The biological importance of vitamins K in relation to blood coagulation processes is generally known. Although extensive biological studies show that vitamins K take part in the electron transfer system 1-5, a clear explanation for their role in this system has not been established. Electrochemical studies of such compounds seem to be worthwhile to provide useful results in solving such biological problems.
The majority of works on electrochemical characteristics of vitamins K has dealt with those of vitamin K 3 (VK3, 2-methyl-l,4-naphthoquinone). It is reduced at a mercury electrode to the corresponding hydroquinone, and several electroanalytical techniques have been examined for the determination of VK3 6-12
Vitamin K2 (VK2) is a general term for the derivatives of 2-methyl-l,4naphthoquinone having a long side chain of some isoprene units in 3-position. The fact that the electrochemical behavior of VK 2 has so far not been studied, in spite of an interest in obtaining information of the role of a long side chain in the electrode reaction of quinones, has led us to the present work.
Recently, O'Brien and Olver 13 examined cyclic voltammetry of ubiquinone (coenzyme Q) in aqueous methanol to clarify the adsorption situation on a mercury electrode. They concluded that the electrode redox reaction involves an adsorption process due to the unsaturated side chain of the ubiquinone molecule.
Adsorption of side chain is also expected for VK 2. The d.c. polarogram of VK 2 exhibits only a single wave due to a two-electron reduction in aqueous alcoholic media, but the a.c. polarogram and drop-time curves suggest that adsorption of VK 2 takes place on the electrode surface. The present paper is concerned with the cyclic voltammetry of VK 2 to elucidate the contribution of adsorption of the side chain to the electrode processes. The role of water in the solution, the adsorbability of VK 2 and its hydroquinone, and the orientation of adsorbed molecule at the electrode surface have been discussed. EXPERIMENTAL D.c. and a.c. polarograms were recorded on a Yanagimoto pen-recording polarograph model 102 in the usual manner. The mercury flow rate and the drop time of the dropping mercury electrode used were 0.926 mg s-1 and 7.32 s in air-free base electrolytic solution at open circuit and at 25Β°C. Linear sweep cyclic voltammetry curves were recorded on a Yokogawa