Structures of cationized proline analogues: evidence for the zwitterionic form.
Structures of cationized proline analogues: evidence for the zwitterionic form.
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DOI：
10.1021/jp0466800
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发表时间：
2005-02
期刊：
影响因子：
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通讯作者：
A. Lemoff;M. F. Bush;E. Williams
中科院分区：
文献类型：
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作者：
A. Lemoff;M. F. Bush;E. Williams
锂化和钠化的α-甲基-脯氨酸（α-Me-Pro）和结构异构体的结构，无论有和没有水分子，研究使用黑体红外辐射解离（BIRD）和密度泛函理论。从作为温度的函数测得的BIRD动力学数据，结合这些数据的主方程建模，阈值解离能从这些集群的水分子的损失。锂化和钠化的α-Me-Pro的这些能量分别为77.5 +/- 0.5和53 +/- 1 kJ/mol。对于非两性离子异构体脯氨酸甲酯，这些值高3.0-4.5 kJ/mol。这些结果提供了令人信服的实验证据表明，α-Me-Pro是两性离子在这些集群。在温度校正的B3 LYP/6-311++G**//B3 LYP/6-31++G** 水平下的理论表明，锂化和钠化的α-Me-Pro的盐桥或两性离子形式的能量比非两性离子或电荷溶剂化形式低17至23 kJ/mol，并且单个水分子的附着不会显著改变这些簇的结构或相对能量。脯氨酸的质子亲和力比α-Me-Pro的质子亲和力高8 kJ/mol，表明锂化和钠化的单水合脯氨酸也应该是两性离子的。
The structures of lithiated and sodiated alpha-methyl-proline (alpha-Me-Pro) and structural isomers, both with and without a water molecule, are investigated using blackbody infrared radiative dissociation (BIRD) and density functional theory. From the BIRD kinetic data measured as a function of temperature, combined with master equation modeling of these data, threshold dissociation energies for the loss of a water molecule from these clusters are obtained. These energies are 77.5 +/- 0.5 and 53 +/- 1 kJ/mol for lithiated and sodiated alpha-Me-Pro, respectively. For the nonzwitterionic isomer, proline methyl ester, these values are 3.0-4.5 kJ/mol higher. These results provide compelling experimental evidence that alpha-Me-Pro is zwitterionic in these clusters. Theory at the temperature corrected B3LYP/6-311++G**//B3LYP/6-31++G** level indicates that the salt-bridge or zwitterionic forms of lithiated and sodiated alpha-Me-Pro are between 17 and 23 kJ/mol lower in energy than the nonzwitterionic or charge-solvated forms and that attachment of a single water molecule does not significantly change the structure or the relative energies of these clusters. The proton affinity of proline is 8 kJ/mol higher than that of alpha-Me-Pro, indicating that lithiated and sodiated singly hydrated proline should also be zwitterionic.