REVERSIBLE PHOTOCHEMISTRY IN A CHLOROPHYLL MODEL SYSTEM

A reversible, endothermic photochemical redox reaction, sensitized by chlorophyll a and related compounds, has been demonstrated in a hetero geneous particulate system. The oxidant, 5,5/-dithiobis(2-nitrobenzoat e) (DTNB), is photoreduced to the thiolate which remains primarily in an aqueous phase. Reductants are trisubstituted hydrazines, capable of oxidation to tetrazanes in the hydrocarbon particle phase. In the cou rse of three days in the dark, thiolate and tetrazane react to regener ate DTNB in yields approaching 100%. A novelty of the present system i s that photoreaction often takes place in discrete rate regimes, which are related to the presence of spectrally identifiable associations o f chlorophyll pigments, Mg-containing and free bases. Among the associ ations that promote photochemical activity are those of chlorophyll an d pheophytin with themselves and with each other. Perhaps more active are associations of a Mg rhodochlorin allomerization product of chloro phyll with its free base. Contributing to the associations is the stab ilizing presence of amphiphiles that both ligate the Mg of chlorophyll strongly and hydrogen-bond to carbonyls: 2-tridecylimidazole, 2-tride cylimidazoline, and (2-aminoethyl)myristamide. Results of this work de monstrate the possibility of generating reaction center models in an a rtificial heterogeneous system, and of conducting reversible photochem ical reactions with them.