Energy transfer from retinal to amino acids — a time-resolved study of the ultraviolet emission of bacteriorhodopsin

Two-step excitation of retinal in bacteriorhodopsin by visible
light is followed by an energy transfer to amino acids that is seen
as fluorescent emission around 350 nm. The fluorescence spectrum
obtained after two-step excitation (2 × 527 nm) differs from the
fluorescence spectrum obtained after one-step ultraviolet
excitation (263.5 nm) by a strongly quenched emission with a
fluorescence lifetime of 10 ± 5 ps and a smaller spectral width.
The two-step absorption process presumably selects tryptophan
residues which strongly couple to the retinal chromophore.