Comparison of the acid–base properties of purine derivatives in aqueous solution. Determination of intrinsic proton affinities of various basic sites

Abstract

The acidity constants of protonated 7,9-dimethylguanine, 7-methylguanosine, 7,9-dimethylhypoxanthine, 7-methylinosine, 9-methyladenine, 1,9-dimethyladenine, 7,9-dimethyladenine and 1-methyladenosine were determined in aqueous solution at 25 °C and I = 0.1 M (NaNO₃). In those instances where pK_a > 2 potentiometric pH titrations were used for the determinations; when pK_a < 2, UV spectrophotometric and ¹H-NMR shift measurements were employed (25 °C). In these latter instances, where I is often larger than 0.1 M, the H₀ scale was applied to define the H⁺ activity of the strong acid (HClO₄; HNO₃). A combination of the present results with values taken from our earlier work allowed us to quantify the intrinsic acidic properties in aqueous solution of the (N1)H^{0 or +} and (N7)H⁺ sites via micro acidity constant schemes for seven purine derivatives and to calculate the tautomeric ratios regarding the monoprotonated species, that is N7–N1·H versus H·N7–N1 meaning that in one isomer H⁺ is at the N1 site and in the other at N7. A plot of the micro acidity constants pk^N7–N1_H·N7–N1, which quantify the acidity of the (N7)H⁺ site, versus the macro acidity constants pK_a/(N1)H, which largely refer to the release of the proton from the (N1)H unit, results in a straight line for the guanine and hypoxanthine derivatives. This fact allows estimation of the micro acidity constant for any related derivative provided a value for pK_a/(N1)H is known. The presented results are also meaningful for nucleic acids because they quantify the acid–base properties of their individual sites.