Distance dependence of photoinduced electron transfer along α-helical polypeptides

alpha -Helical polypeptides containing a pair of L-1-pyrenylalanine and L-p-nitrophenylaianine that are separated by 0-8 amino acid units were synthesized. The rates of photoinduced electron transfer (ET) from the pyrenyl group to the nitrophenyl group were evaluated from the decay curves of pyrenyl fluorescence recorded at different temperatures from -58 to +30 degreesC. The rate constants showed a complex dependence on the number of spacer amino acids. In particular, recoveries of the ET rates with increasing the number of spacer amino acids from 1 to 2 and from 5 to 6 were found. The ET rate constants, however, exhibited a simple exponential dependence on the edge-to-edge distance between the two chromophores, with a distance decay factor beta = 0.66 +/- 0.1 (Angstrom (-1)). The ET data on the alpha -helical polypeptides were analyzed on the basis of the tunneling pathway model. The optimum ET pathways from the pyrenyl group to the nitrophenyl group were searched, and the relative values of the ET matrix elements were evaluated for each polypeptide with different number of the spacer units. The calculated distance dependence was in reasonable agreement with the experimental one when jumps through hydrogen bonds were taken into account.