Modification of pK values caused by change in H-bond geometry.

Abstract

The competition between various groups for a proton is studied by ab initio molecular orbital methods. Reorientations of the 2 groups involved in a H-bond can reverse the equilibrium position of the proton shared between them. Specifically, the carbonyl and hydroxyl groups were modulated by H2CO and HOH. In the H-bond between these 2 groups, association of the proton with the carbonyl (H2COH.cntdot..cntdot..cntdot.OH2)+ is favored over the hydroxyl (H2CO.cntdot..cntdot..cntdot.HOH2)+ when the latter group is situated along a lone pair of the carbonyl oxygen. Displacement of the water to the C.dbd.O axis between the 2 carbonyl lone pairs reverses the situation and (H2CO.cntdot..cntdot..cntdot.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 2 groups point toward one another, the proton prefers the Schiff base to the amine, i.e., (H2CHN.cntdot..cntdot..cntdot.HNH3)+ is more stable than (H2CHN.cntdot..cntdot..cntdot.HNH3)+. 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.