ABSTRACT
The generation of electronically excited molecules in the singlet state as a result of chemical or biochemical reaction has been known for a very long time, as attested to by chemiluminescence and bioluminescence. Since the late fifties, we have been interested in the possibility of biochemical generation of electronically excited triplet species. Molecules in the triplet state have a much longer intrinsic lifetime than those in the corresponding excited singlet state, whereby the former are much more suitable to participate in bimolecular processes, such as reactivity and energy transfer. This is expected to be especially important in biochemical/biological systems, where possible reactants or enery acceptors may exist in low concentration. Therefore, triplet species might play useful and/or deleterious roles in these systems. Progress had to wait for the demonstration that triplet species can also be generated chemically. This came with the advent of dioxetane/dioxetanone chemistry in the late sixties/early seventies. The cleavage of these fourmembered cyclic peroxides can yield a carbonyl group in the triplet state. We were able to demonstrate in the middle seventies that several peroxidase-catalyzed reactions that consume oxygen generate products of the type expected from a dioxetane intermediate and that a triplet carbonyl compound is generated in high yields. These species were shown to participate in uniand bimolecular processes, thus paving the way for a "photobiochemistry without light", the biochemical conterpart of "photochemistry without light" developed by EH White and associates at Johns Hopkins University. The generation of triplet carbonyl compounds and the consequent promotion of photochemical-like processes has also been demonstrated in organized structures, e.g., organelles and even in intact cells. As a whole, the work opens the possibility of understanding the natural occurrence of certain photochemical-like processes - both useful and deleterious - in the absence of light. Several applications can be envisaged for biochemically generated triplet species.