METAL ION-BINDING PROPERTIES IN AQUEOUS-SOLUTION OF THE NUCLEOSIDE ANALOG, 5,6-DICHLORO-1-(BETA-D-RIBOFURANOSYL)BENZIMIDAZOLE (DRB)

The stability constants of the 1:1 complexes formed between Mg2+, Ca2, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ or Cd2+ (= M2+) and 5,6-dic hloro-1-(beta-D-ribofuranosyl)benzimidazole (DRB) were determined by p otentiometric pH titrations in aqueous solution (25 degrees C; I = 0.5 M, NaNO3). The acidity constant of H(DRB)(+), the proton being at N3, was measured by the same method and the result was confirmed via spec trophotometry. Based on previously established [L. E. Kapinos, B. Song , H. Sigel, Inorg. Chim. Acta 280, in press (1998)] log K-ML(M) versus pK(HL)(H) straight-line plots for complexes of imidazole-type ligands it is shown for the Mn(DRB)(2+) and Zn(DRB)(2+) complexes, as example s, that the benzene ring of the benzimidazole residue exerts a steric inhibition for metal ion binding at N3; i.e., the data points for the M(DRB)(2+) complexes fall clearly below the straight lines defined by the imidazole-type ligands.