Comparison of the Excited-State Dynamics of Five- and Six-Chlorophyll Photosystem II Reaction Center Complexes

Abstract

Spectral hole-burning experiments have been performed at liquid He temperature on the Qy-band of isolated reaction center complexes of photosystem 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 was measured at 682 nm as a function of temperature between 1.2 and 4.2 K. It follows a T1.3±0.1 power law in both complexes and extrapolates to the fluorescence lifetime-limited value, τfl = (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 λexc 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 l...