LONG-DISTANCE SINGLET ENERGY-TRANSFER ALONG ALPHA-HELICAL POLYPEPTIDE-CHAINS

alpha-Helical polypeptides carrying L-4-biphenylalanine and L-1-naphth ylalanine that are separated by up to 11 amino acid units or 19.5 Angs trom were prepared by stepwise peptide synthesis. Singlet energy trans fer from the biphenyl group to the naphthyl group was investigated wit h steady-state fluorescence spectroscopy, and the efficiency of energy transfer was evaluated as a function of the number of spacer units be tween the two nonnatural amino acids. The observed efficiency showed q ualitative agreement with theoretical values calculated from interchro mophore distances that were predicted from the helix geometry. When th e biphenylnaphthyl pair is separated by a single amino acid, higher tr ansfer efficiency than that predicted by theory was observed. The high efficiency was interpreted in terms of contributions from higher mult ipole interactions. An effect of a phenyl group inserted between the d onor and acceptor was also studied. The intervening phenyl group sligh tly enhanced the energy transfer when the aromatic rings of the donor, mediator, and acceptor groups were nearly in a face-to-face configura tion.