Photoinduced Electron Transfer on β-Sheet Cyclic Peptides

Gramicidin S analogues that contain a single pyrenyl group and a single p -nitrophenyl group at different positions were synthesized. Photoinduced electron transfer (ET) from an excited pyrenyl group located at the fourth position to a nitrophenyl group located at five different positions was investigated on the β-sheet chains of the cyclic peptides. The observed ET rate constants displayed a complex dependence on the number of spacer units or on the edge-to-edge distance between the two aromatic groups. The ET rate constants, however, showed a reasonable linear relationship with the equivalent σ-distances calculated from the Beratan−Onuchic tunneling pathway model, if through-hydrogen-bond ET was taken into account. The results support the applicability of the tunneling pathway model on both helix and β-sheet model peptides.