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

The acidity constants of the threefold protonated acyclic 9-(4-phosphonobut yl)adenine, H-3(dPMEA)(+), as well as the stability constants of the M(H;dP MEA)(+) and M(dPMEA) complexes with the metal ions M2+=Mg2+, Ca2+, Sr2+, Ba 2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ or Cd2+, have been determined by potentiom etric pH titrations, in aqueous solution at I=0.1 M (NaNO3) and 25 degrees C. Application of previously determined straight-line plots of log K-M(R-PO 3(M)) versus pK(H(R-PO3)(H)) for simple phosph(on)ate ligands, R-PO32-, whe re R represents a residue without an affinity for metal ions, proves that t he primary binding site of dPMEA(2-) is the phosphonate group with all the metal ions studied; in fact, in most instances the stability is solely dete rmined by the basicity of the phosphonate residue. Only for the Ni(dPMEA), Cu(dPMEA) and Cd(dPMEA) systems a stability increase due to macrochelate fo rmation with the adenine residue occurs; the formation degrees are 21 +/- 1 5%, 31 +/- 14% and 29 +/- 18%, respectively. In these three instances the a dditional interaction of the phosphonate-coordinated M2+ occurs most probab ly with N7; hence, dPMEA(2-) is more similar in its metal ion-binding prope rties to the parent nucleotide adenosine 5'-monophosphate (AMP(2-)) than to the antivirally active and structurally more related dianion of 9-[2-(phos phonomethoxy)ethyl]adenine (PMEA(2-)). This result agrees with the observat ion that replacement of the ether O atom in PMEA by a CH2 unit leads to a c ompound, 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 isomer ic 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 t he ether oxygen (formation degree ca. 34%) and also N3 of the adenine moiet y (ca. 41%).