Picosecond decay kinetics and quantum yield of fluorescence of the photoactive yellow protein from the halophilic purple phototrophic bacterium, Ectothiorhodospira halophila

The photoactive yellow protein (PYP) has been previously shown to be partially bleached and red shifted (in less than 10 ns) by a pulse of laser excitation at the wavelength maximum (445 nm), to further bleach ( k = 7.5 × 10 3 s -1), and then to slowly recover in the dark ( k = 2.6 s -1) (Meyer, T. E., G. Tollin, J. H. Hazzard, and M. A. Cusanovich. 1989. Biophys. J. 56:559–564). The quantum yield for the formation of the fully bleached form was found to be 0.64. We have now shown that the yellow protein is weakly fluorescent with an emission maximum at 495 nm (which mirrors excitation at 445 nm) and a fluorescence quantum yield of 1.4 × 10 -3. Measurement of the picosecond kinetics of the fluorescence decay shows that ∼90% of the emission occurs with a lifetime of 12 ps. This is in good agreement with the quantum yield determination, which suggests that a single quenching process (presumably the photochemical event) is primarily responsible for the excited state decay. The lifetime of the excited state of PYP is remarkably similar to that for the rise of the first photochemical intermediate of bacteriorhodopsin, and underscores the fundamental similarity in their photocycles despite a lack of structural relationship.