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, -PO32–, 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: pKH;H4PM ≃ 0 is due to deprotonation of H+(N7), where appropriate; pKH;H3PM ≃ 1.1 to 1.3 is due to -P(O)(OH)2; pKH;H2PM ≃ 4.1 to 6.6 is due to H+(N1) or H+(N3); and pKH;HPM ≃ 6.9 to 7.8 is due to -P(O)2(OH)–. Determination of pKH;H2PM and pKH;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 PM2– occur to some extent in an orientation similar to the anti conformation of 5′-AMP2–; 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 pKa value of about 7.8 compared with pKa ≃ 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).