it is well known that mixed ligand ternary complexes of some metals play an important role in the activation of enzymes . it is studied that mixed ligand complexes are biologically active against pathogenic microorganisms [ 2 , 3 ] ; further , metal complexes , which include 8-hydroxyquinoline as a primary ligand , exhibit biological activity . ternary complexes containing an amino acid as a secondary ligand have a significance as they are potential models for enzyme metal ion substrate complexes . numerous uranium complexes and their mixed chelates have been studied [ 6 , 7 ] . a large number of complexes with varying geometries of dioxouranium(vi ) , the coordination numbers ranging from 7 to 12 for metal chelates of uo2(vi ) and th(iv ) have been reported [ 9 , 10 ] . recently , it was stated that the uo2(vi ) complexes show antimicrobial activity [ 11 , 12 ] . this paper reports the synthesis , characterization , and antibacterial studies of mixed ligand dioxouranium(vi ) complexes prepared with 8-hydroxyquinoline as a primary ligand and amino acids such as l - threonine , l - tryptophan , and l - isoleucine as secondary ligands . these complexes have been screened for their antibacterial properties against the pathogenic bacteria s. aureus , c. diphtheriae , s. typhi , and e. coli . l - threonine , l - tryptophan , l - isoleucine , and 8-hydroxyquinoline were obtained from s.d . solvents like ethanol and dimethylformamide and laboratory grade chemicals whenever used were distilled and purified according to standard procedures [ 13 , 14 ] . mixed ligand dioxouranium(vi ) complexes were prepared from uranyl nitrate hexahydrate , 8-hydroxyquinoline ( hq ) as a primary ligand , and different amino acids such as l - threonine , l - tryptophan , and l - isoleucine as secondary ligands . to an aqueous solution ( 10  ml ) of uranyl nitrate hexahydrate ( 502  mg , 1  mmol ) , ethanolic solution ( 10  ml ) of 8-hydroxyquinoline ( 145  mg , 1  mmol ) was added . the mixture was stirred and kept in a boiling water bath for 10  min . to this hot solution , an aqueous solution ( 10  ml ) of amino acid ( 1  mmol ) was added with constant stirring . the mixture ( 1  :  1  :  1 molar proportion ) was again heated in a water bath for 10  min till the temperature reached to 50c . the complexes were precipitated by raising the ph of the reaction mixture by adding diluted ammonia solution . the mixture was cooled , and solid complex obtained was filtered and washed with water followed by ethanol . the complexes were analyzed for c , h , and n contents on thermo finnigan elemental analyzer model no . the molar conductance values were measured in dmf ( 10 m ) on an equiptronics autoranging conductivity meter model no . room temperature magnetic susceptibilities were measured by a guoy method using hg[co(scn)4 ] as a calibrant at the department of chemistry , i.i.t . the electronic absorption spectra of all the complexes in dmf solution ( 10 m ) in the ultraviolet and visible region were recorded on shimadzu uv / vis-160 spectrophotometer . ft - ir spectra were recorded in kbr disc on a perkin - elmer ft - ir spectrophotometer model 1600 at department of chemistry , i.i.t . nmr spectra were recorded on jeol-300  mhz instrument using tms as an internal standard at the institute of science , mumbai . thermal analysis ( tg and dta ) were carried out in controlled nitrogen atmosphere on a perkin - elmer diamond tg - dta instrument at department of chemistry , i.i.t . , mumbai by recording the change in weight of the complexes on increasing temperature up to 900c at the heating rate of 10c per minute . in the agar cup method , a single compound can be tested against a number of organisms or a given organism against different concentrations of the same compound . the method was found suitable for semisolid or liquid samples and was used in the present work . in the agar cup method , a plate of sterile nutrient agar with the desired test strain was poured to a height of about 5  mm and allowed to solidify , and a single cup of about 8  mm diameter was cut from the center of the plate with a sterile cork borer . thereafter , the cup was filled with the sample solution ( 1000  g / ml ) in dimethylsulphoxide , and the plate was incubated at 37c for 24  h. the extent of inhibition of growth from the edge of the cup was considered as a measure of the activity of the given compound . by using several plates simultaneously , the activities of several samples were quantitatively studied . the test compound ( 10  mg ) was dissolved in dimethylsulphoxide ( 10  ml ) so as to prepare a stock solution of concentration 1000  g / ml . from this stock solution , aliquots of the ranges 5 , 10 , 15 ,  , the test compounds were subjected to in vitro screening against staphylococcus aureus , corynebacterium diphtheriae , salmonella typhi , and escherichia coli using muller hinton broth as the culture medium . bacterial inoculums were prepared in sterilized mueller hinton broth and incubated for 4  h at 37c . this was dispersed ( 5  ml ) in each borosilicate test tube ( 150    20  mm ) . the test sample solution was added in order to attain a final concentration as 5 to 250  g / ml . the bacterial inoculums 0.1  cm of the desired bacterial strain ( s. aureus , c. diphtheriae , s. typhi , and e. coli ) containing 10 bacteria / ml was inoculated in the tube . the tubes were incubated at 37c for 24  h and then examined for the presence or absence of the growth of the test organisms . the lowest concentration which showed no visible growth was noted as minimum inhibitory concentration ( mic ) . the synthesis of mixed ligand uranyl complexes may be represented as follows : 
 ( 1)uo2(no3)26h2o  +  hq  +  hl  [uo2(q)(l)2h2o ]  +  2hno3  +  4h2o , 
 ( where hq is 8-hydroxyquinoline and hl is an amino acid ) . all the complexes are coloured , nonhygroscopic , thermally stable solids ( table 1 ) , indicating a strong metal - ligand bond . the complexes are insoluble in common organic solvents such as ethyl alcohol , acetone , and chloroform but are partially soluble in dmf and dmso .   the elemental analysis data ( table 2 ) of uranyl complexes are consistent with their general formulation as 1  :  1  :  1 , mixed ligand of the type [ uo2(q)(l)2h2o ] . the molar conductance values ( table 2 ) of the complexes in dmf at 10 m concentration are found to be 0.0010.002 mhos cm mol indicating their nonelectrolytic nature . the magnetic moment ( table 3 ) of the complexes was calculated from the measured magnetic susceptibilities after employing diamagnetic corrections and revealed their diamagnetic nature . the electronic spectra of the metal complexes in dmf were recorded in the uv - visible region . the spectra show three transitions in the range 3636436765  cm , 2976230303  cm , and 2525326316  cm ascribed to    * , n  * , and the charge transfer transitions from the ligands to the metal , respectively . the ft - ir spectra of the metal complexes were recorded for kbr discs over the range 4000400  cm . on the basis of the reported infrared spectra of amino acids , 8-hydroxyquinoline , and its metal complexes [ 1820 ] , a broad band was observed in the region between 3460 and 3431  cm due to asymmetric and symmetric o h stretching modes and a band in the range 16001585  cm due to h  o h bending vibrations indicating the presence of coordinated water molecules further confirmed by thermal studies . a strong (co ) band observed in the range between 1106 and 1105  cm indicates the presence of oxine moiety in the complexes coordinated through its nitrogen and oxygen atoms as uninegative bidentate ligand . the (c = n ) mode observed at 1580  cm in the spectra of free hq ligand is found to be shifted to lower wave number , in the range of 14981497  cm in the spectra of complexes . a negative shift in this vibrational mode on complexation indicates the coordination through ternary nitrogen donor of hq . the in - plane and out - of - plane ring deformation modes observed at 506  cm and 786  cm respectively , confirm coordination through nitrogen atom of hq with the metal . h ( asymmetric ) and n  h ( symmetric ) vibrations of free amino acid moiety are shifted to higher wave numbers , in the range 31773140  cm and 30503025  cm , respectively , in the spectra of metal complexes , suggesting coordination of the amino group through nitrogen with the metal ion . the asymmetric(coo ) band of the free amino acid , that is , 16101590  cm , is shifted to lower wave number , in the range of 15711566  cm , and the symmetric(coo ) mode observed at ~1400  cm in the spectra of free amino acids is found to be shifted to lower wave number 1378  cm , in the spectra of complexes indicating the coordination of the carboxylic acid group via oxygen with the metal ion . an important feature of infrared spectra of the metal complexes with 8-hq is the absence of band ~3440  cm due to the o this observation leads to the conclusion that complex formation takes place by deprotonation of the hydroxyl group of hq moiety . the ft - ir spectra of the uranyl complexes show no absorption bands near 1352  cm where ionic nitrate is known to absorb , indicating absence of ionic nitrate . other bands observed at ~1468 , ~1278 , ~1035 , and ~734  cm corresponding to 1 , 4 , 2 , and 3 vibrations agree with frequencies reported for bidentate nitrate group [ 24 , 25 ] . the bands at 898889  cm and 821820  cm were assigned to asymmetric(o  u  o ) and symmetric(o  u  o ) vibrational modes of linear o = u = o moiety [ 26 , 27 ] . some new bands of weak intensity observed in the regions around 604  cm and 486  cm may be ascribed to the m  o and m it may be noted that these vibrational bands are absent in the infrared spectra of hq as well as amino acids . h nmr spectra of complexes in dmso exhibits a singlet at  2.8  ppm ( 2h ; nh2 ) due to amino group protons and broad multiplet in the region  6.67.8  ppm ( 6h ; aromatic protons ) due to the aromatic ring protons . the presence of water molecules in the complexes is confirmed by the appearance of a new signal around  3.5  ppm , attributed to h2o protons .   in case of complex with l - threonine it shows doublet at d 1.26  ppm ( j = 7.5  hz ) for three protons of methyl group , doublet at  2.52  ppm ( j = 7.5  hz ) for one proton of ch , multiplet at  2.902.96  ppm ( ch  ch3 ) for one proton of ch , and singlet at  10.1  ppm for hydroxyl proton which was d2o exchangeable . the complex with l - tryptophan shows doublet at  2.76  ppm ( j = 8.0  hz ) for two protons of methylene group , triplet at  2.94  ppm ( j = 8.0  hz ) for one proton of ch , singlet at  5.25  ppm for one proton of nh which was d2o exchangeable , and multiplet at  6.67.2  ppm ( indole ring protons for aromatic protons . the complex with l - isoleucine shows triplet at  0.93  ppm ( j = 7.0  hz ) ( ch2ch3 ) for three protons of methyl group , doublet at  1.15  ppm ( j = 7.0  hz ) ( ch  ch3 ) for three protons of another methyl group , multiplet at  1.501.57  ppm for two protons of ch2 , another multiplet at  1.601.68  ppm for one proton of ch , and doublet at  2.92  ppm ( j = 7.0  hz ) for one proton of another ch . the tg and dta studies of the uranyl complexes have been recorded in the nitrogen atmosphere at the constant heating rate of 10c per minute . the tg of the uranyl complexes shows that they are thermally quite stable to varying degree . the complexes show gradual loss in weight due to decomposition by fragmentation with increasing temperature as presented in table 4 . the thermogram of these complexes shows the loss in weight corresponding to two water molecules in the temperature range 110190c , followed by weight loss due to amino acid moiety in the range 250445c . the final step of the decomposition observed in the range 450690c corresponds to the weight loss of hq moiety present in the complexes . the dta of the complexes displays an endothermic peak in the range 110190c which indicates the presence of coordinated water molecules . as the temperature is raised , the dta curve shows a small exotherm in the range 250445c and a broad exotherm in the range 450690c attributed to decomposition of amino acid and 8-hydroxyquinoline moieties present in the complexes , respectively . the formation of a broad exotherm is possibly due to simultaneous decomposition of ligand moieties and their subsequent oxidation to gaseous products like co2 and h2o and so forth . like most of the metal organic complexes , these complexes also decompose to a fine powder of metal oxide , that is , uo2 . the constant weight plateau in tg after 700c indicates completion of the reaction . the uo2 form was confirmed by x - ray diffraction pattern of the decomposed product .   on the basis of the physicochemical studies , the bonding and structure for the uranyl complexes may be represented as shown in figure 1 . all the metal complexes were screened against staphylococcus aureus , corynebacterium diphtheriae , salmonella typhi , and escherichia coli . the studies based on agar cup method revealed that the complexes are more active against s. typhi and e. coli and less active against s. aureus and c. diphtheria ( table 5 ) . the minimum inhibitory concentration ( mic ) of ligand and the metal salts ranges between 50 and 110  g / ml while that of metal complexes ranges between 5 and 25  g / ml ( table 6 ) . the complexes are found to be more active against s. typhi and e. coli as compared to s. aureus and c. diphtheria . as compared to standard antibacterial compound tetracycline , the complexes show moderate activity against selected strains of microorganisms . the results show that , as compared to the activity of metal salts and free ligand , the metal complexes show higher activity . the activity of metal complexes is enhanced due to chelation . the chelation reduces considerably the polarity of the metal ions in the complexes , which in turn increases the hydrophobic character of the chelate and thus enables its permeation through the lipid layer of microorganisms . the higher decomposition temperatures of the complexes indicate a strong metal - ligand bond , and electrical conductance studies show nonelectrolytic nature of the complexes , respectively . ir spectra show bonding of the metal ion through n- and o- donor atoms of the two ligands . h nmr study reveals the chemical environment of protons and presence of water molecules in the complexes . thermal analysis confirms the presence of coordinated water molecules .   on the basis of the above results , the antibacterial study shows that complexes are found to be more active against s. typhi and e. coli as compared to s. aureus and c. diphtheria . compared to standard antibacterial compound , tetracycline
mixed ligand complexes of dioxouranium ( vi ) of the type [ uo2(q)(l)2h2o ] have been synthesized using 8-hydroxyquinoline ( hq ) as a primary ligand and amino acids ( hl ) such as l - threonine , l - tryptophan , and l - isoleucine as secondary ligands . 
 the metal complexes have been characterized by elemental analysis , electrical conductance , magnetic susceptibility measurements , and spectral and thermal studies . 
 the electrical conductance studies of the complexes indicate their nonelectrolytic nature . 
 magnetic susceptibility measurements revealed diamagnetic nature of the complexes . 
 electronic absorption spectra of the complexes show intraligand and charge transfer transitions , respectively . 
 bonding of the metal ion through n- and o - donor atoms of the ligands is revealed by ir studies , and the chemical environment of the protons is confirmed by nmr studies . 
 the thermal analysis data of the complexes indicate the presence of coordinated water molecules . the agar cup and tube dilution methods have been used to study the antibacterial activity of the complexes against the pathogenic bacteria s. aureus , c. diphtheriae , s. typhi , and e. coli .