Direct electrochemistry of bovine heart cytochrome c facilitated by cysteine derivatives and analogues. Some effects of facilitator structure

Bovine heart cytochrome c electrochemistry was investigated by cyclic volta mmetry using gold electrodes modified by self-assembled monolayers of cyste ine and some of its derivatives, including glutathione. Glutathione shows a peculiar dependence of the facilitating ability on the oxidation state of the sulfur functionality. Whereas the disulfide form acts as an efficient f acilitator, the thiol form is completely inactive. This behavior was accoun ted for by the lower stability of adsorbed thiol layers as compared to disu lfide layers. Apparently, small molecules of cysteine derivative form rathe r stable adsorbed layers with a high degree of dimer formation. Conversely, reduced glutathione is not able to turn into the disulfide form in the ads orbed layer. Consequently, only the layer produced by the adsorption of the disulfide form itself is stable enough for promoting direct electron trans fer reactions of cyt c.