FEMTOSECOND SPECTROSCOPIC STUDIES ON THE RED LIGHT-ABSORBING FORM OF OAT PHYTOCHROME AND 2,3-DIHYDROBILIVERDIN

The excited state behavior of the red light-absorbing form of phytochr ome (Pr) was studied on the femtosecond time scale, After excitation o f Pr with 75 fs laser pulses at 616 nm the kinetics of the transient a bsorption changes was recorded at selected wavelengths probing mainly the bleaching of the Pr ground-state absorption and the stimulated emi ssion, The kinetic data obtained indicate the population of an excited state with a 3 ps lifetime immediately after excitation, This state p recedes the formation of another excited state with a 32 ps lifetime, The decay of the latter state is followed by the appearance of a first product state that is assumed to represent lumi-R, In addition, 2,3-d ihydrobiliverdin, which is considered to be an adequate model of the P r chromophore, was included in the femtosecond studies, The absorption difference spectra recorded at various delay times show an immediate bleaching of the ground-state absorption, Simultaneously with bleachin g a broad transient absorption appears between 410 and 525 nm, The dat a analysis yields similar kinetic components as they were observed in the decay of Pr, It is suggested from this finding that within the fir st tens of picoseconds after excitation the excited-state properties o f Pr are mainly determined by the properties of the chromophore itself .