Orientation of reaction center and antenna chromophores in the photosynthetic membrane of rhodopseudomonas viridis

The geometry of the dimeric primary donor seems to be affected by the presence of a negative charge in the reaction center.

Abstract

Whole cells of Rhodopseudomonas viridis were oriented in a magnetic field. The degree of orientation of the cells was determined by using a photoselection technique. In order to deduce the orientation of the antennae and chromophores of the reaction centers with respect to the membrane plane, we performed linear dichroism measurements of absolute spectra and light induced difference spectra linked to states P+I and PI− on oriented cells. These measurements lead to the following conclusions: The antennae bacteriochlorophyll molecular plane is nearly perpendicular to the membrane. The Qy and Qx transitions moments of these molecules make respectively angles of 20 and 70°ith the membrane plane. The antenna carotenoid molecules make an angle of 45°ith the membrane. The primary electron donor possesses two transition moments centered respectively at 970 and 850 nm. The 970 nm transition moment is parallel to the membrane plane, the 850 nm transition is tilted out of the plane. Upon photooxidation of this primary electron donor, a monomer-like absorption band appears at 805 nm. Its transition makes an angle smaller than 25° with the membrane. The photooxidation of the dimer also induces an absorption band shift for the two other bacteriochlorophyll molecules of the reaction center. The absorption band shifts of the two bacteriochlorophyll molecules occur in opposite direction. One bacteriopheophytin molecule is photoreduced in state PI−. This photoreduction induces an absorption band shift for only one bacteriochlorophyll molecule. Finally, the geometry of the dimeric primary donor seems to be affected by the presence of a negative charge in the reaction center.