Electron−Phonon Coupling in Solubilized LHC II Complexes of Green Plants Investigated by Line-Narrowing and Temperature-Dependent Fluorescence Spectroscopy

Abstract

Line-narrowed and temperature-dependent fluorescence spectra are reported for the solubilized trimeric light-harvesting complex of Photosystem II (LHC II). Special attention has been paid to eliminate effects owing to reabsorption and to ensure that the line-narrowed fluorescence spectra are virtually unaffected by hole burning or scattering artifacts. Analysis of line-narrowed fluorescence spectra at 4.2 K indicates that the lowest Qy-state of LHC II is characterized by weak electron−phonon coupling with a Huang−Rhys factor of ∼ 0.9 and a broad and strongly asymmetric one-phonon profile with a peak frequency ωm of 15 cm-1 and a width of Γ = 105 cm-1. The 4.2 K fluorescence data are further consistent with the assignment of the lowest Qy-state at ∼ 680.0 nm and an inhomogeneous width of ∼80 cm-1 gathered from a recent hole-burning study (Pieper et al. J. Phys. Chem. A 1999, 103, 2412). The temperature dependence of the fluorescence spectra of LHC II is simulated using the low-energy Qy-level structure reported in the latter study as well as the parameters of electron−phonon coupling determined in the present study. Up to a temperature of 120 K, the calculations reveal that this model satisfactorily describes the basic features of the fluorescence spectra such as thermal broadening and, especially, the blue-shift of the fluorescence peak with increasing temperature. An unexpected red-shift of the fluorescence peak above 150 K is attributed to conformational changes of the protein environment. The shape of the temperature-dependent fluorescence spectra indicates that the low-energy Qy-states are populated according to a Boltzmann distribution representing the thermal equilibrium of excitation energy.