Steady-state and time-resolved polarized light spectroscopy of the green plant light-harvesting complex II

The major chlorophyll a/b light-harvesting complex from spinach thylakoid membranes was analyzed by steady-state polarized light spectroscopy at 4 K and by one-color and two-color pump-probe spectroscopy at room temperature. Steady-state absorption, linear dichroism and circular dichroism spectra indicate that the Chl Q_y(0-0) absorption region is characterized by at least six transitions with significant differences in absorption, orientation and rotational strength. Steady-state low-temperature fluorescence spectra suggest that the fluorescence arises for a large part from several energetically similar species that form a circularly degenerate oscillator in the plane constituted by the two long axes of the particle. The possible presence of special red-absorbing pigments at low temperature is discussed. The time-resolved data suggest that the kinetics of chlorophyll b → a excitation energy transfer, as well as those of downhill excitation transfer among chlorophyll a spectral forms, are heterogeneous with both sub-picosecond and picosecond lifetime components.