marine 
 mussels and sandcastle worms use a suite of proteins rich 
 in 3,4-dihydroxyphenyl - l - alanine ( dopa ) to attach to diverse 
 hard surfaces in the sea . in 2006 , an elegant 
 demonstration of a strong ( 1 nn ) but reversible interaction 
 between a single tethered dopa and a wet tio2 surface raised 
 the possibility that any polymer backbone functionalized with dopa 
 might be rendered adhesive . for a variety of reasons for example , a synthetic acrylamide copolymer functionalized with 
 dopamine achieved less than 70 pn of adhesion per catechol on tio2 , and most poly(ethylene glycol ) 
 constructs with dopa show adhesion to biological surfaces only following 
 oxidation with periodate . to better understand the interplay between the surface 
 activity of dopa and its associated macromolecular backbone , lin 
 et al . examined the adhesion to mica of 
 two mussel foot proteins ( mfp ) with similar dopa contents using a 
 surface forces apparatus and identified two distinctly different dopa - mediated 
 adhesive modes , that is , a bridging and a nonbridging adhesive mode 
 ( figure 1 ) . in the first mode , molecules span 
 the gap between two solid surfaces with adhesive interactions on both 
 sides ; in the second , molecules confine most of their adhesive interactions 
 to one surface . one protein , mfp-3f , exhibited strong and rapid bridging 
 adhesion between the mica surfaces , whereas the other , mfp-1 , bound 
 to one mica surface only with little to no tendency to bridge over 
 to the other side , except under high shear . only two mussel foot proteins , 
 mfp-3 and mfp-5 , exhibit strong bridging adhesion . not surprisingly , 
 these proteins are localized to the interface between the plaque and 
 the substratum . of the mussel foot proteins , mfp-5 has the highest 
 dopa content ( 30 mol % ) and achieves an adhesion energy per 
 unit area on mica that approaches the interaction between tethered 
 biotin and avidin . on the basis of the 
 known primary sequence of mfp-5 , we explore the following questions 
 here : ( 1 ) do the amino acid sequences flanking dopa affect bridging 
 adhesion ? ( 2 ) do interactions besides those involving dopa help to 
 mediate the bridging adhesion of mfps ? ( 3 ) is bridging adhesion dependent on chain length and surface type ? cartoon 
 of a molecule interacting with substrates in strong and 
 minimal bridging adhesive modes . in the first mode , molecules span 
 the gap between two solid surfaces with energetic interactions on 
 both sides ; in the second , molecules confine most of their energetic 
 interactions to one surface . to address these questions , three mfp-5-inspired peptides 1516 
 residues long were prepared with and without enzymatic modification 
 of tyrosine ( y  y * denotes tyr to dopa ) : vgsgy*dgy*sdgy*y*dg 
 ( pep - pi-4 ) , hy*hsggsy*hgsgy*hg ( pep - pi-6.5 ) , gy*kgky*y*gkg 
 kky*yy*k ( pep - pi-10 ) ( figure 2 and figure s1 ) . the effect of chain length on bridging 
 adhesion was tested by also preparing a peptide consisting of the 
 pep - pi-4 sequence repeated twice in a linear fashion ( pep - pi-4-dimer ) . our rationale in selecting these particular peptides was that they 
 represent more than half the sequence and most of the sequence diversity 
 of mfp-5 with the smallest possible number of amino acids . the repulsive 
 and attractive forces of these peptides on mica and gold surfaces 
 were investigated with a surface forces apparatus ( sfa ) . dissecting mussel adhesive protein mfp-5 into distinct 
 peptides . ( a ) an adult mussel attached to a glass surface by a byssus containing 
 many threads and adhesive plaques . ( b ) schematic zoom of a plaque , 
 showing the  super glue ( c ) three unique sequences with 1516 amino acid residues and 
 representing about three - fifths of mfp-5 were selected from the parent 
 sequence . a longer dimeric 
 peptide was created by connecting two pep - pi-4 monomers in tandem . mushroom tyrosinase is known to o - hydroxylate 
 peptidyl tyr to dopa ( figure 3b ) and was thus 
 used for this purpose in the peptides . the conversion 
 by tyrosinase requires safeguards to minimize the 
 formation of undesirable side products such as 3,4,5-trihydroxyphenylalanine 
 ( topa ) and dopaquinone ( figure 3b ) . by reversibly capturing dopa immediately after 
 it has formed , borate significantly improves dopa yield and decreases 
 side - reactions . after stopping the reaction , modified peptides were 
 separated by reverse phase hplc , which typically eluted three types 
 of modified peptides , i.e. , those that were ( a ) partially modified 
 ( some unmodified tyr residues remain ) , ( b ) completely modified ( every 
 tyr residue is modified ) , or ( c ) hypermodified ( one or more tyr residues 
 were converted to topa ) peptides . electrospray ionization mass spectrometry 
 ( esi ) and matrix assisted laser desorption ionization ( maldi ) with 
 time - of - flight ( tof ) were used to measure the peptide masses in the 
 eluting fractions ( figure s2 ) . those fractions 
 with masses matching the calculated mass of completely modified ( type 
 b ) peptides were pooled for amino acid analysis and sequencing by 
 tandem ms . amino acid analysis results demonstrate that the compositions 
 of pure modified pep - pi-4 , pep - pi-6.5 , and pep - pi-4-dimer are consistent 
 with calculated masses : only traces of tyr remained , and no topa was 
 detectable . for pep - pi-10 , there is one tyr that persistently eludes 
 conversion to dopa : this is y-14 , the central member in the y - triad . contact between tyrosinase and y-14 is presumably impeded by the bulky 
 borate groups on y*-13 and y*-15 during the modification . ( a ) tyr residues in the three different peptides were enzymatically 
 hydroxylated to dopa . however , ( b ) mushroom tyrosinase action is not 
 limited to o - hydroxylation of tyr but further converts 
 dopa to dopa quinone or topa . to avoid these reaction products , ( c ) pie chart of amino acid composition of each modified 
 peptide after enzymatic conversion and purification . bridging adhesion was measured in the surface 
 forces apparatus 
 ( sfa ) using an asymmetric configuration ; that is , a monolayer of peptides 
 ( diameter of hydration 23 nm ) was adsorbed to one 
 mica surface which was then brought into contact with the clean surface , 
 compressed , and separated . modified peptides pep - pi-4 , pep - pi-6.5 , 
 and pep - pi-10 all adhered to mica surfaces , with the strongest adhesion 
 force , 4 mn / m ( corresponding to a work of adhesion of 0.64 
 mj / m ) , associated with pep - pi-10 . pep - pi-6.5 , although 
 having the lowest dopa content ( only 3 dopa out of 15 residues ) , exhibited 
 an intermediate adhesion force of 3.0 mn / m , and the weakest 
 adhesion force ( 2.6 mj / m ) was observed for pep - pi-4 ( figure 4a ) . these work of adhesion values are notable in 
 being more than an order of magnitude less than full - length mfp-5 , 
 even though each peptide represents approximately a fifth of the native 
 sequence . the most likely explanation for this vastly decreased adhesion 
 relative to the native sequence is that the shortened 1517 
 residue peptide chains have a much smaller probability of adsorbing 
 in configurations where  loops  or  tails  
 of the peptides adsorbed onto one mica surface are of sufficient length 
 to make a bridging contact with the opposing mica surface . as a result , 
 most of the short peptides end up interacting intimately with one 
 mica surface , and with few remaining groups available for intimate 
 contact with the other surface , they rely on longer range coulombic 
 attractions , e.g. , positive lys or his to negative siloxyl groups , 
 for adhesive bridging ( figure 1 ) . ( a ) force  distance profiles of three short peptides in asymmetric 
 configuration on mica surfaces . ( b ) summary of adhesion energy of 
 three short peptides measured in ph 3 , ph 7.5 , and ph 3 following 
 periodate oxidation .  0  ,  +  , and represent respectively the neutral , positive , and negative charge 
 the peptides carry at each condition . ( c ) comparison of the adhesion 
 energy of three short peptides with and without tyrosinase modification 
 and mfp-5 . the adhesion energies of three short peptides shown in 
 ( b ) and ( c ) are averaged from between 6 and 12 repetitive force runs 
 from two different experiments at each treatment . consistent with previous work , the results confirm 
 that dopa plays 
 an essential role in the adhesion of each of the peptides to mica 
 surfaces . all three peptides adhered most strongly to mica surfaces 
 at ph 3 where dopa is stable , and increasing the solution ph to 7.5 , 
 where much of the dopa is quickly oxidized to the nonadhesive quinone 
 form ( figure 3b ) , significantly reduced the 
 measured adhesion : the work of adhesion dropped by 80% , 60% , and 35% 
 for pep - pi-6.5 , pep - pi-4 , and pep - pi-10 , respectively , at ph 7.5 ( figure 4b ) . previous tests on mussel proteins have shown that dopa undergoes auto - oxidation at ph 7.5 , leading 
 to diminished mfp adhesion . given their derivation from mfp-5 sequences , 
 the dopa - containing peptides should be prone to similar oxidation 
 tendencies . notably , adhesion between two mica surfaces is never eliminated 
 under dopa oxidizing conditions , even for pep - pi-4 at ph 7.5 , where 
 both the peptide and the mica surface are negatively charged . in addition 
 to this residual adhesion , dopa oxidation alone also fails to provide 
 an explanation for why the ph affects the three short peptides so 
 differently . in addition to issues associated with dopa auto - oxidation 
 at alkaline 
 ph , changes in peptide adhesion also reflect changing electrostatic 
 interactions due to the protonation and/or deprotonation of amino 
 acids and surface siloxyl groups . increasing 
 ph significantly changes the charge densities ( or the overall charge ) 
 in the three short peptides . at ph 3 , pep - pi-6.5 and pep - pi-10 are 
 both positively charged due to the histidine and lysine residues , 
 respectively , so both can favorably interact with the negatively charged 
 mica surface through attractive electrostatic interactions . increasing the solution ph to 7.5 greatly reduces 
 the positive charges in pep - pi-6.5 ; therefore , pep - pi-6.5 loses both 
 the hydrogen - bonding ability of dopa to mica surfaces ( due to oxidation ) 
 and the attractive electrostatic interactions offered by positive 
 amino acid residues ( by histidine deprotonation ) , which results in 
 the largest decrease in terms of the percentage of the adhesion force 
 ( 75% decrease ) . in comparison , the sign and density of the charges 
 in pep - pi-10 remain unchanged at ph 7.5 , which enables pep - pi-10 to 
 maintain attractive interactions between two mica surfaces , despite 
 depletion of dopa by oxidation ( 35% decrease of adhesion ) . notably , 
 pep - pi-4 maintains high bridging adhesion between mica 
 surfaces upon increasing the ph from 3 to 7.5 , which would not be 
 expected with both dopa oxidation and carboxylate formation in pep - pi-4 , 
 resulting in electrostatic repulsion between pep - pi-4 and mica . one plausible 
 explanation for this apparent anomaly is that interfacial proton concentration 
 is rarely the same as that of the bulk and that a strongly acidic interface such as that formed between 
 mica and pep - pi-4 can shield interfacial dopa from the oxidation expected 
 in solution at ph 7.5 . thus , the bidendate binding of intact dopa 
 would counteract the repulsion by the acidic groups . the contribution 
 of electrostatic interactions relative to h - bonding 
 was explored by injecting periodate , an artificial oxidant , into the 
 solution between two mica surfaces at ph 3 : after periodate injection , 
 all three peptides exhibited reductions in the measured attractive 
 forces . further , pep - pi-6.5 and pep - pi-4 exhibited higher adhesions 
 at ph 3 with periodate than were measured at ph 7.5 . the work of adhesion 
 measured for pep - pi-6.5 at ph 3 in the absence of dopa residues is 
 0.28 mj / m , almost 3 times the value measured at ph 7.5 
 ( 0.1 mj / m ) . the work of adhesion for periodate - treated 
 pep - pi-4 was measured to be about twice as much as that at ph 7.5 . both of these observations implicate the involvement of electrostatic 
 interactions in the binding of peptides to mica surfaces ( figure 4b ) . to further resolve the relative contributions 
 of charged residues 
 and dopa to the adhesion of three short peptides , sfa tests were performed 
 on the unmodified peptides ( without converting tyrosines to dopa ) . 
 the work of adhesion of the three unmodified peptides to mica surfaces 
 ranged from 0.27 to 0.35 mj / m ( figure 4c ) , with trends similar to those seen with dopa containing 
 peptides , that is , pep - pi-4 < pep - pi-6.5 < pep - pi-10 , which 
 come from electrostatic interactions as well as multiple tyr - mediated 
 monodentate h - bonds . tyrosine conversion to dopa as much as doubled 
 the adhesion energies on the same surface which further confirms the 
 importance of dopa for the bridging adhesion of all the peptides . after oxidizing the dopa group in the peptides by periodate , the adhesion 
 energies of three modified peptides dropped to values approaching 
 the unmodified peptides , again indicating the effect of enzymatic 
 modification ( figure 5a ) . no change of adhesion 
 energy was observed for unmodified pep - pi-10 ( figure 5b ) . ( a ) adhesion 
 of three short peptides after periodate oxidation resembles the adhesion 
 energy of three unmodified peptides . the adhesion energies are averaged 
 from the values of 612 repeating force runs under each condition . ( b ) force  distance curves of pep - pi-10 before and after adding 
 periodate but keeping the ph unchanged ( ph = 3 ) . adding periodate 
 did not change the adhesion force of unmodified pep - pi-10 , but the 
 hard wall increased slightly . to evaluate how peptide length affects adhesion and to provide 
 a better functional synthetic model system for bridging adhesion on 
 mica , the pep - pi-4-dimer sequence was subjected to evaluation by sfa . increasing chain length should enhance the formation of loops and 
 tails in the polymer chain . as -cation interactions between 
 aromatic and cationic residues in the peptides can complicate their 
 conformation as well as self - association , the pep - pi-4 sequence was chosen for the dimer 
 study as the only peptide sequence without ( ph - dependent ) positively 
 charged residues . the adhesive properties of this peptide dimer were 
 characterized before and after mushroom tyrosinase modification by 
 depositing the modified and unmodified peptide dimers onto a single 
 freshly cleaved mica surface and then measuring the adhesive bridging 
 of this peptide dimer to another mica surface . at ph 3 , pep - pi-4-dimer 
 promotes adhesion between the mica surfaces of 6 mn / m ( corresponding 
 to a work of adhesion of 1 mj / m ) ( figure 6 ) , which is approximately twice the adhesion exhibited 
 by the pep - pi-4 monomer sequence . given that the pep - pi-4 monomer 
 and dimer were deposited at the same solution mass fractions ( as opposed 
 to molar concentrations ) , surface dopa amounts in two cases should 
 be roughly equal . higher adhesion of dimer should be due to its higher 
 tendency of forming  loops  and  tails  
 which are favorable for bridging . as in the case of the short peptides , 
 increasing ph to 7.5 decreased adhesion of pep - pi-4-dimer to mica 
 by approximately 65% to a value of about 0.35 mj / m due 
 to dopa oxidation . since both the pep - pi-4-dimer sequence and the mica surface should 
 be negatively charged at this ph , the presence of residual adhesion 
 is unexpected . interfacial 
 ph  explanation that was invoked for the pep - pi-4 monomer . the adhesion of unmodified pep - pi-4-dimer ( without dopa ) on mica at 
 ph 3 was measured to be 0.4 mj / m ( figure s4 ) , which is approximately half the adhesive energy 
 measured for the dopa - containing dimer . interestingly , the adhesion 
 measured for the unmodified pep - pi-4-dimer was seen to be independent 
 of ph for the ph 3 and 7.5 conditions tested in the current study 
 ( figure s4 ) . tyrosine residues do not undergo 
 autoxidation or deprotonation at ph 7.5 and may interact favorably 
 with muscovite mica surfaces through either monodentate hydrogen bonds 
 ( analogous to the bidentate bonding exhibited by dopa ) or cation- 
 interactions with the k ions that are present at mica 
 surfaces . force profiles of the dopa - modified pep - pi-4-dimer on mica at ph 
 3 and 7.5 . adhesion is approximately double that of the corresponding 
 monomer in figure 2 and shows similar losses 
 due to auto - oxidation at ph 7.5 . to test for bridging adhesion between dissimilar surfaces , sfa 
 measurements were performed where one of the mica surfaces was replaced 
 with a smooth gold surface ( 0.5 nm rms roughness ) . pep - pi-10 mediated 
 the strongest adhesion between two mica surfaces ( figure 4a ) and hence was selected for adhesive bridging 
 between asymmetric mica / gold surfaces . pep - pi-10 was first deposited 
 onto mica where many of the dopa groups form strong , specific interactions 
 with the mica substrate , and this film was then compressed against 
 an opposing gold surface at ph 3 ( figure 7 ) . the adhesion of pep - pi-10 in this gold / peptide / mica geometry ( 3 
 mj / m ) was much stronger than the adhesion measured in 
 the mica / peptide / mica geometry . this strong adhesion is attributed 
 to the high surface energy of gold , which has strong so - called  soft 
 epitaxial  binding with dopa , the other amino acid side chains , 
 and backbone amides of protein / peptides . as a result , pep - pi-10 binds gold surfaces , even 
 when most of the dopa side chains have been recruited to the mica 
 surface during the initial film deposition . force distance 
 profile of pep - pi-10 coated on one mica surface 
 against a gold surface . pep - pi-10 showed 5 times stronger adhesion 
 force in comparison to the adhesion force between two mica surfaces . in 
 previous sfa studies of mfp - based adhesion , little attention 
 was given to the electrostatic interactions between mfps and substrates . 
 in this work , however , the range of peptide ionizations enabled a 
 systematic study of electrostatic contributions to adhesion in dopa - containing 
 systems . these measurements were performed at high salt concentration 
 ( 0.35 m ) that , like seawater , typically reduces the debye length to 
 0.5 nm . here , we show that electrostatic interactions between 
 mfps and substrates make a significant contribution to bridging adhesion . these results also highlight the importance of electrostatic interactions 
 in highly concentrated electrolyte solutions , which are typically 
 ignored . electrostatic interactions are advantageous in that they 
 are nonspecific and long - range in nature ( potentially extending over 
 several nanometers ) , whereas dopa - mediated interactions have angstrom 
 length scales and require complementary target geometry for h - bond 
 or coordination bond formation . as most biological molecules and surfaces 
 are charged , long - range electrostatic interactions may serve to guide 
 mussel adhesive proteins and various mussel inspired adhesive polymers 
 toward target binding sites on substrate surfaces , paving the way 
 for dopa residues to make strong , specific bonds . another major finding of this study is that increasing peptide 
 length enhances dopa - mediated bridging adhesion on surfaces . with 
 dopa residues scattered along the sequence , adhesive proteins or peptides 
 have to be long enough to adopt appropriate conformations on one mica 
 surface that enable the presentation of dopa containing side chains 
 to the other surface . all three mfp-5-derived short peptides showed 
 adhesion energies that are at least an order of magnitude lower than 
 the adhesion energy of intact mfp-5 , about 7.1 mj / m ( adjusted 
 according to the derjaguin approximation f/2r ) , measured by the sfa . mfp-5 
 consists of about 74 amino acid residues , 20 of which are dopa ( 30 
 mol % ) . given their size and flexible extended conformations , mfp-5 
 molecules are likely to have wormlike structures on the mica surface . 
 the longer protein backbone and many dopa side chains give mfp-5 molecules 
 an excellent opportunity to bridge two surfaces and to have dopa residues 
 from the same backbone firmly planted on both sides ( figure 8) . ( a ) different conformations 
 of peptides 
 on surface : train loop and tail . ( b ) peptide monomers and dimers bridging 
 two mica surfaces . ( c ) the red side chains represent dopa , green side chains represent 
 positive charge ( lys ) , and blue ones represent side chains of the 
 other amino acids . the peptides , in contrast , 
 are much shorter , with between 3 and 
 5 dopa groups in their sequence . during the deposition , an entire 
 peptide may stick as a train to one mica surface , leaving few or no 
  loops  or  tails  with dopa groups to 
 bridge to the other mica surface . a critical peptide 
 length in adhesion is supported by the sfa results of pep - pi-4-dimer , 
 which demonstrates that doubling the length of the peptide sequence 
 roughly doubles the adhesive forces . this mechanism is consistent 
 with polymer scaling theory , where , in the simplest picture , the mean - squared 
 radius of a solvated polymer in solution , r , scales as n , where n is the number of monomers along 
 a polymer backbone . thus , doubling the length of a 1517 residue 
 peptide should be expected to increase r by a factor of 1.5 , and increasing 
 the length of a 15 residue peptide by a factor of 5 ( mfp-5 is a 75-residue 
 protein ) should be expected to roughly increase r by a factor of 2.63 . both 
 cases demonstrate that increasingly compact and  looped  
 solution configurations of the proteins and peptides become highly 
 favored with increasing length . dls ( figure s5 ) shows pep - pi-4 monomer and dimer have similar hydrodynamic diameter 
 in solution , which indicates dimer has more compact configuration 
 than monomer . since solution configurations are intimately connected 
 to adsorption configurations in cases where polymers quickly and irreversibly 
 bind to surfaces , we believe that on surfaces the dimer adopts a more 
 compact configuration , too , which results in more  loops  
 and  tails  structure to stick to the second surface . 
 when a peptide is deposited onto a surface , entropy maximization will 
 be in competition with the chemical free energy reduction change by 
 dopa / surface binding . with longer peptides , the entropic contribution 
 to the free energy offered by adopting a random structure will win 
 out over the free energy decrease due to chemical binding , which would 
 result in peptides lying flat on surfaces . when the opposing surface 
 is presented , a flexible random structure is more favorable for the 
 peptide to bridge two surfaces when adhesion depends on forming dopa 
 mediated interactions on both sides . the unevenly distributed 
 dopa binding of pep - pi-10 is further suggested 
 by the strong bridging adhesion ( 3 mj / m ) of pep - pi-10 
 in a gold / peptide / mica geometry by measuring the adhesion of a pep - pi-10 
 deposited mica surface against an opposing gold surface in ph 3 buffer 
 ( figure 7 ) . on the peptide / gold interface , 
 pep - pi-10 interacts with the gold surface through  soft epitaxial  
 binding ; adding dopa functions as a  surface 
 anchor  . thus , strong adhesion 
 can be promoted between the mica and gold surfaces , even if few dopa 
 residues are available to interact with the gold surface . further , 
 assuming that the binding of pep - pi-10 to mica is the weakest link 
 in the peptide mediated bridge between gold and mica , the adhesion 
 of 3 mj / m is about 40% of the value of intact mfp-5 on 
 the mica surface . mussel adhesive proteins have inspired numerous 
 investigations 
 of dopa- or catechol - functionalized synthetic materials for various 
 applications including wet adhesives , antifoulant coatings , magnetic imaging agents , tissue glues , 
 and ph - sensitive hydrogels . most of these materials have been designed 
 by a trial - and - error approach after many costly and time - consuming 
 iterations . we maintain that a systematic understanding of factors 
 that contribute to bridging or coating adhesion in mussels proteins 
 can accelerate the practical implementation of important design concepts . the design and synthesis of mussel - inspired peptides allowed us 
 to investigate the role of electrostatic  especially cationic electrostatic 
 effects in the adhesion of mfp-5 were not previously detected , perhaps 
 because of steric constraints imposed by the long native protein backbone . the chain - length - dependent adhesion of pep - pi-4-dimers and monomers 
 explicitly illustrates the significance of peptide length on promoting 
 adhesion between two surfaces when the adhesion of peptides to each 
 of the surfaces is dominated by dopa - mediated interactions . increasing 
 chain length promotes increasingly disordered structures , rather than 
 the flattened conformations where most , if not all , the dopa - containing 
 side chains are bound to a single surface , and thus promotes interaction 
 with a second approaching surface . in this light , the strong adhesion 
 of mfp-5 might result not only from its long sequence but also to 
 synergistic or cooperative effects between amino acid residues over 
 the whole protein sequence . given the fact that nature has optimized 
 mussel proteins for adhesion over millions of years , these synergistic 
 effects should not be so surprising . another important message conveyed 
 by this study is that , in practice , when the second surface ( e.g. , 
 gold ) does not rely exclusively on dopa for bridging adhesion , relatively 
 short molecules can mediate bridging between different surfaces via 
 different binding mechanisms even though dopa residues have been mostly 
 occupied by the first surface ( e.g. , mica ) . analysis of adhesion by 
 short mfp - derived peptide sequences provides a simpler way to design 
 better mussel inspired adhesives for complex / heterogeneous surface 
 chemistries and is more cost - effective from the engineering perspective . peptide modification and purification : the peptides 
 hyhsggsyhgsgyhg ( pep - pi-6.5 ) , vgsgydgysdgyydg 
 ( pep - pi-4 ) , gykgkyygkgkkyyyk ( pep - pi-10 ) , and vgsgydgysdgyydgvgsgydgysdgyydg 
 ( pep - pi-4-dimer ) were synthesized by genscript ( piscataway , nj ) using 
 routine solid - phase synthesis that included n - terminal acetylation 
 and c - terminal amidation and were provided as a desalted solid . mushroom 
 tyrosinase ( 3000 u / mg ) was from aldrich - sigma , and all other reagents 
 were of analytical grade . enzymatic modification : 
 the peptides ( 1 mg ) were dissolved in 1 ml of ph 7.0 , 20 mm borate , 
 0.1 m phosphate - ascorbate buffer , in an eppendorf microfuge tube . 
 after adding mushroom tyrosinase ( 0.3 mg ) , the tube was shaken for 
 4 h at ambient room temperature and pressure . each reaction was stopped 
 by adding 40 l of glacial acetic acid , and the resulting product 
 was subjected to reverse phase hplc column , eluted with a linear gradient 
 of aqueous acetonitrile . eluant was monitored continuously at 230 
 and 280 nm , and 0.33 ml fractions containing peptides were pooled 
 and freeze - dried . sample purity and hydroxylation were assessed by 
 amino acid analysis and electrospray ionization mass spectrometry . fractions with pure hydroxylated peptides were dissolved in 0.1 m 
 acetic acid ( sigma - aldrich ) and 0.25 m potassium nitrate ( sigma - aldrich ) 
 buffer ( ph 3 ) with a peptide concentration of 100 g / ml and 
 kept in a 80 c freezer . mass spectroscopy : a micromass qtof2 tandem mass 
 spectrometer ( waters corp . ) with an electrospray ionization source 
 and matrix assisted laser desorption ionization ( maldi ) with time - of - flight 
 mass ( tof ) ( bruker microflex lrf ) were used to obtain spectrometry 
 of peptides . for qtof2-esi , samples were infused at 10 l / min 
 via a harvard apparatus syringe pump . ms / ms experiments were carried out with argon as collision 
 gas and a collision voltage of 10 v. for maldi - tof , the n2 laser ( 337 nm ) fires at 60 hz . 12 l of sample peptide 
 was spotted onto the gold - plated sample plate and vacuum - dried , on 
 top of which 1 l of matrix solution ( -cyano-3-hydroxycinnamic 
 acid in aqueous 50% acetonitrile and 0.1% trifluoroacetic acid ) was 
 added . external calibrant was peptide calibration standard mixture 
 from bruker containing seven standard peptides with a mass range between 
 1000 and 3500 da . amino acid analysis : the amino acid composition 
 of the hydrolyzed peptides was determined on a hitachi l8900 amino 
 acid analyzer system with ninhydrin detection . polypeptides were hydrolyzed 
 in 100 l of 6 m hcl with 8% phenol in vacuum at 158 c 
 for 40 min . after being washed with water and methanol , the hydrolyzed 
 products were dissolved in 0.2 m hcl and injected into amino acid 
 analyzer . surface forces apparatus ( sfa ) : the 
 adhesion of 
 each peptide to mica was then measured by a sfa in a reported configuration . 
 prior to each experiment , 10 l of peptide solution was added 
 on top of 1 mica surface , letting the peptide adsorb on mica for 20 
 min followed by rinsing excessively with the same buffer to remove 
 the nonadsorbed peptide molecules . the adhesion force measured was 
 normalized by the radius of curved disks used in the sfa experiment , 
 typically 2 cm , and further converted into adhesion energy ( or work 
 of adhesion ) by using the derjaguin approximation : e = f/2r . dynamic 
 light scattering : the sizes of pep - pi-4 
 monomer and dimer in solution ( 2 mg / ml ) in 100 mm acetic acid and 
 100 mm acetic acid containing 0.25 m kno3 were obtained 
 using the malvern nano zs , which is calibrated regularly using malvern 
 zeta potential transfer standard ( p /
the 3,4-dihydroxyphenylalanine ( dopa)-containing 
 proteins of marine 
 mussels provide attractive design paradigms for engineering synthetic 
 polymers that can serve as high performance wet adhesives and coatings . 
 although the role of dopa in promoting adhesion between mussels and 
 various substrates has been carefully studied , the context by which 
 dopa mediates a bridging or nonbridging macromolecular adhesion to 
 surfaces is not understood . 
 the distinction is an important one both 
 for a mechanistic appreciation of bioadhesion and for an intelligent 
 translation of bioadhesive concepts to engineered systems . on the 
 basis of mussel foot protein-5 ( mfp-5 ; length 75 res ) , we designed 
 three short , simplified peptides ( 1517 res ) and one relatively 
 long peptide ( 30 res ) into which dopa was enzymatically incorporated . 
 peptide adhesion was tested using a surface forces apparatus . 
 our 
 results show that the short peptides are capable of weak bridging 
 adhesion between two mica surfaces , but this adhesion contrasts with 
 that of full length mfp-5 , in that ( 1 ) while still dependent on dopa , 
 electrostatic contributions are much more prominent , and ( 2 ) whereas 
 dopa surface density remains similar in both , peptide adhesion is 
 an order of magnitude weaker ( adhesion energy ead  0.5 mj / m2 ) than full length 
 mfp-5 adhesion . between two mica surfaces , the magnitude of bridging 
 adhesion 
 was approximately doubled ( ead  1 mj / m2 ) upon doubling the peptide length . 
 notably , the short peptides mediate much stronger adhesion ( ead  3.0 mj / m2 ) between 
 mica and gold surfaces , indicating that a long chain length is less 
 important when different interactions are involved on each of the 
 two surfaces .