Voltammetry of 6,6 '-dithiodinicotinic acid on a self-assembled phospholipid monolayer

This paper reports a voltammetric study of 6,6'-dithiodinicotinic acid (CPD S) across a biomimetic membrane system consisting of a monolayer of dioleoy lphosphatidylcholine, deposited on mercury. Because of the low solubility o f this compound and its potential decomposition in alkaline media, estimati on of pK values for the carboxyl and amino groups of the pyridine ring of t he CPDS entailed using the Hammett equation. UV spectra seem to confirm the presence of the dianionic form of CPDS above pH=3-4. Differential capacity and cyclic voltammetry measurements were made in order to characterize the voltammetric behavior directly on mercury and through a monolayer of diole oylphosphatidylcholine. Estimation of the CPDS hydrophobicity degree from t he partition coefficient in octanol/water suggests no penetration of the di anion into the monolayer and supports the fact that the named dianion under goes protonation at the phospholipid/solution interface to give a neutral s pecie which penetration into the phospholipid region, favored by its higher hydrophobicity, is followed by electrochemical reduction at the mercury su rface. key words: 6,6'-dithiodinicotinic acid, phospholipid monolayer, volt ammetry.