[Tolerance of 5,10-methenyltetrahydrofolate to ultraviolet radiation]

Aeration of aqueous solutions of 5,10-methenyltetrahydrofolic acid (MTHF) during exposure to ultraviolet irradiation (lambda 300-390 nm, 240 W/m2, 30 min) slowed down photolysis in comparison with deaerated solutions. The rate of photolysis in the presence of oxygen depended on buffer composition. It did not exceed 6% of the starting amount of MTHF. Photolysis of MTHF included opening of the imidazoline ring, dehydrogenation of the tetrahydropterin portion, and elimination of the p-aminobenzoylglutamate moiety. 6,7-Dimethyltetrahydropterin was used as a model compound to show that protonation of the reduced pterin heterocycle increased its tolerance to oxidation, and UV irradiation did not accelerate this process. The stabilizing effect of protonation of the pterin portion and the presence of the positively charged imidazoline moiety are assumed to hamper MTHF oxidation and photolysis. It is assumed that these factors favored the choice of MTHF molecules as photosensors in light-sensitive proteins in the course of evolution.