Ultrafast energy transfer in LHC-II trimers from the Chl a b light-harvesting antenna of Photosystem II

Time-resolved absorption difference profiles were obtained for LHC-II trimers, isolated from Photosystem II in spinach with n-dodecyl β-d-maltoside, using one-color and two-color pump-probe techniques. The one-color isotropic signals are predominantly excited state absorption at 640 nm, and a combination of photobleaching and stimulated emission at wavelengths ≥ 665 nm. At intermediate wavelengths, dynamic red-shifting due to downhill energy transfer among the chlorophyll (Chl) spectral forms produces a bipolar signal, in which prompt photo-bleaching/stimulated emission is superseded at later times by excited state absorption. Triexponential analyses of these profiles yield the lifetime components 2-6 ps (associated with the spectral shifting), 14-36 ps (possibly due to energy transfer between LHC-II monomers), and several hundred picoseconds. The one-color anisotropy decays are resolvable at 665-675 nm, with lifetimes of 4.3 to 6.3 ps. They are unresolvably fast (i.e., exhibit subpicosecond lifetimes) at 640-650 nm. The two-color isotropic absorption difference signals show clear spectral evolution arising from equilibration among the LHC-II spectral components for excitation wavelengths shorter than 670 nm. However, most of this spectral evolution occurs within less than 2.5 ps. No resolvable anisotropy decay was observed in the two-color experiments. Taken together, the one-color and two-color experiments indicate that both picosecond and subpicosecond energy transfer steps occur in this antenna. The faster processes appear to dominate the spectral equilibration; slower processes occur in isoenergetic energy transfers among the longer-wavelength Chl a spectral forms that absorb between 665 and 675 nm. The values of the long-time anisotropic r(x), measured in the one-color and two-color experiments, are qualitatively consistent with static linear dichroism spectra of these preparations.