Influence of oligopeptide flexibility on donor-acceptor distance distribution by frequency-domain fluorescence spectroscopy

We studied energy transfer between tryptophan (donor, D) and dansyl (acceptor, A) separated by three different hexapeptides: hexaglycine, hexaalanine and hexaproline. In each case the donor tryptphanamide was located on the C-terminus, and the dansyl acceptor on the N-terminus amino group. In the absence of acceptor, in propylene glycol at 20°C, tryptophan donors show single exponential fluorescence intensity decays with mean lifetimes about 5.2 ns. In the presence of dansyl, the lifetimes become shorter and intensity decays heterogeneous. However, the extent of heterogeneity is different for each labeled oligopeptide. The strongest heterogeneity was observed for the hexaglycine donor-acceptor pair. Only small deviations from single exponential decay were found for the proline D-A pair. These results suggested a higher degree of conformational heterogeneity for the flexible glycine D-A pair, or composed to the more rigid proline D-A pair. We analyzed the data in terms of Gaussian donor-acceptor distance distributions. The recovered average distances R_av, are 10.7, 15.9 and 23.4 A and full widths at half maximum, hw, are 18.1, 11.0 and 3.9 A for hexaglycine, hexaalanine and hexaproline chain, respectively. The results indicate that flexibility of peptide chain has a strong influence on end-to-end distance distribution. The possible effects of the orientation factor κ² are discussed, along with the linits on κ² consistent with the time-dependent anisotropy decays. These distance distributions provide the basis for comparisions and/or refinement of the rotational potential functions of the amino acids.