SPIN-FORBIDDEN EXCITATION TRANSFER AND HEAVY-ATOM INDUCED INTERSYSTEM CROSSING IN LINEAR AND CYCLIC-PEPTIDES

Four dimeric peptides containing one fluorescent chromophore, beta-(1'-naphthyl)-L-alanine or beta-(1'-naphthyl)-D-alanine, and one heavy atom perturber, p-broMo-L-phenylalanine, were synthesized as two pairs of diastereoisomers-one cyclic pair and one linear pair. The backbone atoms of the cyclic peptides form a stable six-membered diketopiperazine ring which provides a rigid molecular framework onto which the side chains are affixed cis or trans to each other. The fluorescence emission (steady state and time resolved) and the time-resolved triplet-triplet absorption of the naphthyl residue in these peptides were monitored, revealing remote electronic interactions between the naphthyl and the bromophenyl groups. All peptides exhibited fluorescence quenching. A nominally spin-forbidden singlet-triplet energy transfer and a heavy atom induced enhanced intersystem crossing were established as two simultaneously active mechanisms for the fluorescence quenching of the naphthyl residue. An understanding of the energy-transfer rate and the extraction of its electronic matrix element is straightforward. But the heavy atom enhanced intersystem crossing, which reflects the quantum mechanical participation of the bridging peptide groups, shows unexpected trends. Subtle factors arising from the detailed properties of the intervening peptide backbone evidently contribute to this remote heavy atom effect. Here we propose a perturbation analysis, including an integration over intermediary vibronic states, in which the remote heavy atom effect arises through two virtual energy transfer interactions. This provides a general model for radiationless relaxation processes driven by indirect electronic interactions. The electronic coupling responsible for the exchange-mediated remote heavy atom effect in the peptide series is examined. This electronic coupling is a measure of the very weak electronic delocalization common to all exchange-mediated interactions, including electron transfer.