Solution properties of antiviral adenine-nucleotide analogues. The acid–base properties of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA)† and of its N1, N3 and N7 deaza derivatives in aqueous solution

Abstract

The pD dependence of the ¹H NMR chemical shifts of the aromatic and aliphatic hydrogens of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA) and of its 1-, 3- and 7-deaza derivatives have been measured in D₂O at 25 °C (I = 0.1 mol dm^–3, NaNO₃; at pD < 1 I increases to 0.3 mol dm^–3) in order to determine the sites of protonation as well as the acidity constants. The most basic site in all these PMEAs (= PM) is the phosphonate group, -PO₃^2–, followed by N1 in PMEA, 3- and 7-deaza-PMEA. In 1-deaza-PMEA the formation of H₂PM^± occurs by protonation of N3. Further protonation in strongly acidic medium is possible with all four PMEAs. All acidity constants measured in D₂O have been transformed to H₂O as solvent: pK^H;_H4PM ≃ 0 is due to deprotonation of H⁺(N7), where appropriate; pK^H;_H3PM ≃ 1.1 to 1.3 is due to -P(O)(OH)₂; pK^H;_H2PM ≃ 4.1 to 6.6 is due to H⁺(N1) or H⁺(N3); and pK^H;_HPM ≃ 6.9 to 7.8 is due to -P(O)₂(OH)^–. Determination of pK^H;_H2PM and pK^H;_HPM by potentiometric pH titrations in water (H₂O; I = 0.1 mol dm^–3, NaNO₃; 25 °C) give the same results. As in various instances the buffer regions of two successive equilibria are overlapping, a micro acidity constant scheme has been developed and the constants for the various sites calculated; it is concluded, e.g. that about 80% of the H(7-deaza-PMEA)^– species carry the proton at the phosphonate residue and 20% at N1. The ¹H NMR data indicate that the PMEAs in the form PM^2– occur to some extent in an orientation similar to the anti conformation of 5′-AMP^2–; i.e. the phosph(on)ate group is close to H8. For H(3-deaza-PMEA)^– the monoprotonated phosphonate group is in the vicinity of H2 in a hydrophobic region and it is suggested that this is the reason for the relatively high pK_a value of about 7.8 compared with pK_a ≃ 6.9 to 7.0 for HPM^– of the other PMEAs. Finally, the acid–base properties of the PMEAs are compared with those of 5′-AMP and of tubercidin 5′-monophosphate (= 7-deaza-5′-AMP).