Binding energies of water to sodiated valine and structural isomers in the gas phase: the effect of proton affinity on zwitterion stability.
Binding energies of water to sodiated valine and structural isomers in the gas phase: the effect of proton affinity on zwitterion stability.
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DOI：
10.1021/ja034544n
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发表时间：
2003-10
影响因子：
15
通讯作者：
A. Lemoff;M. F. Bush;E. Williams
中科院分区：
文献类型：
--
作者：
A. Lemoff;M. F. Bush;E. Williams
本文从理论和实验两个方面研究了钠离子与缬氨酸（瓦尔）和甲氨基异丁酸（Maiba）的结合结构。计算表明，在没有水的情况下，钠化的瓦尔形成电荷溶剂化结构，其中钠离子与氮和羰基氧配位（NO-配位），而Maiba形成盐桥结构，其中钠离子与两个羧酸氧配位（O-配位）。一个单一的水分子的加入并没有显着影响的相对能量或结构的电荷溶剂化和盐桥形式的集群，虽然在麦巴的钠离子结合的模式略有改变的水分子。Maiba在这些复合物中采用两性离子形式的偏好与其较高的质子亲和力一致。在实验上，用黑体红外辐射离解法（BIRD）测量了瓦尔.Na（+）（H（2）O）和麦巴.Na（+）（H（2）O）团簇的水分蒸发速率.从瓦尔和Maiba复合物的解离速率进行比较，从模型复合物的已知结构在很宽的温度范围内的水蒸发速率。BIRD动力学数据的主方程建模产生钠化缬氨酸的水损失的阈值解离能为15.9 +/- 0.2 kcal/mol，钠化Maiba的水损失的能量为15.1 +/- 0.3 kcal/mol。瓦尔·Na（+）（H（2）O）的阈值解离能与电荷溶剂化模型异构体相同，而盐桥模型复合物与Maiba·Na（+）（H（2）O）具有相同的阈值解离能。这些结果表明，从这些盐桥络合物中损失水分子的阈值解离能比从电荷溶剂化络合物中损失水的阈值解离能小约1千卡/摩尔。
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.