Photophysical Properties of PS-2 Reaction Centers and a Discrepancy in Exciton Relaxation Times

Abstract

Exciton relaxation in the PS-2 reaction center of green plants is studied to explain a discrepancy of 2 orders of magnitude in certain relaxation times. Structural information from an earlier computer model and a recent low resolution structural study are combined with circular dichroism (CD) at 77 K, fluorescence line narrowing spectra at 1.6 K, and pump−probe spectra at 77 K to make predictions on the linear absorption spectrum at 77 K, fluorescence spectrum at 5 K, and absorption difference spectrum for modified pheophytin of the D_2 branch at 5 K. A width of the inhomogeneous distribution function of pigment energies of Δ_(inh) = 300 cm^(-1) and a mean vertical pigment transition energy corresponding to 669 nm are assumed for all pigments, based on the CD data, and a correlation radius of 5 A for the protein vibrations is deduced from the pump−probe data. The two peripheral chlorophylls resolved in a recent structure are calculated to give rise to optical dephasing times larger than 10 ps at wavelengths shorter than 670 nm, in agreement with dephasing times obtained in hole burning experiments. It is proposed, thereby, that the 2 orders of magnitude discrepancy between the fastest transfer times observed in hole burning and these in pump−probe experiments is due to the peripheral chlorophylls. Non-Markovian effects in the exciton-vibrational coupling are found to contribute significantly to optical line shape functions in the form of vibrational sidebands but to have minor influence on the relaxation dynamics of the excitons.