Unfolding of cytochrome c immobilized on self-assembled monolayers. An electrochemical study.

The TE process of progressively unfolded bovine cytochrome c (cyt c), immobilized on different self-assembled monolayers, was investigated. Direct electrochemical measurements were performed on cyt c adsorbed on mercaptopyridine (MP) and mixed 11-mercapto-1-undecanoic acid/11-mercapto-1-undecanol (MUA/MU) at varying temperature, in the presence of urea as unfolding agent. Under strongly unfolding conditions, a non-native form of cyt c, in which the Met ligand is replaced by a His, was observed on both MP and MUA/MU SAMs. The E° of the native form, in which the haem is axially coordinated by Met and His, slightly shifts to negative values upon increasing urea concentration. However, the non-native bis-His species shows a much lower E° value (by approximately 0.4 V) which is by far enthalpic in origin and largely determined by axial ligand swapping. Analysis of the reduction enthalpies and entropies and of the ET rate constants indicate that the nature of the SAM (hydrophilic or anionic) results in changes in the conformational rearrangement of the cyt c under unfolding conditions.