Adsorption Orientation of Horse Heart Cytochrome c on a Bare Gold Electrode Hampers Its Electron Transfer

Direct electron transfer of horse heart cytochrome c (HHCC) adsorbed on a bare metal electrode has been shown to be hard to achieve. The reason for that has been discussed in terms of conformational changes, protein unfolding, and even denaturation of the protein. We explored the adsorption of HHCC on a bare Au electrode by surface-enhanced infrared absorption spectroscopy on the molecular level. Our results revealed that the native secondary structure of adsorbed HHCC was kept even on the bare Au surface. The hampered electron transfer was mainly caused by its adsorption orientation. The adsorption of HHCC on the bare gold surface led the tyrosine 97 residue to be close to the surface. In this orientation, most of the a-helices of adsorbed HHCC run parallel to the bare Au surface with a flatter orientation of the heme relative to its orientation on an 11-mercaptoundecanoic acid-modified Au surface.