Complexes of Ga3+ and In3+ with the N,N″-bis(butylamide) derivative of diethylenetriaminepentaacetic acid: stability constants and nuclear magnetic resonance studies in aqueous solution

Abstract

Potentiometric titrations showed that the sum of the first three protonation constants of diethylene-triaminepentaacetate N,N″-bis(butylamide)(L¹) decreases by units when compared to the diethylenetriaminepentaacetate (dtpa), and that the complexes of Ga³⁺ and In³⁺ of L¹ have stability constants 6.1–6.3 long units lower than those of dtpa. The decreased basicity of the bis (amide) derivative thus correlates with the lower stability of its complexes. However, at pH 7.4 the lower overall basicity of L¹ compared to dtpa partly compensates the lower stability of its complexes. thus causing their conditional stability constants to be comparable to those of the dtpa complexes. The efficacy of L¹ in binding Ga³⁺ and In³⁺ under physiological conditions is discussed by comparing pM values at pH 7.4, showing That this compound can withstand the competition of transferring for biological media relevant to medical applications. The ¹³C and ¹H N M R shifts were measured for the complexes of Al,³⁺, Ga³⁺, and Y³⁺ with L¹and compared with ¹³C and are hexadentate, possibly of octahedral geometry, and form various structuraly isomers, whereas those of In³⁺ and Y³⁺, with a single structural isomer of eight-co-ordination, are very similar to the lanthanide complexes. The complexes of Al³⁺ and Ga³⁺ of L¹ have considerable populations of isomers with bound amide groups, whereas the single structural isomer of the complexes of In³⁺ and Y³⁺ with both amide groups co-ordinated yield a variety of enantimoers in solution.