Deactivation of excited states of kynurenine covalently linked to nitroxides

Due to ability of stable nitroxides to interact with free radicals, they are used as antioxidants for therapeutic and research goals in biology and medicine. A modern trend in medical chemistry is the design of multifunctional molecules such as UV absorbers covalently bound to nitroxides, which provides both UV protection and antioxidant properties combined in the same molecule. In the present work, we report the synthesis of conjugates of a natural UV filter kynurenine (KN) with nitroxides (KN-RNO(•) conjugates) and the study of their photochemical properties in aqueous and methanol solutions. Due to the spin-exchange interaction between KN and nitroxide moieties, the triplet lifetimes in conjugates are much shorter than in KN molecule, but the triplet quantum yields are significantly higher. The reaction of intramolecular electron transfer between photoexcited KN and nitroxide moieties is the main factor determining the quantum yield of KN-RNO(•) conjugates photodecomposition. Consequently, KN-RNO(•) conjugates in aqueous solution are photochemically less stable than the parent KN molecule. Nevertheless, the photostability of KN-RNO(•) conjugates is much higher than that of cinnamates which are widely used as UV absorbers in modern sunscreen formulations. Thus, the combination of the endogenous chromophore KN with nitroxides is very promising for medical applications.