of this extracted fluid is measured and is proportional to that of blood. This technology, in keeping with its nearly non-invasive description, is commonly associated with some discomfort and requires at least twice daily calibrations against conventional blood glucose measurements (e.g., invasive lancing). Other nearly non-invasive blood glucose monitoring techniques similarly involve transcutaneous harvesting for interstitial fluid measurement. Other technologies for disrupting the skin barrier to obtain interstitial fluid include: 1) dissolution with chemicals; 2) microporation with a laser, sound, or electrical stimulation; 3) penetration with a thin needle; and/or 4) suction with a pump. Minimally invasive blood glucose monitoring can also involve the insertion of an indwelling glucose monitor under the skin to measure the interstitial fluid glucose concentration. These monitors typically rely on optical or enzymatic sensors. Technologically innovative, these in situ sensors have had limited success. Implantable glucose oxidase sensors have been limited by local factors causing unstable signal output, whereas optical sensors must overcome signal obfuscation by blood constituents as well as interference by substances with absorption spectra similar to glucose. Moreover, inflammation associated with subcutaneous monitoring may contribute to systematic errors requiring repositioning, recalibration or replacement, and more research is needed to evaluate the effects of variable local inflammation at the sensor implantation site on glucose concentration and transit time. Interstitial fluid glucose concentrations have previously been shown to be similar to simultaneously measured fixed or fluctuating blood glucose concentrations (Bantle et al., Journal of Laboratory and Clinical Medicine 130:436-441, 1997; Steprnberg et al., Diabetes Care 18:1266-1269, 1995). Such studies helped validate non-invasive/minimally invasive technologies for blood glucose monitoring, insofar as many of these technologies measure glucose in blood as well as interstitial fluid. A non-invasive glucose monitor that is portable, simple and rapid to use, and that provides accurate clinical information is highly desirable. In particular, the ability to derive primary and secondary order information regarding real time, dynamic glucose metabolism (such as the direction and rate of change of bioavailable glucose distributed within the blood and interstitial fluid space) is highly desirable. Part SUMMARY OF THE INVENTION: In vivo fluorescence (autofluorescence) has been used for a number of years to determine the metabolic state and to monitor pharmaceutical effects in cells and tissues (Dellinger et al., Biotechnol Appl Biochem, 28(Pt. 1): 25-32, 1998). Consideration of the photophysics involved in autofluorescence rapidly leads one to the conclusion that the use of autofluorescence alone as the analytic probe imposes some severe limitations on any measurement technique. Fluorescence techniques are capable of detecting molecular species at picomole levels or less. This sensitivity arises because of the simplicity of detecting single photons against a dark background. This advantage disappears if there are other fluorescent species in the detection volume. Fluorescence intensity is also not an absolute technique and must be referenced to some internal standard using a ratiometric or comparative method. It has been shown that fluorophores, or colored dyes utilizing absorption spectroscopy can be used to measure glucose in solution or serum by using series of separate reagents. These generic reagents include glucose oxidase (which oxidizes glucose forming hydrogen peroxide); peroxidase (generally, horseradish peroxidase: HRP) used to create an oxidizing reaction in the presence of hydrogen peroxide with the dye or fluorophore; and a dye reagent or fluorophore, which changes its color or fluorescence spectrum when brought in contact with hydrogen peroxide, and peroxidase. The resultant colored or fluorescent species is measured with a colorimeter or fluorimeter and the amount of glucose in solution is calculated. In addition, other analytical techniques have been shown to be commercially useful for measuring hydrogen peroxide generated from the reaction of glucose oxidase and glucose. The methods and compositions of the present invention effectively determine the glucose concentration in blood for a living organism by non-invasive, in vivo measurement of the glucose level in skin by means of fluorescence measurements of metabolic indicators/reporters of glucose metabolism. Disclosed are dyes used as metabolic indicators that allow for specific in vivo monitoring of metabolites, which are used as indicators of metabolic activity. Dyes characterized by this invention are referred to herein as a small molecule metabolite reporters (“SMMRs”) or alternatively, “small molecule multi-domain reporters (“SMMDRs”.) In many cases the description in the specification will apply to both terms. The invention relates in one aspect to SMMRs which comprise novel xanthene-based boronic acid compounds. The SMMRs may be used in sensor compositions for, e.g., direct measurement of glucose, and in other diagnostic or analytical methods as described herein. In an embodiment these compounds are of the following formula (I): wherein D is a heteroatom; R1 and R2 are different and are selected from the group consisting of H, OH, NH2, NO2, OCH3, N(CH3)2, A, or, R1 and R2, taken together with the ring to which they are attached, form R7; R3 and R4 are different and are selected from the group consisting of H, OH, B(OH)2, M, or R3 and R4, taken together with the ring to which they are attached, form R8; R5 and R6 are different and are selected from the group consisting of H or wherein Q is H, COOH, B(OH)2, or M; A is OH, NH3, R7 is R8 is M is L, when present, is an amino-containing linking moiety; R1 and R2, and R3 and R4, are adjacent to each other on the rings on which they reside; and at least one boronic acid moiety is present; and salts thereof. “Amino-containing linking moieties” may include moieties comprising a substituted or unsubstituted amino group, an amido group or a sulfonamido group. In an embodiment, xanthene-based boronic acid compounds of the invention include those of the formula (II): wherein R1 and R2 are different and may be A, or, R1 and R2, taken together with the ring to which they are attached, form R7; R3 and R4 are different and are selected from the group consisting of H, OH, B(OH)2, M, or R3 and R4, taken together with the ring to which they are attached, form R8; R6 is A is R7 is R8 is M is L, when present, is an amino-containing linking moiety; and R1 and R2, and R3 and R4, are adjacent to each other on the rings on which they reside; and salts thereof. In another embodiment, xanthene-based boronic acid compounds of the invention include those of the formula (III): wherein R1 and R2 are different and are selected from the group consisting of H, OH, NH2, NO2, OCH3, N(CH3)2, A, or, R1 and R2, taken together with the ring to which they are attached, form R7; R3 and R4 are different and are selected from the group consisting of H, OH, M, or R3 and R4, taken together with the ring to which they are attached, form R8; R5 and R6 are different and are selected from the group consisting of H or wherein Q is H or M; A is OH, NH3 R7 is R8 is M is L, when present, is an amino-containing linking moiety; and R1 and R2, and R3 and R4, are adjacent to each other on the rings on which they reside; and salts thereof. Examples of xanthene-based SMMRs of the invention include: The invention relates in another aspect to SMMRs which comprise novel phenalene-based boronic acid compounds. In an embodiment these compounds are phenalene-1-one compounds of the following formula (IV): wherein M is and L, when present, is an amino-containing linking moiety; and salts thereof. Examples of such phenalene-based boronic acid compounds include: The invention relates in another aspect to SMMRs which comprise novel boronic acid-containing coumarin or carbostyril derivative compounds. In an embodiment these compounds include those of the following formula (V): wherein D is a heteroatom (e.g., O or N); R9 is H, OH, CH3, CF3, M, or an amino or substituted amino group; R10 is H, CH3, or M; R11, R12, and R13 are individually H, OH, alkoxy, M, or an amino or substituted amino group; R14, when present, is H or CH3; M is at least one boronic acid moiety is present; and salts thereof. A substituted amino group may include where R11, R12, and R13, taken together with the ring to which they are attached, form a nitrogen-containing polycycle. Examples of such boronic acid-containing coumarin or carbostyril SMMRs include: It will be noted that the structure of some of the compounds of the invention includes asymmetric carbon atoms. It is to be understood accordingly that the isomers arising from such asymmetry (e.g., all enantiomers and diastereomers) are included within the scope of the invention, unless indicated otherwise. Such isomers can be obtained in substantially pure form by classical separation techniques and by stereochemically controlled synthesis. Alkenes can include either the E- or Z-geometry, where appropriate. Tautomeric forms of compounds of the invention are also intended to be included within the scope of the invention, unless indicated otherwise. In another aspect, a chromophore of the following rational design structure is disclosed: wherein Het represents a heteroatomic group, e.g., containing N, O, or S; B(OH)2; M or R8 (as set forth in Formula (I))); or mono or di-substituted N, NO2 or N(CH3)2; which groups may be identical or different. Heteroatomic groups may include amino, amido, carbonyl, hydroxyl, thiol, and thio. This invention also provides for fluorescence measurements of extracellular and intracellular reporter molecules placed into the cytosol, nucleus, or organelles of cells within intact, living, tissue that track the concentration of blood glucose in an organism. When any one of a series of metabolites is measured using this technique, the molar concentration of blood glucose can be calculated. Direct or indirect fluorescence measurements of glucose using one or more of the following measurements is described: pH (as lactate/H+), membrane reduction-oxidation electric potential, NAD(P)H (nicotinamide adenine dinucleotide (phosphate), reduced form) for energy transfer, FAD+ (flavin adenine dinucleotide, oxidized form) for energy transfer, ATP/ADP ratio, Ca2+-pumping rate, Mg2+-pumping rate, Na+-pumping rate, K+-pumping rate, and vital mitochondrial membrane stains/dyes/molecules fluorescence response. These analytes, measured in skin using the techniques taught herein, are used to provide a complete picture of epidermal skin glycolytic metabolism where local epidermal analyte (glucose) quantities are proportional to the concentration of glucose in systemic blood, specifically the capillary fields within the papillary layer of the dermis (corium). Temperature and/or nitric oxide measurement may also be combined with the above measurements for better calibration and determination of glucose concentrations. The invention further provides sensor compositions that are applied to at least one surface of living tissue, organs, interstitial fluid, and whole organisms and transported into the tissue at an effective concentration. The sensor composition can include at least one SMMR at an effective concentration such that when the at least one SMMR is brought in contact with one or more specific metabolites or analytes, a change in fluorescence or absorption occurs, thereby allowing quantification of the change in fluorescence or absorption. The primary embodiment of this invention utilizes a series of molecules (SMMRs) specifically designed for topical delivery onto tissue, such as the viable epidermis, which when applied to the tissue will report glucose concentration using any one or more of several reporting mechanisms. The most significant advantage of the present invention is increased sensitivity in reporting glucose concentration, while eliminating the requirement to draw body fluid from the skin as is required by current conventional techniques. The in vivo information obtained when the SMMR is brought in contact with the one or more metabolites or analytes can include, but is not limited to, assessment of metabolic function; diagnosis of metabolic disease state; monitoring and control of disease state; stress status of cells, tissues and organs; determination of vitality and viability of cells based on metabolic function; critical care monitoring; diagnosis and monitoring of cardiovascular diseases, autoimmune disorders, neurological disorders, degenerative diseases; determination of metabolic concentration; and cancer diagnosis, detection, staging and prognosis. For example, the in vivo information obtained may provide detailed information on glucose metabolism, fructose metabolism and galactose metabolism; advanced-glycosylated end products; monitoring and control of diseases such as diabetes, cancer, stress and organ transplantation. The sensor compositions used in these methods for monitoring the concentration of one or more metabolite(s) or analyte(s) can be formulated as, but are not limited to, emulsions, ointments, disposable gel film patches, reservoir devices, creams, paints, polar solvents, non-polar solvents, or any combination thereof. Penetration of the sensor composition can be accomplished using an active transport technique or a passive transport technique, such as, for example, electroporation, laser poration, sonic poration, ultrasonic poration, iontophoresis, mechanical-poration, solvent transport, tattooing, wicking, microneedle or pressurized delivery. In addition, penetration of the sensor composition to the desired depth can be accomplished by combining the composition with various molecular size attachments. Typically, the quantification of the change in fluorescence or absorption is monitored using fluorescence or absorption spectroscopy. An effective concentration of the sensor composition is, for example, at least between 0.01 to 500 μg/ml, between 0.1 to 500 μg/ml, between 1.0 to 150 μg/ml, between 1 to 100 μg/ml, and between 10 to 100 μg/ml. The SMMR can be introduced in a low concentration in a range from 10 μM to 1000 μM and in a volume from 200 μL to 0.1 μL, respectively (e.g., introducing the SMMR at a concentration in the range of 200 μL of a 10 μM SMMR solution to 0.1 μl of a 1000 μM SMMR solution). One specific application of the sensor composition is, for example, a 5 μL volume of a 400 μM SMMR solution, or a 10 μL volume at 200 μM concentration. Once one or more SMMRs are activated as a result of placement within the skin, fluorescence measurements monitor the response of the skin cells to glucose. As described herein, the fluorescence mechanism used is either a direct or indirect indication of the glucose concentration in the target cell environment. Fluorescence is typically measured using an optical reader. The optical reader calculates the skin response to glucose, applies first principles mathematical models to the response, and provides a determination of the blood glucose levels. Choosing the particular commercially available or custom designed optical reader that is compatible for use with the methods and compositions of this invention is within the ability of one skilled in the art. One embodiment of this invention utilizes indirect means to measure skin and blood glucose in vivo by placing one or more SMMRs into the viable epidermis to form a fluorescent product. This fluorescent product is provided by one of many specifically described reporting mechanisms, whereby the SMMR fluorescent signal changes with respect to the effects of glucose concentration on cell metabolism. The quantity of fluorescence, or the fluorescent ratio at two or more emission wavelengths, is indicative of the total glucose concentration within the skin, either intracellular or extracellular as described here. The skin glucose thus determined is used to infer blood glucose levels as calibrated and described herein. In another embodiment, a method for monitoring in vivo blood glucose levels uses SMMRs that directly bind or respond to glucose itself. The mechanisms of glucose reporting thus does not use cell metabolism, as in the first embodiment, but rather the SMMR responds to glucose by one of several direct mechanisms to produce a fluorescent product. The measured fluorescence is thus a direct reporter of the interstitial fluid or extracellular glucose concentrations. Thus, the skin glucose level directly determined in vivo is used to infer blood glucose levels as calibrated and described within this invention text. This invention describes the unique physicochemical, photochemical, photophysical and biological properties of SMMR molecules, as well as their design, synthesis, and application. The use of an SMMR enables fluorescence measurements from picomolar through millimolar in vivo glucose levels in living skin tissue, or interstitial fluid, either of which are indicative of the blood glucose levels. The invention described here relates to the indirect or direct determination of skin glucose levels for use in the monitoring and control of diabetes mellitus. Embodiments of the invention use SMMR fluorescence to measure skin glucose levels without withdrawing bodily fluids. When the SMMR-based skin glucose measurements are made, the blood glucose levels are directly inferred. The quantity of glucose in the epidermis is supplied by mass transport from the blood vessels and capillary fields located within the dermis, immediately beneath the epidermis. The movement of glucose from the blood stream to the epidermis is concentration dependent and non-insulin regulated providing the basis for measurement of blood glucose as a direct inference from skin glucose measurement. The rate of glucose transport into the epidermis is indicative of the differential concentration between skin glucose and blood glucose levels. The rate of transport into the extracellular spaces between human skin cells allows an accurate first principles mathematical extrapolation of blood glucose levels. Once modeled, the kinetics of blood glucose transport to the skin from the blood enables the determination of the precise first principles mathematical relationship between the rate of change of skin glucose and the rate of change of blood glucose. Thus rapid blood glucose concentration changes up or down can be accurately tracked by knowing the skin glucose mean concentration levels and the rate of change of skin glucose levels. First principles mathematical models can be developed for the individual case, preferably for small local populations, and most preferably for a universal patient case. The SMMR-derived fluorescence reports glucose levels within or surrounding human keratinocyte cells as an indication of blood glucose levels. The movement of glucose from the interstitial fluid surrounding the keratinocytes into the keratinocytes of the epidermis is concentration dependent and non-insulin regulated. That is, the glucose is transported into these cells via non-insulin regulated glucose transporter GluT1 (GenBank Accession Number: K03195), not insulin regulated glucose transporter GluT4 (GenBank Accession Number: M91463). This transport mechanism provides the basis for measurement of blood glucose as a direct inference from intracellular keratinocyte glucose measurement. Also provided are noninvasive methods for monitoring in vivo blood glucose levels. According to these methods at least one small molecule metabolic reporter is applied to at least one surface of skin for a predetermined period of time causing penetration of the one or more SMMRs to a depth of about 10 μm, wherein the depth corresponds with the bottom of the dead stratum corneum layer, to about 175 μm, wherein the depth corresponds with the top of the dermal layer, into the epidermis. The one or more SMMRs come in contact with one or more metabolites or analytes and a change in the concentration of the one or more metabolites or analytes is monitored by detecting changes in the SMMRs using an optical reader. The change in the concentration of the one or more metabolites or analytes is then correlated with in vivo blood glucose levels. Also included in the invention is a reagent strip for use in a glucose measuring instrument comprising a polymer strip and a known concentration of at least one small molecule metabolic reporter, wherein when a sample of a biological fluid containing an amount of glucose is interacted with the reagent strip, a change in fluorescence or absorption of the one or more molecular sensor proteins occurs, and the change is measured by the glucose measuring instrument, thereby detecting the glucose concentration of the biological fluid. The change in fluorescence or absorption can be monitored using fluorescence or absorption spectroscopy. Those of ordinary skill in the art will recognize that any fluorescence or absorption spectroscopic techniques can be used in accordance with the invention. The invention also provides sensor systems that include a device having a component that transmits radiation to a material or tissue, a component that detects radiation emitted from a material or tissue, and a component to display the detection results, each component is operably linked. The sensor systems further include an applicator that delivers the sensor composition of the invention to the material or tissue. Typically, there is an air interface between the device and the material or tissue, wherein the air interface measures a resulting excitation radiation emitted from the irradiated sensor composition. The device included in the sensor system can emit radiation at one or more wavelengths that have been chosen to specifically excite the SMMR mixture that is applied to the material or tissue. The sensor composition can include a reporter dye and a marker dye, or alternatively, a dye exhibiting a wavelength shift in absorption or fluorescence emission in the presence of a metabolite. The sensor composition can be present at a depth from the surface of the skin of about 10 μm to about 175 μm in the epidermis in a concentration that is effective for detection of one or more metabolites or analytes in a biological sample. The sensor system can detect radiation at one or more wavelengths that have been chosen to specifically identify fluorescence emission that has been scattered back to the system from the sensor composition. Part DETAILED DESCRIPTION OF THE INVENTION: The features and other details of the invention will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that particular embodiments described herein are shown by way of illustration and not as limitations of the invention. The principal features of this invention can be employed in various embodiments without departing from the scope of the invention. All parts and percentages are by weight unless otherwise specified. DEFINITIONS The singular forms “a”, “and” and “the” include plural referents unless the context clearly dictates otherwise. For example, the term small molecule metabolic reporter “SMMR” and/or “small molecule multi-domain reporter “SMMDR” includes one or more small molecule metabolic reporters “SMMRs” and/or “small molecule multi-domain reporters “SMMDRs”. Those skilled in the art will recognize that the terms “SMMR” and “SMMRs”, and “SMMDR” and “SMMDRs” are used interchangeably herein. The term “biologically active molecule” includes, but is not limited to, enzymes, coenzymes, metabolites, analytes, reactive species, polypeptides, proteins, cofactors, small molecules and other macromolecules of physiological significance including mixtures or active fragments or subunits thereof. A “small molecule” includes a molecule from 100 Da to 250 kDa. Molecules of this molecular weight range have a demonstrated ability for use as quantitative reporters of glucose activity. The terms “small molecule metabolic reporter(s)”, “SMMR (s)”, “analyte enhancing molecules”, “reporter” and “reporters” include, but are not limited to, fluorophores, protein-labeled fluorophores, proteins with a photooxidizable cofactor (such as FADH contained in a glucose oxidase), and proteins with another intercalated fluorophore. A “chromophore” includes a molecule exhibiting specific absorption or fluorescence emission when excited by energy from an external source. This is a more generic term than fluorophore. A “fluorophore” includes a molecule exhibiting specific fluorescence emission when excited by energy from an external source. An “intercalated fluorophore” includes fluorophores that will fluoresce when intercalated with a molecule. For example, Glucose Oxidase-Intercalated Fluorophore (GO-IF) is a molecule with specific glucose binding sites. The fluorescent properties will change when glucose binds to the molecule, causing a measurable change. A “dye” includes molecules having large absorptivity or high fluorescence quantum yield and which demonstrates affinity for certain materials or organic (cellular) structures. A “xanthene dye” includes a molecule having a xanthene-like skeletal structure, which exhibits large absorptivity and high fluorescence quantum yield and which demonstrates affinity for certain materials or organic (cellular) structures. The phrase “energy transfer from reducing equivalents (e.g., NAD/NADH, NAD(P)/NAD(P)H, FAD/FADH2) indicating SMMRs” refers to a use of SMMRs whereby the presence of these reducing equivalents molecules, is detected by excitation of the reducing equivalents molecules from an external source, energy transfer from the reducing equivalents molecule(s) to an SMMR, and detection of the fluorescence emission at the SMMR emission wavelength. The phrase “absorption/diffuse reflection or fluorescence spectrum” refers to two types of spectra measured independently. The absorption/diffuse reflection spectrum refers to the energy reflection spectrum from a material reported in either the dimensions of reflectance or absorbance versus wavelength. The fluorescence spectrum is measured independently as the fluorescence emission intensity or the fluorescence lifetime of a fluorophore following excitation from an external source. The phrase “molecular size attachment” refers to the molecular size in Angstroms (Å), which is related to molecular weight in Daltons (Da), of an attachment added as an adjunct to an SMMR. As used herein, “molecular size attachments” includes adducts to the fluorescent moieties of SMMRs that include, but are not limited to, structural modifications of fluorescence SMMRs as the additions to the fluorescence structure of: acetoxy methyl esters, chloro-methyl derivatives, alkyl chain adducts, highly charged moieties, enzyme substrate mimics, enzyme cofactor tethers, and membrane binding tethers. As used herein, a “reporter” includes an SMMR having the property of optical or fluorescence signal related to the quantity of analyte in the immediate vicinity of the SMMR. Thus, as the analyte quantity increases, the fluorescence signal changes (up or down) in proportion. As used herein, a “marker” includes a molecule having the property of yielding a fluorescence signal that is constant when applied to target cells or tissues. Its main purpose is for use as a reference signal channel. As such, it is applied in a ratiometric measurement for correction of a reporter signal. The variation in physiological and optical characteristics of individual subjects requires a reference channel signal to correct or normalize a reporter channel signal when the ratio of reporter to marker is used for quantitative applications. As used herein, a “sensor” includes a handheld device capable of making absorption or fluorescence measurements at one or more wavelengths, and converting the ratios and sums of these measurements into analyte concentrations. These analyte concentrations are used to infer the rate or quantity of a specific metabolic process. As used herein, a “metabolite” includes a substance produced by a metabolic process, such as glycolysis, which can be quantitatively measured as an indication of the rate or quantity of a specific metabolic process. As used herein, an “analyte” includes a measurable parameter, using analytical chemistry, which can be quantitatively measured as an indication of the rate and quantity of a specific metabolic process. The term analyte is a generic term describing such concepts as metabolites, ions, processes, conditions, physico-chemical parameters, or metabolic results that can be used to infer the rate or quantity of specific metabolic processes. As used herein, a “response range” includes an analyte range (lower and upper limits) over which a metabolic process, and its measured absorption or fluorescence signal, follow a linear or defined mathematical function. The phrase “physico-chemical parameter” refers to a subset of broadly defined analyte parameters specifically related to the physical chemistry constants of materials. These constants can be used in combination with the measurement of other analytes to infer the rate or quantity of specific metabolic processes. Such constants refer specifically to, e.g., atomic mass, Faraday constant, Boltzmann constant, molar volume, dielectric properties, and the like. As used herein, “wicking” includes the flow of a liquid into a solid material via the pull of gravity, Brownian motion, adhesion, mass transport, or capillary action such that a natural movement of a liquid occurs into a solid material. The phrases “direct metabolic reporters,” and “indirect metabolic reporters” refer to the mechanism of action of SMMR for reporting glucose concentration. Direct metabolic reporters report the concentration of glucose directly, whereas indirect metabolic reporters report the concentration of analytes used to infer the concentration of glucose. As used herein, an “octanol-water coefficient (Kow)” includes a measure of the extent to which a solute molecule is distributed between water and octanol in a mixture. The octanol-water partition coefficient is the ratio of a chemical's solubility (concentration) in octanol to that in water using a two-phase vmcguire@example.org. As used herein, “toxicity” includes the degree or quality of being toxic or hazardous to the health and well being of human and other mammalian organisms, organs, tissues, and cells. The phrase “specialized tattoo” or more precisely the “active viewing window” refers to an area of tissue treated with an SMMR. That area is used for viewing the fluorescence ratio measurements of the SMMR interaction with tissue, in order to directly measure, calculate, or otherwise infer the concentration of skin and blood glucose or other metabolites of interest. As used herein, “organ” includes a structure that contains at least two different types of tissue functioning together for a common purpose. Examples of organs in the body include, but are not limited to, the brain, heart, liver, kidneys, pancreas, stomach, intestines, lungs, and skin. As used herein, a “keratinocyte” includes a living cell comprising the majority of the epidermis of mammalian skin. The keratinocyte is unique in both its proximity to the surface of an organism as well as in its glycolytic behavior. The keratinocyte metabolizes glucose in such a way as to produce a number of analytes whereby the glucose concentration within the cell can be inferred. As used herein, a “mammal” includes both a human and a non-human mammal (e.g., rabbit, mouse, rat, gerbil, cow, horse, sheep, etc.). Transgenic animals are also encompassed within the scope of the term. The non-invasive devices, compositions, and methods of the present invention directly yield in vivo information for the assessment of intracellular and extracellular metabolic state, as well as the stress status of cells, tissues, and organisms. In a preferred embodiment, the devices, compositions and methods of the invention can be used to monitor and determine metabolite concentration levels, and more specifically, determine blood glucose concentration levels. The invention provides non-invasive sensor compositions that comprise one or more small molecule metabolic reporters (“SMMRs” or “reporters”). When applied topically to skin, peripheral tissues, or organs, these reporters are able to penetrate the upper tissue layers and interact with a specific biologically active molecule in such a way as to report metabolic or health status, while not interfering with metabolic function. The reporters provide a metabolic signal that can be used for multiple purposes including, but not limited to, assessment of metabolic function (e.g., particularly as related to glucose metabolism); diagnosis of metabolic disease states (e.g., as related to advanced glycosylated end-products); monitoring and control of disease state; stress status of cells, tissues and organs; determination of vitality and viability of cells based on metabolic function; critical care monitoring; diagnosis and monitoring of cardiovascular diseases, autoimmune disorders, neurological disorders, degenerative diseases; determination of metabolic concentration; and cancer diagnosis, detection, staging and prognosis. Specifically, applying the reporters of the invention to living peripheral or epithelial tissue provides detailed information on the state of multiple metabolic pathways in living organisms that can be analyzed using low-cost, hand held instrumentation. The invention provides techniques whereby one or more reporters are applied to solid tissue (i.e., are introduced to the upper cell layers of tissues and organisms following local and/or topical administration). The reporters are added in trace quantities (from about 10 to about 1000 μL of 0.1 to 200 μM, preferably from about 5 to about 100 μL), using a substance that is transparent to visible light and that has a pre-specified temporary residence at the application site (e.g., 2 days-up to 30 days, 24-48 hours, preferably 2-6 hours, more preferably 30 seconds to 5 minutes, and most preferably 5 seconds to 5 minutes). Contemplated diffusion times include periods less than 48 hrs, 24 hrs, 10 hrs, 6 hrs, 2 hrs, 1 hr, 30 min, 15 min, 10 min, 5 min, 1 min, 30 sec, 10 sec, or 1 sec. Reporters that are placed on skin are able to penetrate the skin and be transported to a depth from the surface of from about 10 μm to about 300 μm into the tissue and are brought in contact with a specific metabolite, wherein a change in fluorescence or absorption (e.g., measured using fluorescence or absorption spectroscopy) of the one or more reporters occurs, thereby allowing quantification of the change in fluorescence or absorption that provides detailed in vivo information regarding picomolar through millimolar cellular metabolite and precursor levels for living tissue, organs, interstitial fluid, and whole organisms. The reporters can be monitored non-invasively using any low-cost instrumentation capable of directly analyzing the metabolic state in tissue (e.g., using optical instrumentation). The reporters are chosen to specifically enhance the signal of pre-specified analytes in order to assess metabolic state of a tissue or organism and to yield detailed, real-time information regarding the state of intracellular and extracellular metabolism. Discussion of Properties of SMMR Compounds: Physicochemical, Photochemical, Photophysical and Biological SMMR compounds consist of elements of molecular sub-structure which confer the special properties required to fulfill their specific metabolic reporter function: 1. A fluorescent reporter, with specific photophysical properties, 2. Chemical functional groups that confer affinity to metabolites, enzymes, cell organelles, membranes, or glucose itself, 3. Structural features that confer specificity between the target of interest and similar targets, which are present in the biological medium (e.g., glucose versus fructose). The chemistry of small molecule metabolic reporter (SMMR) compounds combines a number of parameters that, in general, results in the following characteristics: SMMR compounds are non-toxic; they have high molar absorption coefficients, and a high quantum yield of fluorescence. They have a large Stokes shift; they are readily taken up by cells, and are retained in the active form at the target tissue. They undergo a large change in fluorescence in response to the metabolism monitored, they are photostable, they do not exhibit excited state chemistry, and they are eventually lost from the body by shedding of the stratum corneum. The molecular design behind these characteristics is the subject of this invention. Toxicity The metabolic impact of the compounds to be used as SMMRs is low because of a number of properties that are common to these molecules. When minute quantities may be absorbed into the body they are readily eliminated from the system via biotransformation (metabolism) of the aromatic rings through hydroxylation by a variety of nonspecific enzymes contained within the microsomes of the liver endoplasmic reticulum. Small, more water soluble metabolites result, which are then eliminated from the body by passing through the glomeruli into the proximal tubules of the kidney and into the urine. SMMRs have strict requirements relative to toxicity. Four main criteria must be met: (1) they do not bind to DNA, (2) they do not disrupt cell membranes, (3) they are used at low concentration (e.g., 50 μL of 250 μM), and (4) the SMMR is delivered to a limited volume of tissue, typically the viable epidermis. The Activity Index (A.I.) of the SMMR compounds is an indication of the effective dose required to elicit an appropriate response to a metabolic signal or glucose. It is indicated by the ratio of the Toxic Dose (T.D.) to the Effective Dose (E.D.) as in equation (1). Note that the A.I. for any proposed SMMR must be greater than 1.0 and ideally should be 10,000 or more. A . I . = T . D . E . D . ( EQ . ⁢ 1 ) A better indication of the safety of the SMMR would be indicated by the Minimum Activity Index (A.I.min) as the ratio of the Maximum Tolerated Dose (T.D.max) for 100 percent of the tested group, indicating a maximum dose at which no adverse effects occur, to the Maximum Effective Dose used (E.D.max), whereby the maximum signal occurs as equation (2). The larger the A.I.min the better toxicity to effective signal characteristics the SMMR possesses. This number must always exceed 1.0 with values greater than 5.0 considered optimum. A . I ⁢ . min = T . D ⁢ . max E . D ⁢ . max ( EQ . ⁢ 2 ) SMMR molecules with quantum yield (φF) values substantially less than unity (e.g., less than 0.3) or those with longer fluorescent lifetimes may show phototoxicity via a photodynamic effect, a process that is unrelated to the inherent toxicity of the SMMR. Phototoxicity arises from reactions of an excited state or from a reactive intermediate generated by the excited state. Phototoxicity can be minimized by using low light doses combined with high quantum yields (e.g., less than 5 mW excitation with a φF greater than 0.6), thereby decreasing the energy available to form damaging oxygen radicals, and the number of excited states generated, respectively. As a general rule, exogenous materials such as drugs or, with reference to this embodiment, SMMRs that interact with more than one metabolic pathway have a reduced likelihood of becoming clinically significant due to the availability of compensatory pathways if one is inhibited. Interactions, and hence toxicity, are likely to be increased if the SMMR is an inhibitor or inducer of a particular enzyme, if the response of the SMMR is critically dependent on the concentration and particularly if turnover of the SMMR occurs via a single specific pathway (see for example, Johnson, M. D. et al. Clinically significant drug interactions. Postgraduate Medicine 1999; 105(2): 193-222). To a large extent the rationale for SMMR design has avoided many of these problems by targeting normal overall changes in the chemistry of the cell rather than the concentration or activity of a specific metabolite. For example, the monitoring of reducing equivalents within a cell by energy transfer does not affect the overall intracellular concentration of those equivalents nor are there interactions between the enzymes responsible for metabolism and the SMMRs. For some molecules, the toxicity or carcinogenicity is not related to the parent molecule but the metabolite of the molecule. This is often a result of activity of the liver on the molecule. For example, metabolites of benzene are formed by the action of cytochrome P450 in the liver to form, epoxides, phenol, catechol and muconaldehyde. Many of these compounds are extremely reactive and toxic and they may be metabolized further to other toxic materials. Molar Absorption Coefficient The molar absorption coefficient (∈) of a typical SMMR is high (greater than 50,000 dm3 mol−1 cm−1). This implies that the probability of a transition from S0→S1 is high. The amount of light absorbed is given by the Beer-Lambert law as equation (3). log ⁢ ⁢ I 0 I t = A = ɛ ⁢ cl ( EQ . ⁢ 3 ) Where, I0 is the intensity of the incident light, It is the intensity of the transmitted light, A is the absorbance, ∈ is the molar absorption coefficient, c is the concentration and 1 is the pathlength. This expression can also be rearranged to give the fraction of incident light that is absorbed (Ia) as described in equation (4). Ia=1−10−A (EQ. 4) For an SMMR concentration of 10 μM in a sample thickness of 100 μm (typical skin thickness for skin epidermis), a molar absorption coefficient of 50,000 dm3 mol−1 cm−1 results in 11% of the incident light being absorbed. Ultimately, the more light absorbed the more will be converted into fluorescence. In designing an SMMR a high molar absorption coefficient (∈) is important; practically this means values greater than 50,000 dm3 mol−1 cm−1. The probability of the S0→S1 transition occurring can be explained in molecular terms by consideration of the type of bonding that is present in the SMMR. A high probability for the transition requires good overlap between the orbitals in the ground and excited state. Such overlap is found for π-π* transitions and charge transfer states. Transitions involving non-bonding electrons, i.e., n-π* are not as probable and hence the molar absorption coefficients are lower for these type of transitions. However, the electrons in a non-bonding orbital are higher in energy than electrons in a bonding orbital and therefore the n-π* transition occurs at lower energy than the π-π* transition. Because of the low probability of the n-π* transition the excited state generated by such a transition is expected to be longer lived. Since the balance between the rate constants for radiative and non-radiative decay determines fluorescence quantum yield a long lifetime, in general, allows a greater probability for radiationless decay to occur. Hence, n-π* transitions tend to be non-fluorescent. Fluorescence Quantum Yield Following absorption of a photon the SMMR is promoted to an electronically excited state. The molecule undergoes vibrational changes and interactions with the solvent that result in relaxation to the state from which fluorescence occurs. If this state only undergoes spontaneous emission then the yield of fluorescence is high (φF=1). If the state undergoes any other process, such as internal conversion or other photophysical change, then φF<1. The fluorescence quantum yield (φF) of an ideal SMMR is close to unity. Compounds with φF less than unity are likely to be less photostable and more photoreactive. The factors that are common in the design of an SMMR that result in a high quantum yield are: 1. Rigidity of the molecule. Constraining a molecule limits the number of vibrational modes by which the excited state can be deactivated. Binding of ethidium bromide to DNA for example, increases the quantum yield of the molecule by 30 times. 2. Lack of heavy atom effect. Heavy atom substituents, such as iodine or bromine, cause spin orbit coupling in a molecule and facilitate intersystem crossing. The excited singlet state for such a molecule can readily form a triplet state. Not only does this process decrease the fluorescence quantum yield but it also generates a potentially long-lived reactive state. 3. Bonding character. As mentioned above, n-π* transitions are, in general, not only weakly absorbing but also non-fluorescent. π-π* transitions have high molar absorption coefficients and are fluorescent. For molecules that have both an electron donating and withdrawing group attached to the π system, the transitions that can occur are described as charge transfer. Typical electron donating groups include amine and hydroxyl groups and withdrawing groups include carbonyl and nitro groups. The transitions are intense and if the transition is of the lowest energy, they are also fluorescent. From this discussion, it is apparent that the intensity or brightness of fluorescence is determined by the product of the molar absorption coefficient and the quantum yield (∈φF). If brightness is sufficiently high, then even at low concentration, the SMMR absorbs excitation light strongly and efficiently converts this energy to fluorescence. As a general rule then the minimum requirement for an SMMR is that ∈>50,000 dm3 mol−1 cm−1 and φF>0.2; thus brightness or ∈φF≧25,000 dm3 mol−1 cm−1. Stokes Shift The Stokes shift is the difference in energy between the lowest energy absorption and the highest energy emission of a molecule. The advantage in having a large Stokes shift is that it is much easier, from a practical standpoint, to optically eliminate the influence of the excitation light on the detected light, i.e., the bandpass filter requirements are simplified. Consideration of a Jablonski diagram (below) would imply that for an S0 to S1 transition, involving only the lowest vibrational levels, the energy of the fluorescence should be the same as the absorption. This is almost never precisely the case. The main reasons are threefold: 1. Consideration of the bond order in the transition. In a π-π* transition the electron distribution in the excited state involves non-bonding orbitals. As a result the bond length and the magnitude of the vibration in that bond increases. This process results in a loss of energy and therefore the fluorescent transition occurs from an excited state slightly lower in energy that the state generated by the absorption. 2. The lower bond order in the excited state literally causes the molecule to expand. This change in volume is measurable using photoacoustic spectroscopy. For a molecule in solution to expand requires work to be done in pushing back the solvent. This work results in a loss of energy in the excited state and an increase in the Stokes shift. The volume change increases as the size of the π system increases. 3. The greater the degree of flexibility in the molecule the more the number of vibrational modes available to the molecule. For a complex molecule energy may be lost from parts of the molecule not associated with the chromophore. Any mechanism that causes the molecule to lose energy, including solvent and intermolecular interactions, will lead to a decrease in the energy of the observed fluorescence. It is noted that factors that serve to increase the magnitude of the Stokes shift also serve to lower the overall fluorescence quantum yield. A novel method to increase the separation of the excitation and emission wavelengths for an SMMR is to covalently link two fluorescent probes together. In practice the molecule that absorbs the light need not even be fluorescent provided its excited state lifetime is long enough to transfer energy to the acceptor molecule that fluoresces. This kind of system increases the design flexibility of the SMMR. Proposed examples to monitor pH include a molecule where both the donor and acceptor are sensitive to pH and because of electrostatic changes associated with pH changes, the energy transfer process is also sensitive. ratiometric measurements of the donor and acceptor fluorescence show very large changes as a function of pH. An ideal SMMR has a Stokes shift of about 50 nm or more. Xanthene dyes typically have a Stokes shift of 5-15 nm but this value is also dependent on the pKa of the molecule. As an example, BeXan type dyes exhibit Stokes shifts of about 40 nm for the acid form and 60 nm for the basic forms of the dye. Predicting Spectroscopic Properties from Molecular Structure The prediction of spectroscopic properties such as absorption and emission spectra are very difficult. The absorption of a photon involves the promotion of an electron from a ground to excited state. There is currently no molecular calculation algorithm nor automated software package that can take into account the chemical microenvironment of a molecule. The position of the S1←So transition is calculated by considering the energy of
{'bibliographic_information': "{'country': 'US', 'doc-number': '10584821', 'kind': 'B2', 'date': '20041223', 'extension': '544', 'invention_title': 'SMMR (small molecule metabolite reporters) for use as in vivo glucose biosensors'}", 'source_file': './grant/2011/ipg111004.xml', 'source_data': 'USPTO-Grant'}