Relationship between conformational dynamics and electron transfer in a desolvated peptide. Part II. Temperature dependence.

Recent time-resolved lifetime measurements studied the quenching of the fluorescence emitted by a dye covalently bound to the desolvated peptide Dye-Pro(4)-Arg(+)-Trp. This peptide sequence was chosen for study since intramolecular interactions constrain all large-scale fluctuations except for those of the interacting dye and Trp side chain. It was shown that quenching occurred as a result of interaction between the excited dye and tryptophan side chain. These measurements exhibited a temperature dependence that suggested the quenching mechanism was related to electron transfer. This paper presents a comparison of the experimental quenching rate with the Marcus electron transfer model performed with molecular dynamics (MD) calculations. Taking advantage of the AMOEBA force field that explicitly includes polarizability ensures that the intramolecular electrostatic and polarization interactions in this desolvated peptide ion are treated realistically. MD calculations identify both large-scale fluctuations between conformations as well as small-scale fluctuations within a conformation that are shown to be correlated with torsional dynamics of the Trp side chain. Trajectories of the Dye-Trp distance identify the occurrence of close separations required for efficient electron transfer. The temperature dependence of the quenching rate closely follows the rate predicted by the Marcus electron transfer model within uncertainties resulting from statistical averages. Estimates of the energy parameters characterizing the Marcus model indicate the electronic coupling matrix element and the reaction free energy derived from the fits are consistent with published values for transfer in polyproline bridged peptides. These calculations help to provide a molecular basis for investigating conformational changes in desolvated biomolecular ions by fluorescence quenching measurements.