The Interconversion of the 5,6,7,8-Tetrahydro-, 7,8-Dihydro-, and Radical Forms of 6,6,7,7-Tetramethyldihydropterin. A Model for the Biopterin Center of Aromatic Amino Acid Mixed Function Oxidases

The 6,7-blocked pterins 6,6,7,7-tetramethyl-5,6,7,8-tetrahydropterin (1_red). 6,6,7,7-tetramethyl-7,8-dihydropterin (1_ox), and 5,6,6,7,7-pentamethyl-5,6,7,8-tetrahydropterin (2_red) have been synthesized. 1_ox represents the quinonoid product obtained upon 2e⁻ oxidation (electrochemical, Br₂) of 1_red. Two-electron oxidation of 2_red yields 1_ox, rather than 2_ox due to the rapid demethylation of the latter (the pseudo-first-order rate constant determined from elelectrochemical measurements at pH 5.9 is 1.4 × 10⁻² s⁻¹). Solvolysis of 1_ox yields the ring contracted imidazolone 3_ox (Scheme III). The acid-base and spectral properties of 1_red (Scheme I), 1_ox (Scheme II), and 2_red (Scheme IV) are described. The comproportionation equilibrium constant for the formation of the pterin radical 1_sem has been determined by spectral and EPR measurements. The comproportionation constant for formation of 1_sem from 1_red and 1_ox is much smaller (~10⁷-fold at pH 1.0 and ~10²-fold at pH 7.0) than the like constants for flavin radical formation. The pH dependence of the redox potentials associated with 2e⁻ interconversion of 1_red and 1_ox has been determined and the like dependence of the le⁻ redox potentials for the interconversion of 1_red, 1_sem, and 1_ox have been approximated. At scan speeds of 50 mV/s and at pH 5.9, the oxidation of 2_red to 2_ox can be shown to represent two 1e⁻ transfer steps in both anodic and cathodic sweeps. Nernst-Clark plots of potential vs. pH for 1 and 2 are provided (Figure 6). The mechanism of reaction of 1_red and 2_red with O₂ has been explored by comparing ΔG^‡ (from initial rates) to ΔG° values (electrochemical calculations) for 1e⁻ transfer from the tetrahydropterins to O₂. Since the difference ΔG^‡ – ΔG° is very small (13 kJ M⁻¹) and protons are not involved in the critical transition state, it is concluded that the transition state closely resembles the radical pairs {l_semO₂⁻} and {2_semO₂^™·} which must couple to provide 4a-hydroperoxypterins. The hydroperoxide moiety is consumed in the overall autocatalytic oxidation of 1_red and 2_red.