Electric field-induced polarization of photosynthetic membranes and reaction centers of Rhodopseudomonas sphaeroides, strain 1760-1.

Electric fields of 10(5) V/cm cause the polarization of chromatophore and reaction center films prepared from photosynthesizing purple bacteria. Photosynthetic pigments, and in particular carotenoids, the absorption spectra of which are changed in response to electric fields (electrochromism), may serve as intrinsic indicators of the development of a polarized state. Polarization occurs due to changes in the orientation and the spatial position of different charged groups and particles. The field-induced polarized state can be fixed by exposure to low temperature (-120 degrees C). On being heated, the system relaxes to the initial state and this can be seen as a thermodepolarization current in an electric circuit. The effects of hydration, chemical modification or heat treatment on thermodepolarization current indicate the involvement of macromolecular components in the formation of a polarized state. In light-adapted samples the polarization effect is markedly greater, indicating that conformational changes occur during the primary photoact. It is assumed that the polarization of the reaction centers during electron transfer might be involved in the stabilization of separated charges and in the storage of energy.