The Hydrogen-Bonded Dianion of Vitamin K₁ Produced in Aqueous–Organic Solutions Exists in Equilibrium with Its Hydrogen-Bonded Semiquinone Anion Radical

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.