Apruzzese Fabrizio, Bottari Emilio, Festa Maria Rosa
Dipartimento di Chimica, Università 'La Sapienza', P.le A. Moro 5, 00185 Rome, Italy.
Talanta. 2002 Mar 4;56(3):459-69. doi: 10.1016/s0039-9140(01)00570-7.
l-Cystine is the natural least soluble of the naturally occurring amino acids. Its solubility is a function of the hydrogen ion concentration depending on its protonation. This paper presents an investigation on properties of l-cystine in a constant ionic medium of NaCl at different concentrations (3.00,2.00,1.00,0.500,0.150 mol dm(-3)) and at 25 degrees C. In such experimental conditions, solubility and its changes as a function of hydrogen ion concentration were determined. Protonation of cystine was studied in acid and basic solution, by measuring the electromotive force of galvanic cell involving a glass electrode. Four protonation constants for l-cystine were determined in all the selected concentrations of ionic medium. The different values obtained for the constants in ionic medium of different concentration could be explained both with the variation of activity coefficients or with the complex formation between l-cystine and sodium ions. Variations of solubility for different concentration of NaCl were observed as well.
L-胱氨酸是天然存在的氨基酸中溶解度最低的。其溶解度是氢离子浓度的函数，取决于其质子化情况。本文介绍了在25℃下，L-胱氨酸在不同浓度（3.00、2.00、1.00、0.500、0.150 mol dm⁻³）的NaCl恒定离子介质中的性质研究。在这样的实验条件下，测定了溶解度及其随氢离子浓度的变化。通过测量包含玻璃电极的原电池的电动势，研究了胱氨酸在酸性和碱性溶液中的质子化。在所有选定的离子介质浓度下，测定了L-胱氨酸的四个质子化常数。不同浓度离子介质中常数的不同值，既可以用活度系数的变化来解释，也可以用L-胱氨酸与钠离子之间的络合物形成来解释。还观察到不同浓度NaCl下溶解度的变化。
