PHOTOINDUCED ELECTRON-TRANSFER ON A SINGLE ALPHA-HELICAL POLYPEPTIDE-CHAIN - EVIDENCE OF A THROUGH-SPACE MECHANISM

alpha-Helical polypeptides containing a single pair of L-p-(dimethylamino)phenylalanine and L-l-pyrenylalanine were synthesized. The distance and orientation of the (dimethylamino)phenyl (D) and pyreyl (P) group were varied by inserting different numbers (m) of L-alanine units between the two artificial aromatic amino acids. The edge-to-edge distances between D and P groups, predicted from conformational energy calculations, were 5.4 angstrom for m = 0, 9.4 angstrom for m = 1, and 5.5 angstrom for m = 2. This indicates that the D-P pair in the m = 2 polypeptide is separated by a larger number of bonds but located closer than the D-P pair in the m = 1 polypeptide. The D-P pair in the m = 0 polypeptide was arranged in a head-to-tail manner, but that in the m = 2 polypeptide had a face-to-face arrangement. The rates of photoinduced electron transfer (ET) from the D to P* group at -20-degrees-C in trimethyl phosphate solution were evaluated from the decay time of P* fluorescence to be 1.9 x 10(7) (m = 0), 6.6 x 10(5) (m = 1), and 2.1 x 10(7) s-1 (m = 2). The results show that the ET is occurring through space, and the possibility for a helical polypeptide chain as an electron mediator was excluded, at least for this relatively short distance ET. The ET rate was not very sensitive to the orientation of the donor and acceptor groups.