Spectroscopy and photochemistry of spinach photosystem I entrapped and stabilized in a hybrid organosilicate glass

Photosystem I (PSI) is part of the photosynthetic apparatus of higher plants and algae. It is one of the naturally occurring molecular photovoltaic structures whose primary function is to convert solar energy into chemical energy. The immobilization and stabilization of this large transmembrane protein complex in a solvent-limited and optically clear organosilicate glass are reported. This was achieved by a modification of the sol-gel process whereby in the first instance the methanol formed during the hydrolysis of the alkoxide precursor was removed before adding the enzyme preparation. In the second instance, glycerol was used as a nonsurfactant templating agent. It was present at a final concentration of 50% (v/v) after the solvent removal process was completed. In addition to stabilizing PSI, it may also play an important structural role, as its omission results in opaque and brittle glasses. The PSI complexes retained their activity during the immobilization procedure and after 96% (w/w) of the solvent in the sol-gel matrix was removed. The ability of the P700 reaction centers in entrapped PSI to undergo photochemical oxidation indicated that the intramolecular electron-transfer apparatus of the reaction centers was functional. In addition, it was demonstrated that their intermolecular electron-transfer function was also intact, as evidenced by PSI-mediated photodependent hydrogen production. The results demonstrate the validity of this novel approach for investigation of complex biological electron-transfer processes under nonnative conditions.