α-氨基酸在水-乙醇中羟基质子化热力学

α 氨基酸为重要的两性物质 ,其酸碱性质对于维持生物体内的酸碱平衡和蛋白质的生物活性起着十分重要的作用．α 氨基酸在纯水溶剂中的质子化反应热力学性质已进行了广泛的研究［１ -６］,但对球形蛋白来说 ,蛋白亚基间的作用是处在一个大量但不完全由水组成的非水环境中 ,因此 ,研究氨基酸在水 有机物混合溶剂中的质子化热力学性质具有重要意义．但这方面的工作 ,特别是用量热法直接研究溶剂对质子化焓变的影响开展得不多．本文选择乙醇和水组成的混合溶剂模拟蛋白质亚基所处的介质环境 ,利用精密微量流动热量计测定２９８．１５Ｋ时甘氨…

Thermodynamics of Carboxyl Group's Protonation of α-amino Acids in Water-Ethanol Mixtures at 298.15K

Enthalpy changes for the protonation of carboxyl group of four α amino acids(glycine, L –α-alanine, L-valine and L-serine) were measured in water ethanol mixtures (10－70wt％) at 298.15K using LKB-2277 Bioactivity Monitor. The corresponding entropy and Gibbs energy changes were also calculated. The results show that both enthalpy changes and entropy changes are favorable to the protonation of carboxyl groups of the investigated amino acids in water ethanol mixtures. However, the influence of the composition of ethanol in the mixed solvents on the enthalpy change and entropy changes is complicated . Both sσΔH1Θand sσΔS1Θ,the differences of enthalpy changes and entropy changes in mixed solvents and in pure water respectively, show a minimum approximately at xEtOH=0.1.The effects of side chains on the enthalpy change and entropy changes were also investigated using the proton transfer process between glycine and the other three amino acids. The results demonstrate that the proton transfer processes for alanine and valine are spontaneous but not for serine, which could be interpreted in terms of the electrostatic interaction between amino group and carboxyl group within the molecule and the interaction between carboxyl group and the solvent.