Reactions of CO2˙– radicals with pterin and pterin-6-carboxylate ions

Abstract

Pulse radiolysis experiments showed that the reaction of pterin-6-carboxylate (Pn6CB) with CO₂˙^– at pH 7 produced a monohydropterin radical similar to that observed for unsubstituted pterin (Pn), with the rate of the second order reaction for Pn6CB (k= 1.7 × 10⁸ dm³ mol^–1 s^–1) being a factor of 2.5 slower than for Pn. At pH 10, where the 3,4-amide groups of the pterins were deprotonated, reactions of CO₂˙^– with both pterins were too slow to be observed by this method. However, the fact that reactions occur on a slower time scale was established by the observation of products in gamma radiolysis experiments. Formation of these was followed by spectrophotometry and by measurement of CO₂ yields. Over the entire pH range 6–10, titrations of Pn6CB with CO₂˙^– led to a two-electron-equivalent reduction with no incorporation of CO₂. Also the dihydro product was readily oxidised by air, a feature in which it resembled 5,8-dihydropterins reported elsewhere. With Pn at pH 7 there was again a two electron reduction with no incorporation of CO₂, but the product spectrum was completely different from that observed with Pn6CB. Furthermore, at pH 10 the stoichiometry of the titration indicated a four-electron-equivalent reduction, and a deficit in the CO₂ yields demonstrated the incorporation of at least one CO₂ per molecule. The spectrum and redox chemistry of this product strongly resembled those of tetrahydropterins. These effects of pH and the differences between Pn and Pn6CB were discussed and shown to be in accord with the known properties of di- and tetra-hydropterins.