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Fig. 2. Schematic representation of the ET pathway in the A8Pyr/SSA6 bicomponent SAM.In a photocurrent generation experiment, photo-excitation of the pyrene chromophoretriggers an electron transfer (ET) event across the peptide chain to the gold electrode(anodic current). The cycle is closed by a second ET step from a sacrificial electron donorin solution (triethanolamine) to the pyrene radical cation. In the presence of an electronacceptor in solution (methylviologen), the electronic current direction can be easilyreversed, producing a cathodic current, i.e. Au Pyr ET. On/off cycles of the electronicphotocurrent generated by excitation at different wavelengths of the SSA6Pyr SAMimmobilized on a gold electrode in a triethanolamine aqueous solution are shown inFigure 1 (antenna effect).In the case of A8Pyr, the photoactive peptide is embedded in a bicomponent peptideSAM formed by an equimolar A8Pyr and SSA6 solution, and it is not linked to the goldsurface. Direct ET from the pyrene excited state to the gold surface is therefore inhibited bythe high activation energy required for a through-space ET step. A possible mechanismforecasts that the ET process would take place through an intermolecular A8Pyr* SSA6interchain ET step followed by multiple ET steps across the SSA6 peptide chain linked togold (junction effect). This mechanism is pictorially sketched in Figure 2.References1. Toniolo, C., Crisma, M., Formaggio, F., Peggion, C. Biopolymers (Pept.Sci.) 60, 396-419 (2001).2. Karle, I.L., Balaram, P. Biochemistry 29, 6747-6756 (1990).619