The hydrogen-bonded dianion of vitamin K1 produced in aqueous–organic solutions exists in equilibrium with its hydrogen-bonded semiquinone anion radical
Lim, Zhen Hui
Chng, Elaine Lay Khim
Webster, Richard David
Date of Issue2013
School of Physical and Mathematical Sciences
When the quinone, vitamin K1 (VK1), is electrochemically reduced in aqueous-acetonitrile solutions (CH3CN with 7.22 M H2O), it undergoes a two-electron reduction to form the dianion that is hydrogen-bonded with water [VK1(H2O)y2–]. EPR and voltammetry experiments have shown that the persistent existence of the semiquinone anion radical (also hydrogen-bonded with water) [VK1(H2O)x–•] in aqueous or organic–aqueous solutions is a result of VK1(H2O)y2– undergoing a net homogeneous electron transfer reaction (comproportionation) with VK1, and not via direct one-electron reduction of VK1. When 1 mM solutions of VK1 were electrochemically reduced by two electrons in aqueous-acetonitrile solutions, quantitative EPR experiments indicated that the amount of VK1(H2O)x–• produced was up to approximately 35% of all the reduced species. In situ electrochemical ATR-FTIR experiments on sequentially one- and two-electron bulk reduced solutions of VK1 (showing strong absorbances at 1664, 1598, and 1298 cm–1) in CH3CN containing <0.05 M H2O led to the detection of VK1–• with strong absorbances at 1710, 1703, 1593, 1559, 1492, and 1466 cm–1 and VK1(H2O)y2– with strong absorbances at 1372 and 1342 cm–1.
The journal of physical chemistry B