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Factors influencing proton positions in biomolecules
Author(s) -
Scheiner Steve,
Redfern Paul,
Hillenbrand Eric A.
Publication year - 1986
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560290420
Subject(s) - proton , chemistry , biomolecule , chemical physics , ion , shielding effect , electromagnetic shielding , bond energy , bond length , coupling (piping) , atomic physics , computational chemistry , molecule , crystallography , physics , materials science , quantum mechanics , crystal structure , organic chemistry , biochemistry , metallurgy , layer (electronics)
Results of quantum mechanical calculations are presented that suggest a number of mechanisms whereby protons may be shifted from one group to another along an H bond. The first factor to be considered is a stretching of the bond that drastically raises the energy barrier to transfer. It is possible to predict barriers for an arbitrary system based only on results for a simple system and knowledge of the relevant bond length in the isolated subsystems. Factors that increase the intrinsic basicity of the B group in A‐H‐B lead not only to a lowering of the energy of the A‐HB state relative to AH‐B but also to a reduction in the barrier to transfer of the proton from A to B. Ions in the vicinity of the H bond exert a powerful influence and can shift the proton to the less basic group across a gradient of several pK units. Rather than shielding the proton from the external ion, the H bond acts instead to amplify the effects of the electric field. Reorientation of the A and B groups relative to one another, i.e., bends of the H bond, also produce surprisingly large changes in the relative energies of the AH‐B and A‐HB states. Such bends are capable of pushing the proton across to the normally less basic group, providing a mechanism of coupling conformational changes to proton ‘pumping’ activity. It is found that the high and low pH states of a given H bond can have dramatically differnt relative populations of the AH‐B and A‐HB configurations. These observations are explained in terms of fundamental concepts involving electrostatic interaction energies.