Tautomeric Nature of Quinonoid 6,7-Dimethyl-7,8-dihydro-6H-pterin in Aqueous Solution: A ¹⁵N NMR Study

The ¹⁵N chemical shifts of N-1, N-3, N-5, and the NH₂in the parent 6,7-dideuterio-5,8-dihydro-6,7-dimethylpterin, its cation, the two-electron oxidation product quinonoid 6,7-dideuterio-6,7-dimethylpterin, its cation, the rearrangement product 7-deuterio-8-hydro-6,7-dimethylpterin, and fully oxidized 6,7-dimethylpterin were assigned from the ¹⁵N NMR spectra of the pterins with the ¹⁵N isotope in N-5 (95%), N-1 (33%), N-3 (33%), and NH₂ (33%) and the pterins labeled singly in the NH₂ (95%). The ¹⁵N spectrum of the unstable quinonoid dihydropterin was obtained by using dilute Bicine buffer at low temperature combined with deuterium substitution at carbons 6 and 7 to slow rearrangement to the 7,8-dihydro derivative. The spectral shifts of the ¹⁵N resonances observed upon oxidation of the parent compound to the two-electron oxidation product established that the endocyclic quinonoid 6,7-dimethyl-7,8-dihydro-6H-pterin tautomer 5 is the predominant form in aqueous solution at near neutral pH. We propose that in future the structure of the quinonoid species of 7,8-dihydro-6H-pterins which are not further substituted in the pyrimidine ring should be drawn with the 2-NH₂ group and an endocyclic double bond as in 5.