Pulse Polarographic Studies of a-Lipoic Acid and Its Reduced Form

Abstract

In cathodic scan pulse polarography (cspp) with the initial potential (E') of 0 V vs. SCE, a 10-4molidm3 a-lipoic acid (1, 2-dithiolane-3-pentanoic acid) solution buffered at pH 4.5 gave a cathodic wave with a maximum at -0, 495 V which corresponds to the DC polarographic wave of a-lipoic acid. And two small, ill-defined and peak-shaped waves were produced at more negative potentials between -1.1 to -1.2 V. In anodic scan pulse polarography (aspp)with E₁ of -1.5 V, α-lipoic acid gave a cathodic wave at the potential near -1.1 V and a S-shaped cathodic-anodic wave at E_1/2=-0.452 V. In cspp (with E₁ =0 V), dihydrolipoic acid (6, 8-dimercaptooctanoic acid) at concentrations lower than 10^-4mol/dm³ gave an anodic-cathodic wave at E_1/2=0.475 V (cspp- I ) and a small, round-maximum wave with the peak potential (E_p) of -1.27 V (cspp-II). With increasing dihydrolipoic acid concentration, the cspp- I wave became a peak-shaped wave with the peak height approaching a saturation value, and at concentrations higher than 3 × 10^-4mol/dm³, an additional peak-shaped wave was produced at E_p=-0.394 V (cspp-III). In aspp (E₁=-1.5 V) dihydrolipoic acid gave a small wave at near -1.1 V and a S-shaped anodic wave at E_1/2=0.453 V. At concentrations lower than 10^-4mol/dm³, the behavior of a-lipoic acid essentially agreed with that of dihydrolipoic acid.