Ultrashort Processes of Native Phytochrome: Femtosecond Kinetics of the Far-Red-Absorbing Form Pfr

Abstract

The biologically active far-red-absorbing form of the plant photoreceptor phytochrome, Pfr, was studied by femtosecond spectroscopy. The transient absorption changes that occurred upon excitation of Pfr at 730 nm with laser pulses of 150 fs were measured as a function of time at selected probe wavelengths. Immediately after excitation, bleaching of the ground-state absorption bands in the red and blue spectral region was observed. Bleaching was accompanied by an absorption increase at wavelengths between 775 and 825 nm as well as at around 400 nm. The kinetic analysis of the absorption changes ΔOD(t) at different probe wavelengths yielded two decay components, one with a lifetime of about 700 fs and another with about 4.5 ps. A model that explains the relaxation processes following the excitation of Pfr is suggested. It assumes that the initially excited state decays within about 700 fs toward a secondary excited one which is depopulated with a rate constant of (4.5 ps)-1.