MOLECULAR OXYGEN (O_2): REACTIVITY AND LUMINESCENCE

摘要

Luminescence occurs when an electron relaxes from an excited state by photon emission. Electronic excitations require relatively high energies. Emission of a blue photon requires about 60 kcalmol~(-1) of energy. This quantity of energy is significantly greater than the energies liberated in most biochemical reactions. For example, hydrolysis of ATP to ADP yields about 7 kcalmol. Essentially all chemiluminescence (CL) and bioluminescence (BL) phenomena result from O_2-dependent reactions, i.e., dioxygenations and mixed function oxygenations. These reactions are combustions and O_2 is incorporated into the final product. The great electronegativity of oxygen promises reaction exergonicities sufficient for electronic excitation. Compare the reaction of O_2 and chlorine (C1_2) with an organic substrate such as ethylene. Reaction with chlorine (Cl_2) yields about -51 kcal'mol~(-1) of free energy while reaction with O_2 yields about -87 kcalmol~(-1). C1_2 is a potent reactive agent that is biologically lethal in very low concentration, yet we live in an environment that is about 21 % O_2- Chlorination reactions are highly exergonic, yet oxygenation reactions are significantly more exergonic. Chlorine reactivity is spontaneous, but direct oxygenation is not. What protects against spontaneous combustion? Although O_2 is required for CL and BL, the pathway to oxygenation is indirect. The non-radical pathway to luminol luminescence requires dioxygenation, but reaction with O_2 is not direct. Luminol is first dehydrogenated to its diazaquinone by removal of two reducing equivalents, i.e., two electrons (2e~-) and two protons (2H~+). Dehydrogenation is a type of oxidation that does not require the direct participation of O_2. Reducing O_2 by two equivalents generates hydrogen peroxide (H_2O_2). The luminol diazaquinone reacts with H_2O_2 to yield a dioxygenated product that rearranges to electronically excited aminophthalate, and this excited state relaxes by photon (hv) emission. The net reaction is a simply dioxygenation, luminol + O_2 → aminophthalate + N_2 + hv, but luminol reaction with O_2 is indirect. Why are exchanges of reducing equivalents required to ultimately achieve dioxygenation? What limits the direct reactivity of O_2? The answers to these questions can be found in boson-fermion symmetry considerations. The symmetry-based approach described herein is straightforward and consistent with the law of parsimony. Where necessary, fundamental quantum mechanical principles are developed to provide appropriate background material.