Intramolecular electron transfer across amino acid spacers in the picosecond time regime. Charge-transfer interaction through peptide bonds

For a series of alanine-based peptides having 1-3 amino acid residues as spacers, the chromophore, pyrenesulfonyl (Pyr), has been attached at the N-terminus and an electron donor, dimethyl-1,4-benzenediamine (DMPD), covalently bound at the C-terminus. Evidence for an intramolecular charge-transfer interaction involving the electron donor and acceptor groups has been obtained from absorption spectra. Intramolecular electron transfer involving the end groups, Pyr (electron acceptor) and DMPD (electron donor) has been confirmed by ultrafast pump-probe methods. The radical-ion pair states that are generated on Ti/sapphire laser excitation at 400 nm decay in the picosecond to nanosecond time domain and generally show multiexponential decay kinetics. These rates of charge recombination are among the fastest yet observed involving electron transfer between terminal groups for peptide oligomers. The falloff of rate constants for ion pair recombination is irregular in terms of the through-bond distance that separates Pyr and DMPD groups for the various peptide links; i.e., back electron transfer remains fast for the tripeptide, Pyr-Ala-Ala-Ala-DMPD, despite an average through-bond distance between photoactive groups that reaches 18 Å. Molecular modeling studies show that the peptides are free to adopt conformations in essentially random fashion, without showing evidence for long range ordering of the peptide chain.