Characterization of the low-temperature triplet state of chlorophyll in photosystem II core complexes: Application of phosphorescence measurements and Fourier transform infrared spectroscopy

Abstract

Phosphorescence measurements at 77 K and light-induced FTIR difference spectroscopy at 95 K were applied to study of the triplet state of chlorophyll a (3Chl) in photosystem II (PSII) core complexes isolated from spinach. Using both methods, 3Chl was observed in the core preparations with doubly reduced primary quinone acceptor QA. The spectral parameters of Chl phosphorescence resemble those in the isolated PSII reaction centers (RCs). The main spectral maximum and the lifetime of the phosphorescence corresponded to 955±1nm and of 1.65±0.05ms respectively; in the excitation spectrum, the absorption maxima of all core complex pigments (Chl, pheophytin a (Pheo), and β-carotene) were observed. The differential signal at 1667(−)/1628(+)cm−1 reflecting a downshift of the stretching frequency of the 131-keto C=O group of Chl was found to dominate in the triplet-minus-singlet FTIR difference spectrum of core complexes. Based on FTIR results and literature data, it is proposed that 3Chl is mostly localized on the accessory chlorophyll that is in triplet equilibrium with P680. Analysis of the data suggests that the Chl triplet state responsible for the phosphorescence and the FTIR difference spectrum is mainly generated due to charge recombination in the reaction center radical pair P680+PheoD1−, and the energy and temporal parameters of this triplet state as well as the molecular environment and interactions of the triplet-bearing Chl molecule are similar in the PSII core complexes and isolated PSII RCs.