The kinetics of excitation energy transfer processes within the light-harvesting Chl a/b protein complex of Photosystem II (LHC II) from spinach was investigated using femtosecond transient absorption difference spectroscopy at room temperature and 12 K. Upon excitation of Chl b with 640 nm pulses of 120 fs duration, time-resolved absorption changes were measured at different wavelengths within the Qy(0-0) transition band of Chl a. At room temperature the transient spectra showed a single negative band which is due to photobleaching and/or stimulated emission (PB/SE). The peak position of this band was found to be time independent (>-400 fs) at about 680 nm. In contrast, at 12 K the maximum of the PB/SE band shifted from 676 nm at 400 fs to about 680 nm at later times (≥ 100 ps). The absorption difference spectra at 12 K showed a bipolarity at times ≥ 7.7 ps, where excited state absorption and PB/SE predominate at shorter and longer wavelengths, respectively. The dynamic spectral evolution processes at 12 K could be rationalized in terms of excitation energy transfer between excitonically coupled antenna pigments in isolated LHC II complexes. Based on a global analysis of the flash induced absorption changes at 12 K several components have been obtained with lifetimes in the subpicosecond and picosecond time domain. The results of these fits are discussed with respect to the observed time dependent spectral changes.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry