Orientation of the Chromophores in the Reaction Center of Rhodopseudomonas viridis. Comparison of Low-Temperature Linear Dichroism Spectra with a Model Derived from X-Ray Crystallography

Abstract

Linear dichroism (LD) and absorption (A) spectra of reaction centers from Rps. viridis included in the native chromatophores or reconstituted in planar aggregates have been recorded at 10 K. The samples were oriented in squeezed polyacrylamide gels and the primary donor P was in the reduced or (chemically) oxidized state. The LD spectra of reaction centers in these two states are in favour of a dimeric model of P in which excitonic coupling between the two non-parallel QY transitions leads to a main transition at 990 nm (parallel to the membrane plane) and another one of smaller oscillator strength at 850 nm (tilted at 55–60° out of the membrane plane). These assignments are in close agreement with the ones proposed in a previous LD study at room temperature (Paillotin, G., Vermeglio, A. and Breton, J. 1979, Biochim. Biophys. Acta 545, 249–264). The main QX excitonic component of P has a broad absorption peaking at 620 nm and corresponds to dipoles exhibiting the same orientation as those responsible for the 850 nm transition. On the basis of the present LD study and of CD data of chemically oxidized -minus- reduced reaction centers, we propose that the minor QX excitonic component of P is oriented close to the membrane plane and absorbs around 660 nm.