Pterins. Part 9. The structure of quinonoid dihydropterins [2-amino-7,8-dihydropteridin-4(6H)-ones]

Abstract

A comparative study of the rates of oxidation of 2-amino-cis-6,7-dimethyl-5,6,7,8-tetrahydropteridin-4(3H)-one (6), its 2-methylamino (7), 2-dimethylamino (8), 8-methyl (9), and 3-methyl (10) derivatives, and their 6,7-dideuterio-derivatives by molecular oxygen to the corresponding quinonoid 7,8-dihydro-6H-pterins, the rates of rearrangement of these to the respective 7,8-dihydro-3H-pterins, and the reduction of these quinonoid dihydropterins by NADH in Tris chloride buffer at pH 7.6 and 25° are reported. These data together with the lack of significant u.v. spectral shifts in the bands of the transient spectra of quinonoid 7,8-dihydro-6H-pterin, its 6-methyl and 2-dimethylamino-6,7-dimethyl derivatives in buffers between pH 6.8 and 11.6 strongly support the ‘ortho’ quinonoid dihydropterin structure (3) over the ‘exocyclic para’ quinonoid (2) and ‘endocyclic ortho’ quinonoid (4) structures. A mechanism whereby hydride from NADH is transferred to N(5) and a proton to N(3) of quinonoid dihydropterin substrates in the enzymic reduction by dihydropteridine reductase is proposed.