Ab initio theory of hydrogen bonding in vitamin B₆

Abstract

Ab initio MO computations have been carried out for the crystalline form of the B, vitamin, pyridoxinium chloride. In the crystal, the pyridoxine is protonized to pyridoxinium, with the formation of a hydrogen bond N-H+ . * * CI-. For an isolated molecule, the calculations predict a single potential well, with H+ placed close to CI-at the minimum. When neighboring molecules are included in the calculations, a double well is formed, and the lower minimum occurs for the proton placed near the experimentally observed position, in the vicinity of the N atom.Mulliken populations, which depend on the proton position, are described.

Hydrogen bonding in vitamin B6

“…The results of the earlier calculations [2] are epitomized by the two curves in Figure 2, which are newly calculated using the Atmol package [4] on a Cyber 205 in place of GAUSSIAN 76 [5] and confirm the previous results. The present calculations throughout are based on the sTo-3G approximation.…”

“…It was pointed out in the earlier paper [2] that the effect of the environment, which in particular lowers the potential energy curve in the region of the N-H separation dexp can, apparently, be associated in a classical description with the charge shifts introduced as the proton is moved along the hydrogen bond. The relevant charge shifts, obtained from the ab initio calculations, are primarily those from the C1 to the neighboring fragments; these increase as the proton moves from d,,, to dexp, and this increment is consistent throughout the models a, b, c, and d, being 0.057, 0.063, 0.065, and 0.064, respectively, which are differences between corresponding columns of sections A and B of Table I in the first row, which applies to the C1-ion.…”

“…In a previous communication [2] ab initio SCF MO results relating to the nature of the hydrogen bonding of B, were presented. It was shown that, within the s~o-3G approximation implemented in GAUSSIAN 76 and applied to an individual molecule of the crystal, the results obtained failed to give a correct description of the N-H ... C1 hydrogen bond, predicting the position of the proton in the alternative tautomeric N .…”