Distribution of end-to-end distances of oligopeptides in solution as estimated by energy transfer

A homologous series of oligopeptides each containing at its ends a donor and an acceptor of electronic excitation energy was synthesized by the solid-phase method. N-5-(2-Hydroxyethyl)-L-glutamine was the repeating unit, and peptides containing 4, 5, 6, 7, 8, and 9 of these amino-acid residues were prepared. The chromophores naphthalene and dansyl, which were used as donor and acceptor, respectively, fulfil the conditions necessary for energy transfer according to the Förster mechanism. A distance corresponding to 50% efficiency of energy transfer, tro = 22 plus or minus 1 A, was calculated. The kinetics of fluorescence decay of an oligomer containing the naphthalene chromophore only could be described precisely by a monoexponential function. In contrast, the kinetics of the decay curves of the fluorescence of the donor of all of the oligomers containing both donor and acceptor, as measured in viscous solution, deviated markedly from monoexponential behavior. The deviation was interpreted in terms of the great number of different conformations that the various molecules of each of the oligomers attain in solution, leading to characteristic end-to-end distribution functions between the donor and acceptor. Numerical adjustment of the parameters of some of the previously proposed expressions to describe the end-to-end distribution enabled the reconstruction of the kinetics of the fluorescence decay of the donor with great precision. The end-to-end distribution functions for the various oligopeptides were thus evaluated.