Effect of the spacer length and nitroxide sterical shielding upon photostability of spin-labeled kynurenines

The synthesis of the natural molecular UV filters − kynurenines (KN) − covalently bound to nitroxides has recently been proposed as an alternative for traditional combination of antioxidants and UV-filters in sunscreen formulations. Due to the spin-exchange interaction between KN and nitroxide moieties, the triplet lifetimes in such 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. Searching for a method to suppress this side process we synthesized analogues of previously prepared conjugates with the enlarged spacer between the nitroxide and kynurenine parts and with the sterically hindered access to the nitroxide group. The new spin-labeled kynurenines showed at least three-fold higher photostability compared to the previously synthesized compounds of this type.