抄録
α-Lipoic acid shows a two-electron, two-proton reduction wave to dlhydrolipoic acid in its polarogram and the wave is divided into two steps by adsorption phenomenon. The 1st wave nears a constant value by increasing the concentration but the total wave-height is proportional to concentration. When a rotating platinum electrode is used, an oxidation wave appears at E1/2+0.8V vs. N. C. E. α-Lipoic acid shows three A. C. polarographic waves. The 1st wave is Ep+0.02V and corresponds to adsorption of α-lipoic acid, the 2nd wave corresponds to the 1st wave in D. C. and is due to electrolysis and desorption, and the 3rd wave is considered to be due to desorption of dihydrolipoic acid. β-Lipoic acid shows reduction wave accompanied by an adsorption wave and this is due to a four-electron, four-proton reduction to dihydrolipoic acid, An A. C. polarographic wave is obtained at the potential corresponding to the D. C. adsorption wave. β-Lipoic acid is easily hydrolyzed by alkali to form mercaptosulfinoöctic acid and loses reduction wave. The polymer of α-lipoic acid does not show a reduction wave and undergoes depolyrnerization by alkalis. The reduction wave of α-lipoic acid is deformed by the detergent, Tween 20, but its effect is the smaller, the lower the pH. α-Lipoic acid and thiamine propyl disulfide can be determined separately but at the same time in a 0.02% solution of Tween 20 at pH 2.
