Estimation of stability constants of copper(II) chelates with N-alkylated amino acids using topological indices

Protonation (pK(1) and pK(2)) and copper(II) binding stability constants (log beta(110) and log beta(120)) of glycine and five N-alkylated glycines (methyl, dimethyl, diethyl, tert-butyl and tert-butylmethyl) were determined by potentiometric titration. These constants and the constants of eight N-alkylated glycinates, measured previously, were correlated with four topological indices (Wiener index, W, and the first-, second- and third-order valence-connectivity indices, (1)chi(v), (2)chi(v) and (3)chi(v)) calculated for the ligand. The first protonation constants (ascribed to the dissociation of COOH group) do not correlate with any of the chosen topological indices (r(2) < 0.3), but significant correlation was obtained for pK2 (dissociation of amino group) with r(2) values up to 0.920. "Corrected stability constants (obtained by subtracting pK(2) value) yielded a generally better fit than the uncorrected ones. Our stability constants are better correlated with topological indices (max. r(2) = 0.993) than the constants measured previously (max. r(2) = 0.836). From the regression analysis it is possible to predict the values of pK(2), log beta(110) and log beta(120) with an error less than 0.1, 0.3 and 0.3 log units, respectively