Femtosecond time-resolved spectroscopy on biological photoreceptor chromophores

This paper reviews our results on fermtosecond time-resolved spectroscopy (transient absorption, transient-grating and fluorescence spectroscopy) to study the photophysics and photochemistry of the two very important biological photoreceptor chromophores phycocyanobilin (PCB) and protochlorophyllide a (PChla). The compound PCB serves as a model chromophore for the photoreceptor phytochrome. By means of transient-grating spectroscopy where the excitation wavelength was varied over the spectral region of the So -> S-1-absorption the ultrafast processes were studied upon excitation with varying excess energy delivered to the system. On the basis of the results obtained, both the rate of the photoreaction in PCB and the rate of the decay of different excited-state species via different decay channels depend on the excitation wavelength. Furthermore, transient absorption experiments illuminating the excited-state dynamics of PChla, a porphyrin-like compound and, as substrate of the NADPH/protochlorophyllide oxidoreductase (POR), a precursor of the chlorophyll biosynthesis are presented. In addition to pump-energy-dependent measurements performed with PChla dissolved in methanol, the excited-state dynamics of PChla was interrogated in different solvents that were chosen to mimic different environmental conditions. In addition to the femtosecond time-resolved absorption experiments the picosecond time-resolved fluorescence of the system was studied. The transient absorption and time-resolved fluorescence data allow suggesting a detailed model for the excited-state relaxation of PChla describing the excited-state processes in terms of The model suggested to account for the excited-state relaxation processes in protochlorophyllide a upon photoexcitation is presented a branching of the initially excited state population into a reactive and nonreactive path. Thus, the excited-state potential energy surface exhibits two distinct S, and S. minima separated from the Franck-Condon region along two most likely orthogonal reaction coordinates. Finally, the model derived is related to models suggested to account for the reduction of PChla to chlorophyllide a within the natural enzymatic environment of POR.