Kinetics of Formation of Ca(2+) Complexes of Acyclic and Macrocyclic Poly(amino carboxylate) Ligands: Bimolecular Rate Constants for the Fully-Deprotonated Ligands Reveal the Effect of Macrocyclic Ligand Constraints on the Rate-Determining Conversions of Rapidly-Formed Intermediates to the Final Complexes.

Abstract

The apparent bimolecular rate constants, k(1) (M(-)(1) s(-)(1)), for the formation of Ca(2+) complexes of a series of acyclic (edta, egta, cdta) and macrocyclic (dota, teta, do3a) poly(amino carboxylate) ligands were determined in the pH range 7-13 using the fluorescent ligand quin2 in a stopped-flow apparatus to monitor the ligand competition reaction. The k(1) values are observed to reach maximum constant values at high pH, characteristic of reactions involving the fully-deprotonated ligand species. Bimolecular formation constants k(Ca)(L), k(Ca)(HL), and k(Ca)(H)()2(L), characteristic of the reaction of the fully-deprotonated and mono- and diprotonated ligands, respectively, were derived from the pH dependence of the k(1) values. The k(Ca)(L) values of the acyclic ligands are edta, 4.1 x 10(9) M(-)(1) s(-)(1); egta, 2.1 x 10(9) M(-)(1) s(-)(1); and cdta, 2.3 x 10(9) M(-)(1) s(-)(1), while the corresponding values for the macrocyclic ligands are dota, 4.7 x 10(7) M(-)(1) s(-)(1); teta, 1.1 x 10(7) M(-)(1) s(-)(1); and do3a, 1.0 x 10(8) M(-)(1) s(-)(1). The smaller values for the macrocyclic ligands are consistent with ligand-dictated constraints imposed on the conversion of a stable intermediate to the final complex, a process which involves the simultaneous stripping of several water molecules from the first-coordination sphere of the Ca(2+) ion.