Influence of gem-Dimethyl substitution on the stability, kinetics and relaxometric properties of PDTA complexes

The protonation constants of DMPDTA (H₄DMPDTA = 2,2-dimethylpropylenediamine-N,N,N',N'-tetraacetic acid) and the stability and protonation constants of its Ln³⁺ and some divalent metal complexes have been determined by pH potentiometry and spectrophotometry (Cu²⁺) and compared with the corresponding properties of the complexes formed with PDTA (H₄PDTA = propylenediamine-N,N,N',N'-tetraacetic acid). The log K₂^H value of DMPDTA is lower by 1.5 log K units than that of PDTA. The stability constants (log K_ML) of the Ln³⁺ complexes formed with DMPDTA are lower by 1.0-1.5 log K units than those of PDTA. The kinetics of the transmetallation reactions of Gd(DMPDTA)^- and Gd(PDTA)^- with Cu²⁺ and Eu³⁺ have been studied by spectrophotometry. The reactions with Cu²⁺ and Eu ³⁺ occur predominantly by spontaneous and proton-assisted dissociation of the Gd(DMPDTA)^- and Gd(PDTA)^- complexes. The rates of the metal-exchange reactions of Gd(DMPDTA)^- are significantly lower than those of Gd(PDTA)^-. The presence of the two methyl groups on the ligand backbone increases the kinetic inertness of Gd(DMPDTA)^- due to the higher conformational rigidity of DMPDTA. Temperature-dependent ¹⁷O NMR spectra and 1/T₁ ¹H nuclear magnetic relaxation dispersion profiles of the Gd ³⁺ complexes were measured and analyzed to obtain the parameters that influence the water exchange rate and rotational dynamics. The introduction of a gem-dimethyl group on the backbone of the ligand leads to a significant variation in the properties of the corresponding complexes, which originate from a different conformational behaviour.