Transient absorption spectroscopy in the time range from -1 ps to 4 ns, and over the wavelength range from 420 to 550 nm, was applied to the Glu46Gln mutant of the photoactive yellow protein (PYP) from Ectothiorhodospira halophila. This has allowed us to elucidate the kinetic constants of excited state formation and decay and photochemical product formation, and the spectral characteristics of stimulated emission and the early photocycle intermediates. Both the quantum efficiency (~0.5) and the rate constants for excited state decay and the formation of the initial photochemical intermediate (I0) were found to be quite similar to those obtained for wild-type PYP. In contrast, the rate constants for the formation of the subsequent photocycle intermediates (I0(+) and I1), as well as for I2 and for ground state regeneration as determined in earlier studies, were found to be from 3- to 30-fold larger. The structural implications of these results are discussed.
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