Pigment assignment in the absorption spectrum of the photosystem II reaction center by site-selection fluorescence spectroscopy.

The steady state fluorescence properties of the photosystem II reaction center (D1-D2-cyt-b559 complex, PSII-RC) have been investigated by site-selection spectroscopy. The pattern of the vibronic bands in the emission spectra is used to identify the fluorescing species that have their absorption maxima on the red edge of the spectrum (at around 682 nm). At 10 K, even samples with a low content of red absorbing chlorophyll a (Chl) show pure Chl emission upon excitation at 685 nm, whereas at 77 K the fluorescence of the PSII-RCs is contributed to by Chl and pheophytin a (Pheo) in a ratio of roughly 8:2. These results allow an unequivocal distinction between two different spectral decompositions that were recently suggested for the absorption spectrum of the PSII-RC [Konermann, L., & Holzwarth, A. R. (1996) Biochemistry 35, 829]. Only one of these decompositions is compatible with the experimental data presented here according to which the absorption on the red edge of the spectrum is dominated by an accessory Chl.