Nickel(II), copper(II) and zinc(II) complexes of 9-[2- (phosphonomethoxy)ethyl]-8-azaadenine (9,8aPMEA), the 8-aza derivative of the antiviral nucleotide analogue 9-[2-(phosphonomethoxy)ethyl] adenine (PMEA). Quantification of four isomeric species in aqueous solution.
The acidity constants of the twofold protonated acyclic nucleotide analogue 9-[2-(phosphonomethoxy)- ethyl]-8-azaadenine, H(2)(9,8aPMEA)(+/-), as well as the stability constants of the M(H;9,8aPMEA)(+) and M(9,8aPMEA) complexes with the metal ions M(2+) =Ni(2+), Cu(2+) or Zn(2+), have been determined by potentiometric pH titrations in aqueous solution at I=0.1 M (NaNO(3)) and 25. The result for the release of the first proton from H(2)(9,8aPMEA)(+) (pK(a)= 2.73), which originates from the (N1)H(+) site, was confirmed by UV-spectrophotometric measurements. Application of previously determined straight-line plots of log KMM(R-PO(3)) versus PKH(3)(R-HPO(3))' for simple phosph(on)ate ligands, R- PO-, where R represents a residue without an affinity for metal ions, proves that the primary binding site of 9,8aPMEA(2-) is the phosphonate group for all three metal ions studied. By stability constant comparisons with related ligands it is shown, in agreement with conclusions reached earlier for the Cu(PMEA) system [PMEA(2-)=dianion of 9-[2- (phosphonomethoxy)ethyl]adenine], that in total four different isomers are in equilibrium with each other, i.e. (i) an open isomer with a sole phosphonate coordination, M(PA)(op), where PA(2-)=PMEA(2-)or 9,8aPMEA(2-), (ii) an isomer with a 5-membered chelate involving the ether oxygen, M(PA)cl/o, (iii) an isomer which contains 5- and 7-membered chelates formed by coordination of the phosphonate group, the ether oxygen and the N3 site of the adenine residue, M(PA)(cl/O/N3), and finally (iv) a macrochelated isomer involving N7, M(PA)(cl/]N7). The Cu(2+) systems of PMEA(2-) and 9,8aPMEA(2-) behave quite alike; the formation degrees for Cu(PA)(op), CuM(PA)(cl/O), Cu(PA)(cl/O/N3) and Cu(PA)(cl/N3) are approximately 16, 32, 45 and 7%, respectively, which shows that Cu(PA)(cl/N7) is a minority species. In the Ni(2+) and Zn(2+) systems the open isomer is the dominating one followed by M(PA)(cl/O), but there are indications that the other two isomers also occur to some extent.
Gómez-Coca
Raquel B
RB
Department of Chemistry, Inorganic Chemistry, University of Basel, Spitalstrasse 51, Basel CH-4056, Switzerland.
Holý
Antonín
A
Vilaplana
Rosario A
RA
González-Vílchez
Francisco
F
Sigel
Helmut
H