SPECTRAL EVIDENCE FOR PHOTO-INDUCED ISOMERIZATION OF CAROTENOIDS IN BACTERIAL PHOTOREACTION CENTER

Abstract The aim of this work was to determine whether spirilloxanthin and sphaeroidene bound to the same site of the photoreaction center isolated from Rhodospirillum rubrum can display cis–trans isomerization under conditions that lead to formation of triplet state P^R of the primary electron donor. To this end, we monitored changes in the absorption spectrum of these bound carotenoids as induced by red light at low redox potentials. This experiment was performed both with the intact photoreaction center isolated from strain Sl and with the photoreaction center isolated from carotenoidless strain G9 reconstituted with either spirilloxanthin or sphaeroidene. In both preparations, spirilloxanthin exhibited light-induced absorption changes that can be interpreted as a cis–trans isomerization. Under our experimental conditions, the absorption changes attained their full extent in about 1 min and were not or were only partially reversed when the light was switched off. Under our experimental conditions, the extent of these changes indicate that about 15% of the bound spirilloxanthin undergoes isomerization. Sphaeroidene artificially attached to the G9 photoreaction center also undergoes light-induced absorbance changes, but these cannot easily be interpreted as a cis–trans isomerization.