Short, strong hydrogen bonds in the gas phase and in solution:: Theoretical exploration of pKa matching and environmental effects on the strengths of hydrogen bonds and their potential roles in enzymatic catalysis
Short, strong hydrogen bonds in the gas phase and in solution:: Theoretical exploration of pKa matching and environmental effects on the strengths of hydrogen bonds and their potential roles in enzymatic catalysis
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DOI：
10.1021/jo972262y
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发表时间：
1998-07-10
影响因子：
3.6
通讯作者：
Houk, KN
中科院分区：
文献类型：
--
作者：
Chen, JG;McAllister, MA;Houk, KN
短而强的氢键在气相带电系统中很常见，但这种键在酶催化中的重要性一直是相当有争议的主题。关于键强度、氢键的“低势垒”或“无键垒”性质、pK(a)的作用、匹配、键的共价或静电性质以及溶剂化对这些类型氢键强度的作用之间的关系，已经出现了混淆。我们试图通过高层次的计算来定义短-强氢键何时会出现，何时不会出现，从而去除这一领域定义的“爱丽丝梦游仙境”性质。通过从头算量子力学计算，研究了几种阴离子氢键的强度和几何形状。对于一系列烯醇与烯醇酸的氢键，短而强的气相氢键的强度与共用质子的两个阴离子的质子亲和（PA）之间的差成线性关系。结合强度也与O…它们之间的距离是0。在δ PA = 0处没有不连续现象，计算实验表明，当ii键受体的PA超过给体的PA时，氢键变得更强。当δ PA接近0时，包含零点能量的单井最小值的“低势垒”氢键出现，但当双井最小值变为单井时，不产生特殊的稳定性。对Drueckhammer研究的马来酸/富马酸体系和中亚光/香橼酸体系进行了计算研究。采用溶剂腔模型研究了溶剂化对氢键强度的影响。介电常数从气相值（epsilon = 1）的微小增加会迅速降低带电氢键的强度。短而强的氢键只发生在带电体系中，并且只发生在非极性（epsilon < 10）环境中；通常有其他机制可以解释酶催化；讨论了奥罗替丁单磷酸脱羧酶的实例。
Short, strong hydrogen bonds are common in charged systems in the gas phase, but the importance of such bonding in enzymatic catalysis has been the subject of considerable controversy. Confusion has arisen about the relationship among bond strength, the "low-barrier" or "no-banier" nature of the hydrogen bonding, the role of pK(a), matching, the covalent or electrostatic nature of the bonding, and the role of solvation on the strengths of these types of hydrogen bonds. We have attempted to strip away the "Alice in Wonderland" quality of the definitions in this field by defining, through high-level calculations, when short-strong hydrogen bonds do and do not occur. The strengths and geometries of several types of hydrogen bonds involving anions have been investigated by ab initio quantum mechanical calculations. For a series of enols hydrogen-bonded to enolates, the strengths of the short, strong gas-phase hydrogen bonds are linearly related to the differences between the proton affinities (PA) of the two anions which share the proton. The bond strength is also related to the O ... O distance between them. There is bo discontinuity at Delta PA = 0, ana hydrogen-bonding becomes even stronger in a computational experiment when the PA of the II-bond acceptor exceeds that of the donor. "Low-barrier" hydrogen bonds with single-well minima after inclusion of zero-point energies occur when Delta PA is near 0, but no special stability accrues when the double-well minimum becomes single-well. The maleic/fumaric and mesaconic/citraconic systems studied by Drueckhammer have been investigated computationally. The influence of solvation on hydrogen-bond strength was studied using solvent cavity models. Small increases in dielectric constant from the gas-phase value (epsilon = 1) rapidly reduce the strengths of charged hydrogen bonds. Short, strong hydrogen bonds occur only with charged systems, and only then in nonpolar (epsilon < 10) environments; Alternative mechanisms are often available to account for enzymatic catalysis; the example of orotidine monophosphate decarboxylase is discussed.