FS SPECTROSCOPIC STUDIES OF THE PLANT PHOTORECEPTOR PHYTOCHROME

Femtosecond time-resolved spectroscopic studies were performed with th e plant photoreceptor phytochrome in its biologically active form, Pfr . After excitation of Pfr with 150 fs pulses at 730 nm the changes in the optical density Delta OD(t) were measured at selected probe wavele ngths. Following excitation, bleaching of the two ground-state absorpt ion bands, the Q band at 730 nm and the Soret band at 402 nm were obse rved. Moreover, a transient absorption at around 800 nm was detected. Analysis of the experimental data strongly suggests that excited Pfr decays with 700 +/- 100 fs in part to the ground state and in part to another state, which absorbs at 760 nm and decays with a time constant of 4.5 +/- 0.5 ps. These two decays are the fastest light-induced pro cesses observed in the deexcitation of Pfr so far.