PICOSECOND DYNAMICS OF THE EXCITED STATE RELAXATIONS IN PHYTOCHROME

Abstract— A comparative study of the picosecond kinetics of rye ( Secale cereale L.) phytochrome, its 39 and 23 kDa chromopeptides and deuterated rye phytochrome has been carried out. Evidence is presented that the fluorescence decay of Pr contains a very short lifetime component (14 ps) which has escaped detection in the fluorescence studies reported so far. Thus, the overall decay is well described by four exponential components, two rapid (14 and 44 ps) and two slower ones (157 and 690 ps). The fluorescence decays of deuterated Pr and of a 39 and 23 kDa chromopeptide of Pr also require the analysis in terms of four exponentials for a good fit. Some of the lifetime and amplitude values obtained differ significantly from the values estimated for Pr. In the chromopeptides, the two longer components have distinctly slower decays. For the two faster components the lifetimes remain approximately the same, but their relative amplitudes vary greatly. In deuterated Pr, the lifetimes are affected only slightly by deuteration. In contrast, the decay amplitudes are strikingly altered. Moreover, from a rate equation simulation modelling the observed fluorescence kinetics, it turns out that the yields for the various deactivation steps in the chromopeptides and in deuterated Pr reveal differences from the corresponding values in Pr. The implications of the results presented with respect to the influence of the protein moiety of Pr on the picosecond relaxation process are discussed.