Peptide Conformational Distributions As Studied by Electron-Transfer Kinetics

The preparation and electron-transfer (ET) behavior of a homologous series of alanine oligomers bearing naphthoyl groups at the N-termini and biphenylylamide groups at the C-termini is described. Electron pulse radiolysis was used to generate the corresponding radical anions, and the rates of ET were monitored at 700 nm (decay of donor) and at 500 nm (growth of acceptor). Several systems displayed ET decays too fast to measure (k_et > 10¹⁰), and in the others multiexponential decay kinetics were observed. The ET decay of dipeptide 3 could be fit to two exponential described by rate constants of 5.2 X 10⁸ (22%) and 5.6 X 10⁹ s^-1 (78%). In the longer peptides, the fit of the rate constants (and their relative contributions to total intensity) becomes less well-defined, suggesting additional conformational diversity.