Chemiluminescence of lucigenin by electrogenerated superoxide ions in aqueous solutions

The chemiluminescence reaction of lucigenin (Luc(2+)c2NO(3) (-), N,N'-dimethyl-9,9'-biacridinium dinitrate) at gold electrodes in dioxygen-saturated alkaline aqueous solutions (pH 10) was investigated in detail by the use of electrochemical emission spectroscopy. We noted that both O(2) and Luc(2+) are reduced on a gold electrode in aqueous solution of pH 10 in almost the same potential region. From this fact, we expected chemiluminescence based on a radical-radical coupling reaction of superoxide ion (O(2).(-)) and one-electron reduced form of Luc(2+) (Luc.(+), a radical cation). Chemiluminescence was actually observed in the potential range where O(2) and Luc(2+) were simultaneously reduced at the electrodes. The effects were examined upon addition of enzymes, i.e. superoxide dismutase (SOD) and catalase, into the solution and the substitution of heavy water (D(2)O) for light water (H(2)O) as a solvent on the chemiluminescence. In the presence of native and active SOD, chemiluminescence was completely absent. On the other hand, chemiluminescence was observed, unchanged in the presence of either denatured and inert SOD or catalase. In addition, the amount of chemiluminescence in D(2)O solution was about three times greater than that in H(2)O solution. These results, together with cyclic voltammetric results, suggest that O(2).(-) participates directly in the chemiluminescence but H(2)O(2) does not, and the chemiluminescence results from the coupling reaction between O(2).(-) and Luc(.+) under the present experimental conditions. These chemically unstable species, O(2).(-) and Luc.(+), are produced during the simultaneous electroreduction of O(2) and Luc(2+). The coupling reaction between those radical species would lead to the formation of a dioxetane-type intermediate and, finally, to chemiluminescence. The chemiluminescence reaction mechanism is discussed.