Acta Phys. -Chim. Sin. ›› 2012, Vol. 28 ›› Issue (09): 2015-2022.doi: 10.3866/PKU.WHXB201206261

• THERMODYNAMICS, KINETICS, AND STRUCTURAL CHEMISTRY • Previous Articles     Next Articles

Enthalpic Pairwise Interactions of α-Aminobutyric Acid Enantiomers in DMF+Water Mixtures

GUO Zheng, HU Xin-Gen, LIANG Hong-Yu, JIA Zhao-Peng, CHENG Wei-Na, LIU Jia-Min   

  1. College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, Zhejiang Province, P. R. China
  • Received:2012-03-17 Revised:2012-06-25 Published:2012-08-02
  • Contact: HU Xin-Gen E-mail:hxgwzu@126.com
  • Supported by:

    The project was supported by the National Natural Science Foundation of China (21073132).

Abstract:

Dilution enthalpies of two α-aminobutyric acid enantiomers, L-α-aminobutyric acid and D-α-aminobutyric acid, in dimethylformide (DMF)+water mixtures of various compositions are determined by isothermal titration microcalorimetry (ITC) at 298.15 K. Homotactic enthalpic pairwise interaction coefficients for each solvent composition are calculated according to the McMillan-Mayer theory of statistical thermodynamics. From the point of view of solute-solute and solute-solvent interactions, competition equilibria among hydrophobic-hydrophobic, hydrophobic-hydrophilic, and hydrophilichydrophilic interactions in ternary solutions are explored. It is found that all values of hxx are positive across the entire studied composition range of mixed solvents (mass fraction of DMF, wDMF=0-0.3), gradually reducing with the increasing wDMF. It is of interest that the hxx values for the L-enantiomer are universally larger than those of the D-enantiomer (hLL>hDD), which indicates that ITC is useful to discriminate homochiral enthalpic pairwise interaction of enantiomers. Our results show that hydrophobic-hydrophobic and hydrophobic-hydrophilic interactions are predominant in pairwise molecular interaction processes in ternary solutions containing α-aminobutyric acid, water, and DMF, and that the configuration of L-L molecular pair is more advantageous for the approach of hydrophobic side-chains (CH3CH2-) on α-carbon than a D-D pair, where part of the structured water molecules relax to less structured bulky water due to the overlap and partial breaking of hydrophobic hydration cospheres around nonpolar groups. This confirms that the process is spontaneous and is accompanied with positive enthalpy change and obvious increase in entropy (ΔG<0, ΔH>0, and ΔS=(ΔHG)/T>0), consequently releasing more heat upon dilution of the solutions and leading to larger values of hxx.

Key words: α-Aminobutyric acid, Enantiomer, DMF+water mixture, Isothermal ditration microcalorimetry, Dilution enthalpy, Enthalpic pairwise interaction, Chirality discrimination effect

MSC2000: 

  • O645