Picosecond events in the phototransformation of phytochrome — a time-resolved absorption study

Abstract

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.