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 hig h-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 Figm ents of the reaction centers and they are not associated with electron transfer from P to Q(A). It is concluded that, following direct excit ation, heme c-556 becomes five-coordinated. We find no evidence of a s ignificant photooxidation of heme c-559 under the same conditions.