Nuclear magnetic resonance studies of the acid-base chemistry of amino acids and peptides. I. Microscopic ionization constants of glutathione and methylmercury-complexed glutathione
Nuclear magnetic resonance studies of the acid-base chemistry of amino acids and peptides. I. Microscopic ionization constants of glutathione and methylmercury-complexed glutathione
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DOI：
10.1021/ja00790a009
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发表时间：
1973-05
影响因子：
15
通讯作者：
D. Rabenstein
中科院分区：
文献类型：
--
作者：
D. Rabenstein
根据选定的碳键合质子的化学位移，确定完全质子化的谷胱甘肽的两个羧酸基团、巯基和铵基团的电离程度作为pH的函数。结果表明，两个羧酸基团在pH 0.5至6.0范围内同时电离，而巯基和铵基团在pH 7至12范围内同时电离。根据四个酸性基团电离程度的pH依赖性，获得了在分子水平上描述谷胱甘肽酸碱化学的八个微观电离常数。完全质子化谷胱甘肽的L-谷氨酰羧酸基团的酸性是甘氨酰羧酸基团的10.8倍，而双质子化谷胱甘肽的巯基的酸性是铵基团的1.57倍。还通过化学位移测量确定了描述谷胱甘肽的硫配位甲基汞复合物的两个羧酸基团的电离的四个微观电离常数和铵基团的电离常数。完全质子化的甲基汞络合物的L-谷氨酰羧酸基团的酸性是甘氨酰羧酸基团的10.7倍，而铵基团的电离常数与双质子化谷胱甘肽相应反应的微观常数相同。提出了一种评估二元酸微观电离常数的方法，当同时进行电离的每个酸性基团的电离分数可以确定为 pH 的函数时。氨基酸和肽的酸碱化学通常用从 pH 滴定数据获得的宏观电离常数来表征。当取代基的酸度明显不同时，可以将常数分配给各个基团。然而，如果电离同时发生在两个或多个组上，则宏观常数是各个组电离的微观常数的组合。 1 在这种情况下，不
Theextent of ionization of the two carboxylic acid groups, the sulfhydryl group, and the ammonium group of fully protonated glutathione was determined as a function of pH from the chemical shifts of selected carbon-bonded protons. The results indicate that the two carboxylic acid groups ionize simultaneously in the pH range 0.5 to 6.0 while the sulfhydryl group and the ammonium group ionize simultaneously in the pH range 7 to 12. From the pH dependence of the extent of ionization of the four acidic groups, eight microscopic ionization constants which describe the acid-base chemistry of glutathione at the molecular level were obtained. The L-glutamyl carboxylic acid group of fully protonated glutathione is 10.8 times as acidic as the glycyl carboxylic acid group, while the sulfhydryl group of doubly protonated glutathione is 1.57 times as acidic as the ammonium group. The four microscopic ionization constants which describe the ionization of the two carboxylic acid groups of the sulfurcoordinated methylmercury complex of glutathione and the ionization constant of the ammonium group were also determined from chemical shift measurements. The L-glutamyl carboxylic acid group of the fully protonated methylmercury complex is 10.7 times as acidic as the glycyl carboxylic acid group, while the ionization constant of the ammonium group is the same as the microscopic constant of the corresponding reaction of doubly protonated glutathione. A method is presented for evaluating the microscopic ionization constants of diprotic acids when the fractional ionization of each of the acidic groups undergoingionization simultaneously can be determined as a function of pH.The acid-base chemistry of amino acids and peptides is usually characterized in terms of macroscopic ionization constants obtained from pH titration data. When the acidities of the substituent groups are markedly different, theconstants can be assigned tothe individual groups. If, however, ionization occurs simultaneously at two or more groups, the macroscopic constants are a composite of the microscopic constants for ionization from the individual groups. 1 In such cases, it is not