The reaction center of photosystem II studied with polarized fluorescence spectroscopy

Abstract

Low-temperature steady-state emission properties have been analyzed of Photosystem II reaction center (RC) complexes isolated from spinach CP47-RC complexes after a short Triton X-100 treatment and stabilization in n -dodecyl β, d -maltoside. Excitation spectra of the fluorescence anisotropy were detected at the maximum of the single fluorescence band at 683.5 nm and at the vibrational subband of the same emission at 742 nm. The Q 1 transitions of the red-most absorbing pigment(s) showed positive anisotropy with a value of about 0.22. The value is lower than that of the theoretical maximum (0.4) and is explained by a combination of (1) vibrational depolarization effects and (2) by assuming that the red-most absorbing pigments arise from the low-exciton component of P680, that the exciton coupling breaks upon excitation, and that the angle between the monomer Q 1 transitions of P680 is 48 ± 10°. The Q 1 transitions of pheophytin showed negative anisotropy. This result, combined with the results obtained with linear dichroism spectroscopy, suggests that the spatial organization of the Q 1 transitions of pheophytin matches the organization of the bacteriopheophytin residues in the bacterial reaction center. The spatial organization of the y -polarized transitions of pheophytin could be similar in both systems, although these transitions could also be tilted somewhat more towards the membrane plane in PS II. The data furthermore indicate that the accessory chlorophylls in PS II and in bacterial reaction centers have different average orientations, and suggest that at least some of the accessory chlorophylls in PS II have a pheophytin-like orientation.