Dogan Alev, Köseoglu Fitnat, Kiliç Esma
Department of Chemistry, Faculty of Gazi Education, University of Gazi, Ankara, Turkey.
Anal Biochem. 2002 Oct 1;309(1):75-8. doi: 10.1016/s0003-2697(02)00254-3.
Both to demonstrate whether the predominant species are dipolar ion or the neutral form and to predict the change of dipolar form to neutral form ratio in ethanol-water mixtures, the macroscopic protonation constants of eight alpha-amino acid (glycine, L-alanine, L-valine, L-leucine, L-phenylalanine, L-serine, L-methionine, and L-isoleucine) were determined potentiometrically in 20-80% (v/v) ethanol-water mixtures at 25 degrees C with an ionic strength of 0.10 M. The calculation of the constants was carried out using a PKAS computer program. The effect of solvent composition on the protonation constants and the dipolar ionic to neutral form ratio of these acids in the mixed solvents are discussed. One can conclude that the dipolar form of amino acids, HA(+/-), dominates in ethanol-water mixtures.
为了证明主要存在形式是偶极离子还是中性形式，并预测在乙醇 - 水混合物中偶极形式与中性形式比例的变化，在25℃、离子强度为0.10 M的20 - 80%（v/v）乙醇 - 水混合物中，通过电位滴定法测定了8种α - 氨基酸（甘氨酸、L - 丙氨酸、L - 缬氨酸、L - 亮氨酸、L - 苯丙氨酸、L - 丝氨酸、L - 蛋氨酸和L - 异亮氨酸）的宏观质子化常数。常数的计算使用PKAS计算机程序进行。讨论了溶剂组成对这些酸在混合溶剂中的质子化常数以及偶极离子与中性形式比例的影响。可以得出结论，氨基酸的偶极形式HA(+/-)在乙醇 - 水混合物中占主导地位。
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