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Picosecond resolution study of intramolecular energy transfer in lumazine protein

Lumazine protein is a 21200 Da protein containing a single tryptophan residue and a non-covalently bound, highly fluorescent ligand, 6,7-dimethyl-8-ribityllumazine. Visser et al. have reported that excitation into the region of the absorption of the tryptophan residue, around 300 nm, produces a distinct rise in fluorescence emission at 475 nm from the lumazine ligand. They analyzed a rise rate of around 1 ns and attributed this to energy transfer between the tryptophan donor and the lumazine as acceptor. This present report re-investigates this phenomenon using a ten times higher resolution (FWHM = 23 ps) . The fluorescence rise is found to be more complex and can only approximately be fitted by a sum of two exponential processes, with rise times of 0.02 and 0.6 ns. For the fluorescence of the tryptophan measured at 340 nm, no rise is detected but the decay is similarly much more complex than previously recognised with data taken at lower resolution. Global analysis of three 340 nm decay curves taken with time windows of 1.2, 4.8, and 55.6 ps/channel, results in about 5 exponential components being required for a satisfactory fit to the fluorescence decay.