Sammendrag
Homocysteine (Hcy) is a thiol-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-member 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). Adsorbed thiol undergoes decomposition to metal sulfides under anodic polarization in the presence of the Ni or Co ions. Metal sulfides thus formed are responsible for the catalytic hydrogen evolution at potentials characteristic to the Brdicka wave. The same kind of processes has also been detected with cysteine (Cys) but Cys is more stable against decomposition under the above mentioned conditions. In addition, Cys gives rise to a catalytic hydrogen reduction process catalyzed by a five-member Ni2+ complex. This contribution puts into evidence some differences between Hcy and Cys electrochemical behavior and points out possible applications for Hcy determination in the presence of Cys.
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