Comparison of the excited-state dynamics of five- and six- chlorophyll photosystem II

Spectral hole-burning experiments have been performed at liquid He temperature on the Q_y-band of isolated reaction center complexes of pholosystem II (PS II RC) containing five (RC-5) and six (RC-6) chlorophyll a (Chl a) molecules. The aim was to investigate the nature of the redmost shoulder in the absorption spectrum of RC-5 and to identify distributions of "trap" pigments. The "effective" homogeneous line width Γ′_hom was measured at 682 nm as a function of temperature between 1.2 and 4.2 K. It follows a T^1.3±0.1 power law in both complexes and extrapolates to the fluorescence lifetime-limiled value, τ_n = (4 ± 1) ns, for T → 0. These results indicate that the redmost absorbing pigments act as "traps" for the excitation energy. The spectral distribution of these traps was reconstructed from the hole depth measured as a function of excitation wavelength λ_exc and compared to that of RC-6 previously obtained by us (Groot, M. L.; et al. J. Phys. Chem. 1996, 100, 11488). The maximum of the RC-5 trap distribution lies at (682.9 ± 0.2) nm. We discovered a second distribution of fluorescing pigments centered at (673.4 ± 0.5) nm in both RC-5 and RC-6. The dependence of Γ′_hom on the delay time t_δ between burning and probing, for the red- and blue-absorbing pigments, is constant for t_δ ≤ 1 s and increases linearly with log t_δ for longer delay times. The molecules absorbing at ∼674 nm, which can be chemically removed, are not free Chl a but are bound to a protein with the same mass as that of the RC complex.