pH-dependence of the structure and degradation of 8-alkyl-substituted pterins.

Structures of 8-(2-hydroxyethyl)pterins (8-11) investigated using 1H- and 13C-NMR and UV/vis spectroscopies showed a complex dependence on pH, solvent and 6,7-substitution pattern. In acid, only one cation was observed for all the pterins. In neutral aqueous solution, only one neutral form, the normal quinonoid tautomer, was observed for 7-unsubstituted pterins 9 & 11, but two neutral tautomeric forms, quinonoid and 7-exo-methylene, were observed for 7-CH3 substituted pterins 8 & 10 with 70% and 92%, respectively, of quinonoid. The neutral pterins in MeOH, however, showed a different distribution of structural forms: quinonoid and a five-membered intramolecular ether forms were observed for 7-unsubstituted pterins 9 & 11 as 60% and 25%, respectively, of quinonoid, while quinonoid and 7-exomethylene forms were observed for 7-CH3 substituted pterins 8 & 10 as 10% and 50%, respectively, of quinonoid. In base, for 7-unsubstituted pterins 9 & 11 only the intramolecular ether forms were observed, while for the 7-CH3 substituted pterins 8 & 10 two anion forms, the 7-exo-methylene and intramolecular ether, were observed in the ratio 2:1. Investigation of the distinctive proton-resonance pattern of the ethanomoiety of the intramolecular ether anion of 9 using 600 MHz NMR and spectrum simulation, indicated all four protons have different chemical environments. One conformation of the cyclic-ether ring satisfying the experimental data has been deduced, and the conformational energetics of the ring studied further using AM1 semiempirical quantum chemical calculations. Structural distributions of 8-methylpterins 12-15 were also studied in base only. These showed the 7-unsubstituted pterins 13 & 15 existed solely as the hydrated anion forms, while the 7-CH3 substituted pterins 12 & 14 existed predominantly as the 7-exo-methylene anions. Spectroscopic investigations of the degradation processes of 8-(2-hydroxyethyl)pterins and 8-methylpterins in base indicated a complex pattern of oxidation, ring opening and elimination reactions as a function of time. Using authentic samples, the 7-oxo compounds 16 & 17 and ethanolamine were identified, and evidence for ring-opened forms was obtained by comparison with the relevant 2,5-diamino-6-alkyl-aminopyrimidin-4(3H)-ones 1 & 2. Characteristically different degradation pathways for 7-CH3 and 7-unsubstituted compounds were established.