Mechanism of Oxidation of Tetrahydropterins

The oxidation of tetrahydropterins and 7,8-dihydropterins by ferric iron has been examined by both polarographic and spectral techniques. Quinonoid dihydropterin, an intermediate in the oxidation of the tetrahydropterins, is formed rapidly, as is ferrous iron. The quinonoid dihydropterin then rearranges to 7,8-dihydropterin. The rate of oxidation of 7,8-dihydropterins is much slower than that of tetrahydropterins. The reduction potential of the reversible quinonoid dihydropterin – tetrahydropterin couple has been measured and shown to be consistent with the known chemistry of these compounds. Kinetic measurements and structural studies have been used to formulate a mechanism for the oxidation of tetrahydropterins to quinonoid dihydropterins. This mechanism involves abstraction of an electron from position 5, followed by rapid loss of two protons and the other electron.