Energy transfer and trapping in isolated photosystem II reaction centers of green plants at low temperature. A study by spectral hole burning

Spectral hole burning has been performed on the Q_y-region of the isolated reaction center of photosystem II, the D1-D2-cytochrome b559 complex (PSII RC), between 665 and 688 nm, at liquid He temperatures. The "effective" homogeneous line width Γ′_hom at 682 nm, in the red wing of the Q_y-band, follows a T^1.3±0.1 power law between 1.2 and 4.2 K characteristic of glasses and extrapolates to Γ′₀ = (2πT₁)^-1 for T → 0 with T₁ = (4 ± 1) ns, the fluorescence lifetime of the pigments. At these low temperatures, the red-absorbing "trap" pigments are unable to transfer energy to other pigments. The spectral distribution of the traps has been determined from hole depth vs λ_exc experiments. Their linear electron-phonon coupling strength was found to be rather weak, S = 0.73 ± 0.05. For λ_exc < 678 nm, "downhill" energy transfer takes place. Spectral distributions of pigments characterized by decay times of 200 and 12 ps have further been identified in this spectral region. The data have been used to reconstruct the fluorescence excitation and absorption spectra.