PHOTOCHEMISTRY OF A PHOTOSYNTHETIC REACTION-CENTER IMMOBILIZED IN LIPIDIC CUBIC PHASES

Photosynthetic reaction centers, isolated and purified from the facult ative phototrophic bacterium Chloroflexus aurantiacus, were immobilize d in optically transparent lipidic cubic phases composed of 42% (w/w) 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine and 58% (w/w) water. The immobilized photosynthetic protein retains its native properties, as indicated by visible and circular dichroic spectra. The ground sta te visible spectrum of the immobilized reaction centers is very simila r to the corresponding spectrum in aqueous solution, indicating that t he protein pigments are not extracted into the lipidic regions of the cubic phase. The secondary structure of the protein is maintained in t he immobilized state, as determined by far-UV circular dichroism spect roscopy in the 200- to 250-nm range. Moreover, immobilized reaction ce nters retain their photochemical activity: a reversible photo-oxidatio n of the primary electron donor (P) is seen upon continuous illuminati on. Furthermore, the entrappment of reaction centers does not affect t he kinetics of charge recombination between the photo-oxidized primary donor(P+) and the photoreduced primary quinone acceptor, generated by a short flash of light. Reaction centers devoided of the secondary qu inone acceptor can be easily reconstituted in cubic phases by means of their coimmobilization with 1,4-naphtoquinone. Indeed, the kinetics f or charge recombination in reconstituted reaction centers is dramatica lly slower than the corresponding kinetics in the unreconstituted prot ein. Interestingly, immobilized reaction centers are significantly sta bilized as compared with reaction centers in aqueous solution: the int egrity of the protein in the cubic phase is maintained for at least 5 months, whereas in water solution 50% of the activity is lost within 2 months. (C) 1995 John Wiley and Sons, Inc.