Lemoff Andrew S, Bush Matthew F, Williams Evan R
Department of Chemistry, University of California, Berkeley, California 94720-1460, USA.
J Am Chem Soc. 2003 Nov 5;125(44):13576-84. doi: 10.1021/ja034544n.
The structures of valine (Val) and methylaminoisobutyric acid (Maiba) bound to a sodium ion, both with and without a water molecule, are investigated using both theory and experiment. Calculations indicate that, without water, sodiated Val forms a charge-solvated structure in which the sodium ion coordinates to the nitrogen and the carbonyl oxygen (NO-coordination), whereas Maiba forms a salt-bridge structure in which the sodium ion coordinates to both carboxylate oxygens (OO-coordination). The addition of a single water molecule does not significantly affect the relative energies or structures of the charge-solvated and salt-bridge forms of either cluster, although in Maiba the mode of sodium ion binding is changed slightly by the water molecule. The preference of Maiba to adopt a zwitterionic form in these complexes is consistent with its higher proton affinity. Experimentally, the rates of water evaporation from clusters of Val.Na(+)(H(2)O) and Maiba.Na(+)(H(2)O) are measured using blackbody infrared radiative dissociation (BIRD). The dissociation rates from the Val and Maiba complexes are compared to water evaporation rates from model complexes of known structure over a wide range of temperatures. Master equation modeling of the BIRD kinetic data yields a threshold dissociation energy for the loss of water from sodiated valine of 15.9 +/- 0.2 kcal/mol and an energy of 15.1 +/- 0.3 kcal/mol for the loss of water from sodiated Maiba. The threshold dissociation energy of water for Val.Na(+)(H(2)O) is the same as that for the charge-solvated model isomers, while the salt-bridge model complex has the same water threshold dissociation energy as Maiba.Na(+)(H(2)O). These results indicate that the threshold dissociation energy for loss of a water molecule from these salt-bridge complexes is approximately 1 kcal/mol less than that for loss of water from the charge-solvated complexes.
利用理论和实验方法，研究了缬氨酸（Val）和甲基氨基异丁酸（Maiba）与钠离子结合的结构，包括有无水分子存在的情况。计算结果表明，无水时，钠化缬氨酸形成电荷溶剂化结构，其中钠离子与氮原子和羰基氧原子配位（NO配位），而Maiba形成盐桥结构，其中钠离子与两个羧酸根氧原子配位（OO配位）。添加单个水分子对两种团簇的电荷溶剂化和盐桥形式的相对能量或结构没有显著影响，不过在Maiba中，水分子会使钠离子的结合模式略有改变。Maiba在这些配合物中更倾向于采用两性离子形式，这与其较高的质子亲和力一致。实验上，使用黑体红外辐射解离（BIRD）测量了Val.Na⁺(H₂O)和Maiba.Na⁺(H₂O)团簇的水蒸发速率。在很宽的温度范围内，将Val和Maiba配合物的解离速率与已知结构的模型配合物的水蒸发速率进行了比较。对BIRD动力学数据进行主方程建模，得出钠化缬氨酸失去水的阈值解离能为15.9±0.2 kcal/mol，钠化Maiba失去水的能量为15.1±0.3 kcal/mol。Val.Na⁺(H₂O)的水阈值解离能与电荷溶剂化模型异构体的相同，而盐桥模型配合物与Maiba.Na⁺(H₂O)具有相同的水阈值解离能。这些结果表明，这些盐桥配合物失去一个水分子的阈值解离能比电荷溶剂化配合物失去水的阈值解离能大约低1 kcal/mol。
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