α-氨基丁酸对映异构体在DMF+水混合溶剂中的焓对作用

利用等温滴定微量热法(ITC)分别测定了298.15K时L-α-氨基丁酸和D-α-氨基丁酸两种对映异构体在不同组成的二甲基甲酰胺(DMF)+水混合溶剂中的稀释焓.根据统计热力学的McMillan-Mayer理论计算各溶剂组成下的同系焓对作用系数(hxx).从溶质-溶质和溶质-溶剂相互作用的观点出发探讨了三元水溶液中疏水-疏水、疏水-亲水和亲水-亲水作用的竞争平衡.实验发现,在所研究的混合溶剂组成范围内(wDMF=0-0.3),α-氨基丁酸两种对映体的hxx都是较大的正值,且都随wDMF的增大而逐渐减小,而有趣的是L型对映体的hxx值普遍比D型的大(hLLhDD),说明ITC可以区分对映异构体的同手性焓对作用.结果表明:在α-氨基丁酸+水+DMF三元溶液体系中,疏水-疏水和疏水-亲水作用在分子对作用过程中占优势;在α-氨基丁酸的同种对映体发生分子对作用时,L-L分子对比D-D分子对在构型上更有利于α-碳上疏水侧链(CH3CH2-)的靠拢,疏水基团的水合壳层发生交盖,局部破坏而释放出部分结构化的水,过程自发、吸热且伴随显著熵增(ΔG0,ΔH0,ΔS=(ΔH-ΔG)/T0),因而在稀释时释放出更多热量,hxx具有较大的正值.

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

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 h_xx are positive across the entire studied composition range of mixed solvents (mass fraction of DMF, w_DMF=0-0.3), gradually reducing with the increasing w_DMF. It is of interest that the h_xx values for the L-enantiomer are universally larger than those of the D-enantiomer (h_LL>h_DD), 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=(ΔH-ΔG)/T>0), consequently releasing more heat upon dilution of the solutions and leading to larger values of h_xx.