Homocysteine electrochemical reactions at a mercury electrode in the presence of nickel or cobalt ions

Homocysteine (Hcy) is a sulfur-containing amino acid that differs from cysteine (Cys) by one additional methylen group in the side chain. Consequently, stability of Hcy complexes with metal ions is expected to be much lower than in the case of Cys which is known to form very stable, five-membered chelate complexes involving both the amino and thiol group. That is why we have been looking for differences in Hcy and Cys electrochemical behavior in the presence of metal ions like Ni2+ and Co2+. In a phosphate buffer at pH 6.5, Hcy gives rise to a series of voltammetric peaks in the presence of Ni2+ and under the conditions of cathodic stripping voltammetry. The forward scan reveals the following processes: A) cathodic reaction of mercury thiolate (with a Ni2+-dependent peak potential); B and C) catalytic Ni2+ reduction peaks; E) catalytic hydrogen evolution. On the reverse scan, the followings processes were noticed: G) Hcy-assisted anodic reaction of mercury; F) Hcy-catalyzed anodic reaction of Ni(0). Thiol decomposition under anodic polarization in the presence of the metal ion was evidenced by means of the catalytic hydrogen evolution induced by the metal sulfide thus formed. The same kind of processes has also been detected with cysteine (Cys) under similar conditions. This contribution reveals some differences between Hcy and Cys electrochemical behavior and points out possible applications for Hcy determination in the presence of Cys.