Kinetics and mechanism of the chemiluminescence associated with the free radical-mediated oxidation of amino acids

Abstract

When amino acids are incubated in the presence of a free radical source [2,2'-azobis(2-amidinopropane) dihydrocloride], only tyrosine (Tyr) and tryptophan (Trp) produce significant chemiluminescence. The relationship between the observed light intensity, the rate of the oxidation process and the substrate concentration is complex and can not be explained in terms of the formation of excited states in termination processes involving two peroxyl radicals (Russell's mechanism). The observed increase in light emission with the incubation time, for both Trp and Tyr, would indicate the participation of more than one reaction product as intermediates in the pathways leading to the production of excited molecules. However, the fact that after product accumulation a high proportion of the observed luminescence is quenched by Trolox addition, implies that the main chemiluminescent process must involve the interaction of product(s) and free radicals. From the effect of added Ebselen, it is proposed that hydroperoxides and peroxides, formed along the reaction path, are the intermediates whose accumulation leads to the observed increase in chemiluminescence with elapsed time. The observed time profiles and the proposed mechanism strongly resemble those associated with the oxidation of complex biological systems, suggesting that protein oxidation could be one of the main sources of chemiluminescence in biological oxidations.