Modification of pK values caused by change in H-bond geometry.
Modification of pK values caused by change in H-bond geometry.
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H 键几何形状的变化引起 pK 值的改变。
DOI：
10.1073/pnas.82.9.2741
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发表时间：
1985
影响因子：
11.1
通讯作者：
Hillenbrand,EA
中科院分区：
文献类型：
--
作者：
Scheiner,S;Hillenbrand,EA
用从头算分子轨道方法研究了不同基团对质子的竞争。研究发现，氢键中两个基团的重新取向可以使它们之间共享质子的平衡位置发生逆转。具体地，羰基和羟基基团由H2 CO和HOH建模。在这两个基团之间的H-键中，质子与羰基（H2 COH. OH 2）+比羟基（H2 CO.当后一基团沿着羰基氧的孤对电子时，然而，水向两个羰基孤对之间的C = O轴的位移使情况逆转，并且（H2 CO…HOH ~（2+）更稳定。在涉及Schiff碱（以CH 2NH为模型）和胺（NH 3）的H-键中观察到类似的稳定性逆转。在两个基团的孤对相互指向的一种排列中，质子更喜欢希夫碱而不是胺--即，（H2CHNH. NH3）+比（H2 CHN）更稳定。HNH 3）+。另一方面，胺的孤对电子远离席夫碱氮的旋转导致质子转移到胺上。这些稳定性的变化对应于所涉及基团的相对pK的逆转。从计算中出现的一个基本原理是，离子-偶极静电相互作用有利于质子转移到尽可能靠近另一个偶极矩矢量负端的基团。这里提出的想法提出了许多方法，通过这些方法可以利用构象变化来将质子从蛋白质分子（如酶或细菌视紫红质）的残基转移到残基。
The competition between various groups for a proton is studied by ab initio molecular orbital methods. It is found that reorientations of the two groups involved in a H-bond can reverse the equilibrium position of the proton shared between them. Specifically, the carbonyl and hydroxyl groups were modeled by H2CO and HOH. In the H-bond between these two groups, association of the proton with the carbonyl (H2COH...OH2)+ is favored over the hydroxyl (H2CO...HOH2)+ when the latter group is situated along a lone pair of the carbonyl oxygen. However, displacement of the water to the C = O axis between the two carbonyl lone pairs reverses the situation and (H2CO...HOH2)+ is more stable. A similar reversal of stability is observed in the H-bond involving a Schiff base (modeled by CH2NH) and amine (NH3). In one arrangement where the lone pairs of the two groups point toward one another, the proton prefers the Schiff base to the amine--i.e., (H2CHNH...NH3)+ is more stable than (H2CHN...HNH3)+. On the other hand, rotation of the lone pair of the amine away from the Schiff base nitrogen results in proton transfer across to the amine. These shifts in stability correspond to reversal of relative pK of the groups involved. A fundamental principle emerging from the calculations is that ion-dipole electrostatic interactions favor transfer of a proton to the group that is positioned as closely as possible to the negative end of the dipole moment vector of the other. The ideas developed here suggest a number of means by which conformational changes may be utilized to shift protons from residue to residue within a protein molecule such as an enzyme or bacteriorhodopsin.