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(H ₂O)_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 VK ₁(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(H ₂O)_y^2- with strong absorbances at 1372 and 1342 cm^-1.