Oxidation of tetrahydropterins by azide radical and the spectra of trihydropterin radicals

Spectrophotometric titration of tetrahydropterins with incremental amounts of N3 showed that 6,7-dimethyl-7,8-dihydropterin was an intermediate in the oxidation of 6,7-dimethyltetrahydropterin at the two electron-equivalent oxidation point. The eventual end product at four electron equivalents was 6,7-dimethylpterin. In the case of unsubstituted tetrahydropterin, the dihydro form was clearly not an exclusive product at the two electron equivalent point, and this was attributed to the disproportionation of different forms of dihydropterin to tetrahydropterin and pterin. The azide radical oxidized tetrahydropterins to trihydropterin radicals, PnH3, with overall second-order rate constants at pH 7 of 4.1, 3.8, and 2.9 X 109 M-1 s-1 for tetrahydropterin, 6,7-dimethyltetrahydropterin, and 6-carboxylated tetrahydropterin, respectively. At pH 10 the rate constants are slightly larger due to the presence of the deprotonated enolate forms of tetrahydropterins. Spectra vary somewhat with substitution in the pterin molecule, but all species have a strong peak (ε ∼ 9000 M-1 cm-1) near 320 nm and lower absorption above this, with a tail extending to 580 nm