Influence of secondary structure on the decay kinetics of fluorescent donor-acceptor labelled peptides

Time-resolved fluorescence decays from a series of methoxynaphthalene labelled peptides in ethyl acetate were monitored over the temperature range -40 to 60 degrees C. The quenching effect of a piperidone acceptor group placed at various positions along the peptide chain relative to the fluorescent methoxynaphthalene donor was studied. In this moderately polar solvent the mechanism of quenching is most likely electron transfer, although a Dexter exchange mechanism cannot be ruled out. Both donor and acceptor moieties were covalently attached to the side-chains of glutamic acid residues. These were either placed adjacently, in the case of a dipeptide, or separated by three and six amino acids within a 12 and 15 amino-acid oligopeptide, respectively. The presence of the piperidone group resulted in a reduction in the fluorescence lifetime and a change from a simple monoexponential decay to more complex behaviour. This was found to vary reversibly with temperature and not to be caused by impurities. Modelling of the fluorescence decays was carried out using either the sum of two exponentials or a distribution of decays. For the dipeptide the best fit was a distribution while in the case of the 12-mer two clearly distinguishable populations could be observed. The results for the 15-mer were equivocal. Importantly, regardless of the fitting method used the quenching rate was found to be fastest for the 12-mer. The slower quenching rates observed for the dipeptide compared to the oligopeptides provide strong evidence that secondary structure promotes better electronic coupling between the donor and acceptor. The biexponential fluorescence behaviour for the 12 amino-acid oligopeptide is ascribed to two slowly (> 10 ns) interconverting conformational states. Comparison with circular dichroism and infrared obtained in acetonitrile indicates these two conformers are likely to be an alpha-helix and a 3(10)-helix with electronic coupling strongest in the latter case.