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A highly sensitive, convenient, direct spectroscopic method for the measurement of stability constants of Eu3+ ion complexes of multidentate ligands is described. Eu3+ ion complexation is monitored by means of 7F0 → 5D0 excitation spectroscopy. The method involves the competition against one another, of two ligands, one (L‘) of known and one (L) of unknown stability constant for its Eu3+ complex. With laser excitation at a particular wavelength (in the 578−581 nm range), the excitation intensity of EuL and/or EuL‘ is measured by either a time-resolved method, if the excited state lifetimes of EuL and EuL‘ are sufficiently different, or in a non-time-resolved manner, if the emission intensities (λem = 614 nm) of EuL and EuL‘ are significantly different at the exciting wavelength. These data lead to the determination of relative conditional stability constants (K rel = [EuL‘][L] t /[EuL][L‘] t ) of the two ligands which, with knowledge of the protonation constants of the ligands, can be used to calculate the thermodynamic formation constant of the fully deprotonated ligand, L. Using ethylenediaminetetraacetic acid (edta) as the reference ligand, formation constants for the 1:1 complexes of Eu3+ with N-(2-hydroxyethyl)ethylenediaminetriacetic acid (hedta), diethylenetriaminepentaacetic acid (dtpa), and 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (do3a) were determined, all in good to excellent agreement with their literature values. The method was also applied to the Eu3+−dtpa-dien [1,4,7-tris(carboxymethyl)-9,17-dioxo-1,4,7,10,13,16-hexaazacyclooctadecane] system, where the species EuH(dtpa-dien)+, Eu(dtpa-dien), and Eu(OH)(dtpa-dien)¯ exist in different pH regions. The respective formation constants are log K EuL = 17.2 ± 0.1, log K EuHL = 14.2 ± 0.2, and, for the reaction EuL + OH- = Eu(OH)L-, log K OH = 5.7 ± 0.2 (pK a = 8.3 ± 0.2). Advantages of the present method include high sensitivity (1−10 μM concentrations of Eu3+ and ligand), small sample volumes (1 mL or less), and the ability to detect directly and characterize the species present in solution under particular pH conditions using excitation spectroscopy and lifetime measurements.

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Excitation spectroscopy of the (7)F(0) --> (5)D(0) transition of Eu(3+) is used to detect and characterize a kinetic intermediate in the formation of a complex between Eu(3+) and the macrocyclic ligand dtpa-dien (1,4,7-tris(carboxymethyl)-9,17-dioxo-1,4,7,10,13,16-hexaazacyclooctadecane). Both the long-lived intermediate and the final product, [Eu(Hdtpa-dien)(H(2)O)](+), are formed immediately upon mixing the components, as evidenced by separate peaks in the excitation spectrum. The transformation of the intermediate to the final product, monitored by excitation spectroscopy, occurs by both proton-assisted and non-proton-assisted pathways. It is proposed that the intermediate represents a "blind alley" in the pathway to a final nine-coordinate tricapped trigonal prismatic product. The intermediate with an "up, down, up" configuration of carboxylate arms must undergo some decoordination to form the final complex. Molecular mechanics calculations and excited state lifetimes suggest that the intermediate is eight-coordinate with one coordinated water molecule. The stability constant of the final complex is found to be log K = 14.11 +/- 0.05.

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.