Cyclic homodimer formation by singlet oxygen-mediated oxidation of carnosine

Although carnosine (β-Ala-L-His) is one of physiological protectants against <i>in vivo</i> damages caused by reactive oxygen species (ROS), its reactivity against singlet oxygen (¹O₂), an ROS, is still unclear at the molecular level. Theoretically, the reaction consists of two steps: i) oxygenation of the His side chain to form an electrophilic endoperoxide and ii) nucleophilic addition to the endoperoxide. In this study, the end product of ¹O₂-mediated carnosine oxidation was evaluated using 2D-NMR and other analytical methods both in the presence and absence of external nucleophiles. Interestingly, as the end product without external nucleophile, a cyclic homodimer was confirmed under our particular conditions. The reaction was also replicated in pork specimens.