Singlet−Singlet and Triplet−Triplet Energy Transfer in Bichromophoric Peptides

Intramolecular singlet−singlet (SSET) and triplet−triplet (TTET) energy transfer processes were studied in solution in two bichromophoric peptides using absorption, fluorescence, phosphorescence, laser flash photolysis, and molecular modeling/dynamics. Compound I, a dipeptide formed by coupling 2-naphthyl- l -alanine and 4‘-benzoyl- l -phenylalanine (naphthalene and benzophenone chromophores), undergoes intramolecular SSET from the naphthyl chromophore to the benzophenone chromophore as indicated by singlet lifetime measurements as well as a reduction in the intensity of the steady-state fluorescence emission relative to 2-naphthyl- l -alanine itself. Results of the lifetime experiments coupled with modeling studies suggest that SSET is consistent with a Förster mechanism, although other mechanisms cannot be ruled out. Low-temperature phosphorescence and room-temperature laser flash photolysis results indicate that intramolecular TTET from the benzophenone group to the naphthyl moiety proceeds with a rate constant, k > 10 8 s -1 (lower limit). Compound II consists of the same two chromophores appended to the backbone of a 14-residue peptide in which the chromophores are separated by two alanine−α-aminoisobutyric acid−alanine tripeptides and each end of the peptide is capped with an identical tripeptide. Circular dichroism measurements and molecular modeling/molecular dynamics calculations demonstrate an α-helical secondary structure for this peptide in acetonitrile solvent. Intramolecular SSET is again suggested by steady-state and lifetime measurements and, in this case, only the Förster mechanism is required to account for the observed rate. Laser flash photolysis measurements in acetonitrile and 50:50 ethanol/methanol again give evidence for rapid intramolecular TTET, with k > 10 8 s -1 . In contrast, phosphorescence spectra of II in methyltetrahydrofuran and 50:50 ethanol/methanol exhibit strong benzophenone emission consistent with inefficient TTET. This behavior is attributed to the ability of the low-temperature matrix to prevent the chromophores from achieving conformations conducive to good orbital overlap.