Abstract
Chemically reduced reaction centers of the photosynthetic bacteria Rhodopseudomonas sphaeroides wild type give rise to carotenoid triplet-state electron spin resonance signals upon illumination at 85 K. The relative signal intensities are found to depend on both the wavelength and the polarization of the excitation. These data are used to calculate the orientations of the primary donor 870 nm transition moment and the carotenoid 514 nm transition moment with respect to each other and with respect to the carotenoid principal magnetic axis system. The angle between the primary donor and the carotenoid transition moments is found to be 75 ± 8° and taken together with data published previously (Frank, H.A., Bolt, J., Friesner, R. and Sauer, K. (1979) Biochim. Biophys. Acta 547, 502–511 and Frank, H.A., Machnicki, J.P. and Toppo, P. (1984) Photochem. Photobiol. 39, 429–432) is used to calculate the orientation between primary donor and the carotenoid principal magnetic-axis systems.
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