Time-resolved spectroscopy at 10 K of the Photosystem II reaction center; deconvolution of the red absorption band

Abstract

The red absorption band of isolated Photosystem II reaction centers at 10 K and its flash-induced changes due to formation of the primary radical pair, P +I −, and subsequent recombination to the triplet state, P T, were analysed. It is concluded that the primary electron donor P consists of two chlorophyll molecules, which show exciton interaction and are responsible both for the main long wavelength absorption band at 679.6 nm and a shoulder at 683.6 nm (these wavelengths vary somewhat between preparations). The angle between their Q y transitions is about 60° and the exciton splitting about 85 cm −1. Both bands are bleached in the states P + and P T and replaced by a single band at 678 nm, attributed to one of the two chlorophylls, while the oxidized or triplet state is localized on the other, contributing little absorbance in the Q y region. The pheophytin which acts as the intermediary electron acceptor I has a 6.4 nm wide Q y band at 676.5 nm with about 2 3 of the amplitude of a chlorophyll Q y band. For the remaining pigments gaussian curve fitting of the absorption spectrum led to the following tentative assignments. The Q y band of the other pheophytin is spectrally indistinguishable from that of I. All accessory chlorophylls absorb in the 670 nm region; a good fit was obtained with two 4.7 nm wide bands peaking at 672.6 and 669.4 nm, respectively, and at least one Q y band at slightly shorter wavelength. However, neither an artifactual origin of the latter band nor the presence of two such bands can be excluded. The pigments of the PS II reaction center show little exciton interaction.