Enthalpic Pairwise Interactions Of Diols And Amino Acids In Mixed Solvents

Molecular interactions and chiral recognition is a very popularinvestigation subject in chemistry, biology and related disciplines. In orderto exploring weak nonbonding interactions, such as hydrogen bonding,hydrophobic bonding, salt linkage and van der Waals interactions, etc. andtheir energetic effects in chiral molecular solutions, the microcalorimetry isvery useful. In this dissertation, enthalpic pairwise interactions and theirsolvent effects of some chiral and achiral diols,-amino acids in stronglypolar aqueous solutions have been investigated by precise isothermaltitration microcalorimetry （ITC）. The content consists of the following twoparts:In the first part, the dilution enthalpies of two chiral diols,（2R,4R）-（+）-2,4-pentanediol and （2S,4S）-（+）-2,4-pentanediol, in purewater and in dimethylsulfoxide （DMSO）+water mixtures of various massfractions （w=0-0.3）, and the dilution enthalpies of other two achiral diols, glycol and propylene glycol in N,N-Dimethylformamide （DMF）+watermixtures of various mass fractions （w=0-0.3） have been determinedrespectively, using an isothermal titration calorimeter （MicroCal ITC200）at298.15K. According to the McMillan-Mayer theory of thermodynamics,the enthalpic pairwise interaction coefficients (h_XX) of the four diols in themixed solvents of various compositions have been calculated. Therelationship between the enthalpic pairwise interaction coefficient and thecomposition of mixed solvents were examined to explore the possiblefashion of solute-solute interactions or solvent-solvent interactions in thesystems. The results revealed are as follows:（1） across the studiedcomposition range of mixed solvents, the h_XXvalues of（2S,4S）-（+）-2,4-pentanediol are universally larger than（2R,4R）-（+）-2,4-pentanediol, therefore the method of ITC can differentiatethe energetic effects of homochiral pairwise interactions between differentpairs of optical isomers;（2） the values of h_XXare all positive for the fourdiols across the studied composition range of mixed solvents, so it isconcluded that hydrophobic-hydrophobic and hydrophobic-hydrophilicinteractions are predominant in the ternary solutions;（3） the diversevariations of h_XX（increase or decrease） of different diols with the massfractions of cosolvents （DMSO or DMF） depend largely on thecompetition equilibrium among hydrophobic-hydrophobic,hydrophobic-hydrophilic, and hydrophilic-hydrophilic interactions in mixed solvents;（4） DMSO and DMF, as mediating cosolvents, playdifferent roles in the pairwise interactions, which the former works as awater structure-maker in diluter solutions, but a water structure-breaker inmore concentrated solutions, while the latter works mainly as a waterstructure-breaker in the whole studied range of mixed solventscompositions.In the second part, using the same isothermal titration calorimeter（MicroCal ITC200）, the enthalpies of dilution of D-2-aminobutyric acidand L-2-aminobutyric acid in pure water and DMF+H₂O mixtures ofvarious mass fractions （w=0-0.3）, and the dilution enthalpies of glycineand betaine hydrochloride in DMF+H₂O and DMSO+H₂O mixtures ofvarious mass fractions （w=0-0.3） have been determined respectively. Basedon the statistical thermodynamics theory of McMillan-Mayer, thecorresponding enthalpic interaction coefficients were calculated. Theinfluence of the intervening of strongly polar cosolvents, i.e. DMF andDMSO, on the value of enthalpic pairwise interaction coefficients has beenanalysed in detail. The results obtained are as follows:（1） the h_xxvalues ofthe two optical isomers of aminobutyric acid are all positive in the studiedmixed solvents, and become smaller with the increase of mass fractions ofcosovent DMF, illustrating that hydrophobic-hydrophobic andhydrophobic-hydrophilic interactions are predominant in the ternarysolutions, and that the cosolvent DMF works basically as a water structure-breaker;（2） the h_XXvalues of the D-isomer are generally largerthan L-isomer，which validates the ability of ITC to discriminate energeticeffects in homochiral pairwise interactions;（3） the h_xxvalues of glycineand betaine hydrochloride are all negative across the studied compositionranges of mixed solvents, and become smaller with the increase ofcosolvent concentration, indicating that hydrophilic-hydrophilicinteractions are predominant in the ternary solutions, and that both DMSOand DMF are water structure-breaker by and large as cosolvents;（4） the h_xxvalues of betaine hydrochloride are largely negative, which reflects theability of DMF to form intermolecular and intramolecular hydrogenbonding, and the hydrophilic interactions are very marked in solutions.