PICOSECOND EVENTS IN THE PHOTOTRANSFORMATION OF PHYTOCHROME - A TIME-RESOLVED ABSORPTION STUDY

Picosecond time-resolved transient absorption studies with phytochrome in its red light absorbing form (P(r)) isolated from rye (Secale cereale cv. Danae) are reported. After excitation of P(r) with 6 ps pulses at 621 nm the transient absorption spectra were measured at various delay times from 5 ps up to 1 ns. Upon excitation an increase in the transient absorption around 400 nm immediately appeared, which is ascribed to the first excited state absorption of P(r). With a delay of about 15 ps a strong bleaching of the P(r) ground state absorption around 665 nm emerged. After a series of further absorption changes, an increase in absorbance occurred above 685 nm. Since this transient absorption persisted over the 1 ns time-window measured, it is interpreted as being due to the formation of lumi-R. Based on the time evolution of the observed absorption changes it seems likely that two pools of lumi-R were formed. In addition, it was found from the kinetics of the picosecond absorption changes that the relaxation of excited P(r) obviously involves processes which follow a non-exponential rate law. From a comparison with picosecond adsorption spectroscopy on deuterated P(r) and a 39 kDa chromopeptide, conclusions about the first relaxation steps in the phototransformation of P(r) are drawn. It is suggested that they involve rotations at the single bonds in the chromophoric methine bridges.