ELECTRON AND PROTON TRANSFERRING PROPERTIES OF VITAMIN-K-1 ACROSS A SELF-ASSEMBLED PHOSPHOLIPID MONOLAYER

The electrochemical behaviour of vitamin K-1 (VK1) incorporated in a s elf-assembled monolayer of dioleoylphosphatidylcholine (DOPC) deposite d on a hanging mercury drop electrode was investigated by a computeriz ed chronocoulometric technique. The kinetics of VK1 electroreduction t o the corresponding quinol, VK1H(2), and that of VK1H(2) reoxidation t o VK1 were examined for reactant concentrations ranging from 0.5 to 2 mol% by varying the pH from 5.5 to 9 with phosphate and berate buffers . On the basis of a general kinetic approach it was concluded that the reduction of VK1 to VK1H(2) in a DOPC monolayer takes place via the r eversible uptake of one electron, yielding the semiquinone radical ani on VK1(.-), followed by the rate determining protonation of the latter . On the other hand, the oxidation of VK1H(2) to VK1 takes place via t he reversible release of one electron, yielding the semiquinone radica l cation VK1H(2)(+.), followed by the rate determining deprotonation o f the latter. The only effective proton donors in VK1 reduction are th e protons, whereas the main proton accepters in VK1H(2) oxidation are the water molecules. (C) 1998 Elsevier Science S.A. All rights reserve d.