Light-induced conformational change at low temperature in the coordination of heme c-556 in the reaction center of Rhodopseudomonas viridis

Isolated reaction centers of Rhodopseudomonas viridis with the two high-potential hemes reduced were illuminated at 5 K. Difference spectra show a bleaching of the heme c-556 alpha bands and a red shift of the Soret band. These effects are reversed by warming to around 80 K. They are not induced by near infra-red light absorbed by the chlorine pigments of the reaction centers and they are not associated with electron transfer from P to Q_A. It is concluded that, following direct excitation, heme c-556 becomes five-coordinated. We find no evidence of a significant photooxidation of heme c-559 under the same conditions.