The stability constants of Sm(EDTMP) (logKML = 20.71) and Y(EDTMP) (logKML = 19.19) were determined by a competition reaction between the Ln3+ ion (Ln3+ = Sm3+ or Y 3+) and Cu2+ for the EDTMP ligand by spectrophotometry at pH ≈ 10, in the presence of an excess amount of citrate (0.15 M NaCl, 25 °C). For determining the stability constants of Cu(EDTMP) (logKML = 19.36) and Ca(EDTMP) (logKML = 8.71) pH-potentiometry was used. In the pH range 4-9 the EDTMP complexes are present in the form of nonprotonated and mono-, di- and triprotonated species. The Ca2+ ion forms a dinuclear complex with Ln(EDTMP). In a simplified blood plasma model consisting of Sm3+, Ca2+ and Zn2+ metal ions, EDTMP, citrate, cysteine and histidine ligands, Sm3+ is practically present in the form of [Sm(HEDTMP)Ca]2-, whereas Zn2+ predominantly forms [Zn(HEDTMP)]5- and [Zn(H2EDTMP)] 4- complexes. For studying the dissociation rates of the complexes, the kinetics of the metal exchange (transmetallation) reactions between the Ln(EDTMP) complexes and Cu2+-citrate were investigated in the pH range 7-9 by the stopped-flow method. The rates of the exchange reactions are independent of the Cu2+ concentration and increase with the H + concentration. The rate constants, characterizing the proton-assisted dissociation of the Ln(EDTMP) complexes, are several orders of magnitude higher than those of the similar Ln(EDTA) complexes, because the protonation constants of Ln(EDTMP) are high and the protonated Ln(HEDTMP) and Ln(H2EDTMP) species are present in higher concentration. The half-times of dissociation of Sm(EDTMP) and Y(EDTMP) at pH = 7.4 and 25 °C are 4.9 and 7.5 s, respectively. These relatively short dissociation half-time values do not predict the deposition of Ln3+ ions in bones in the form of intact Ln(EDTMP) complexes. It is more probable that sorption of the EDTMP ligand and Sm3+ or Y3+ ions occurs independently after the dissociation of complexes.
ASJC Scopus subject areas
- Inorganic Chemistry