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

The pD dependence of the H-1 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 degrees 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: pK(H4PM)(H) similar or equal to 0 is due to deprotonation of H+(N7), where appropriate; pK(H1PM)(H) similar or equal to 1.1 to 1.3 is due to -P(O)(OH)(2); pK(H2PM)(H) similar or equal to 4.1 to 6.6 is due to H+(N1) or H+(N3); and pK(HPM)(H) similar or equal to 6.9 to 7.8 is due to -P(O)(2)(OH)(-). Determination of pK(H2PM)(H) and pK(HPM)(H) by potentiometric pH titrations in water (H2O; I=0.1 mol dm(-3), NaNO3; 25 degrees 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 H-1 NMR data indicate that the PMEAs in the form PM2- 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) similar or equal to 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).