Anti-cooperativity in the two electron oxidation of the S118C disulfide dimer of azurin

We have constructed a disulfide dimer of S118C azurin, in which two copper centers are coupled through a relatively short covalent pathway, and studied its electron transfer properties. The dimer exhibits intriguing mechanistic properties. Due to the strain in the molecule, caused by the limited accessibility of Cys118, anti-cooperativity occurs in the two step oxidation of the dimer with a difference in redox potential between the two half reactions of 33 mV. Upon oxidation, the dimer favours the semi-reduced over the fully oxidized state, as the Cu(I) site in the semi-reduced dimer is able to stabilize the strained dimer complex. The internal electron transfer is surprisingly slow, which could be partially due to an increase in reorganization energy.