UV resonance Raman studies of cis-to-trans isomerization of glycyglycine derivatives
UV resonance Raman studies of cis-to-trans isomerization of glycyglycine derivatives
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DOI：
10.1021/ja984003b
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发表时间：
1999-04-21
影响因子：
15
通讯作者：
Asher, SA
中科院分区：
文献类型：
--
作者：
Holtz, JSW;Li, PS;Asher, SA
我们报告的动力学紫外共振拉曼测量的pH值依赖性的活化势垒的反式-顺式异构化甘氨酰甘氨酸（Gly-Gly）。我们确定了反式-顺式基态能量差Δ G和反式-顺式异构化活化势垒E-a的电荷态依赖性。pH 5.7时两性离子Gly-Gly的ac（3.0 +/-0.5 kcal/mol）低于pH 10.5时阴离子Gly-Gly的ac（4.4 +/-0.7 kcal/mol）。阳离子Gly-Gly在pH 3.0（3.6 +/- 0.4 kcal/mol）下的Δ G测量值介于两性离子和阴离子值之间。阳离子、两性离子和阴离子的反式-顺式活化势垒分别为13.3 +/- 1.0、12.6 +/- 0.9和13.9 +/- 1.1 kcal/mol，在实验误差范围内相同。然而，两性离子Gly-Gly（13 +/- 1.9 s（-1））顺反异构化速率常数大于阳离子（3 +/- 0.4 s（-1））和阴离子（2 +/- 0.3 s（-1））的速率常数，表明活化势垒降低了1 kcal/mol。能量和活化势垒的小差异有点令人惊讶，因为两性离子形式可以独特地形成离子对，这应该影响反式、顺式和活化的络合物能量。
We report kinetic UV resonance Raman measurements of the pH dependence of the activation barrier for the trans-cis isomerization of glycylglycine (Gly-Gly). We determined the charge-state dependence of the trans-cis ground-state energy difference, Delta G, and the trans-cis isomerization activation barrier, E-a. ac for zwitterionic Gly-Gly at pH 5.7 (3.0 +/- 0.5 kcal/mol) is below that of pH 10.5 anionic Gly-Gly (4.4 +/- 0.7 kcal/mol). The measured value of Delta G for cationic Gly-Gly at pH 3.0 (3.6 +/- 0.4 kcal/mol) lies between the zwitterion and anion values. The measured trans-to-cis activation barriers of 13.3 +/- 1.0, 12.6 +/- 0.9, and 13.9 +/- 1.1 kcal/mol for the cation, the zwitterion, and the anion, respectively, are identical within experimental error. However, the larger zwitterionic Gly-Gly (13 +/- 1.9 s(-1)) cis-to-trans isomerization rate constant over those of the cation (3 +/- 0.4 s(-1)) and anion (2 +/- 0.3 s(-1)) suggests a similar to 1 kcal/mol decreased activation barrier. The small differences in energy and activation barriers are somewhat surprising since the zwitterionic form can uniquely form ion pairs, which should impact the trans, the cis, and the activated complex energies.