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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.

Peptides composed of alanine (Ala) and tryptophan (Trp), modified with the (nitro)pyrenesulfonyl chromophore (Pyr and NPyr) at the N-terminus have been examined by nanosecond laser flash photolysis. A common phototransient for Pyr-AlaOEt and Pyr-Ala-TrpOEt was observed that exhibited broad absorption at 410−550 nm and decay time constants in the range, τ1/2 = 20−40 μs. This species was assigned to the triplet excited state that is local to the pyrene chromophore (3Pyr). For the conjugates having a stronger electron acceptor group at the N-terminus, NPyr-Ala-TrpOEt and NPyr-Ala-Ala-TrpOEt, the local triplet was replaced with a phototransient whose principal feature is a sharp band at 440 nm (assigned to the NPyr-• radical anion). The radical ion transients for the NPyr peptide derivatives were assigned to intermediates that result from the intramolecular electron transfer quenching of NP excited species by pendant groups (i.e., the indole ring of tryptophan). The lifetimes observed for the radical ion transients associated with the NPyr series were relatively long (extending to ca. 400 ns) and depended in an interesting way on the structure of the peptide linkage. A mechanism of electron transfer in the singlet manifold and recombination yielding a local Pyr triplet state is important for the Pyr series.

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