||Schuler B, Lipman EA, Steinbach PJ, Kumke M, Eaton WA. Polyproline and the "spectroscopic ruler" revisited with single-molecule fluorescence. Proc Natl Acad Sci U S A. 2005 Feb 22 102(8):2754-9. p.2757 caption to fig.4 & p.2758 left column top paragraphPubMed ID15699337
||Abstract: "To determine whether Forster resonance energy transfer (FRET) measurements can provide quantitative distance information in single-molecule fluorescence experiments on polypeptides, [investigators] measured FRET efficiency distributions for donor and acceptor dyes attached to the ends of freely diffusing polyproline molecules of various lengths."
||P.2757 right column 2nd paragraph: "A quantitative determination of the influence of chain bending on the mean FRET efficiency also requires consideration of the time scale of the end-to-end distance fluctuations and the donor fluorescence lifetime (refs 35, 36). Fig. 4 summarizes the relevant times. The typical duration of a photon burst in [investigators'] experiments is ≈1 ms, much longer than the donor fluorescence lifetime or the end-to-end distance correlation time, resulting in complete conformational averaging during the observation time for an individual molecule. The fluorescence lifetimes of the donor, as determined by time-correlated single-photon counting measurements, range from ≈0.4 to 3.6 ns for the different labeled peptides. The reorientational correlation time of the donor chromophore is ≈0.3 ns as determined from the anisotropy decay of donor-labeled peptide in the absence of transfer."