title: The hydrogen-bonded dianion of vitamin K1 produced in aqueous-organic solutions exists in equilibrium with its hydrogen-bonded semiquinone anion radical.
abstract: 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)y(2–)]. 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)y(2–) 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)y(2–) with strong absorbances at 1372 and 1342 cm(–1).
authors: Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371.
authors: Zhen Hui
authors: ZH
authors: Lim
authors: Elaine Lay Khim
authors: EL
authors: Chng
authors: Yanlan
authors: Y
authors: Hui
authors: Richard D
authors: RD
authors: Webster
mesh_terms: D000097:Acetonitriles / Q000737:chemistry*; D000838:Anions / Q000737:chemistry; D016227:Benzoquinones / Q000737:chemistry*; D055664:Electrochemical Techniques; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D006860:Hydrogen Bonding; D017550:Spectroscopy, Fourier Transform Infrared; D010837:Vitamin K 1 / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D000097:Acetonitriles; D000838:Anions; D016227:Benzoquinones; D014867:Water; C025232:semiquinone radicals; D010837:Vitamin K 1; C032159:acetonitrile