Metal ion-binding properties of 9-(4-phosphonobutyl)adenine (dPMEA), a sister compound of the antiviral nucleotide analogue 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA), and quantification of the equilibria involving four Cu(PMEA) isomers

Abstract

The acidity constants of the threefold protonated acyclic 9-(4-phosphonobutyl)adenine, H₃(dPMEA)⁺, as well as the stability constants of the M(H;dPMEA)⁺ and M(dPMEA) complexes with the metal ions M²⁺ = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ or Cd²⁺, have been determined by potentiometric pH titrations, in aqueous solution at I = 0.1 M (NaNO₃) and 25 °C. Application of previously determined straight-line plots of log K^M_M(R-PO3)versus pK^H_H(R-PO3) for simple phosph(on)ate ligands, R-PO₃²⁻, where R represents a residue without an affinity for metal ions, proves that the primary binding site of dPMEA²⁻ is the phosphonate group with all the metal ions studied; in fact, in most instances the stability is solely determined by the basicity of the phosphonate residue. Only for the Ni(dPMEA), Cu(dPMEA) and Cd(dPMEA) systems a stability increase due to macrochelate formation with the adenine residue occurs; the formation degrees are 21 ± 15%, 31 ± 14% and 29 ± 18%, respectively. In these three instances the additional interaction of the phosphonate-coordinated M²⁺ occurs most probably with N7; hence, dPMEA²⁻ is more similar in its metal ion-binding properties to the parent nucleotide adenosine 5′-monophosphate (AMP²⁻) than to the antivirally active and structurally more related dianion of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA²⁻). This result agrees with the observation that replacement of the ether O atom in PMEA by a CH₂ unit leads to a compound, i.e. dPMEA, devoid of any biological activity. In addition, use is made of the stability enhancement obtained for the Cu(dPMEA) system due to macrochelate formation to analyze the equilibria regarding the four isomeric complex species possibly formed in the Cu(PMEA) system. It is shown that a macrochelated isomer involving N7 of the adenine residue occurs with Cu(PMEA) only in trace amounts; the important isomers in this system involve the ether oxygen (formation degree ca. 34%) and also N3 of the adenine moiety (ca. 41%).