Stability of 7,8-dihydropterins in air-equilibrated aqueous solutions

6-Substituted 7,8-dihydropterins (= 2-amino-7,8-dihydropteridin-4 (1H)-ones) are heterocyclic compounds that occur in a wide range of living systems and participate in relevant biological functions. In air-equilibrated aqueous solutions, these compounds react with dissolved O-2 (autooxidation). The rates of these reactions as well as the products formed strongly depend on the chemical structure of the substituents. 7,8-Dihydro-6-methylpterin and 7,8-dihydro-6,7-dimethylpterin that bear electron-donor groups as substituents are the most reactive derivatives and undergo oxidation of the pterin moiety to yield the corresponding oxidized derivatives (6-methylpterin and 6,7-dimethylpterin, resp.). The oxidations of 7,8-dihydrobiopterin, 7,8-dihydroneopterin, and 7,8-dihydrofolic acid are slower, and they yield 7,8-dihydroxanthopterin as the main product. 7,8-Dihydroxanthopterin, 6-formyl-7,8-dihydropterin, and sepiapterin are rather stable, and their consumption in air-equilibrated solutions is negligible for several days. The pseudo-first-order rate constants of the reactions between these compounds and 02 at 25 degrees and 40 degrees are reported. The biological implications of the results obtained are also discussed.