Novel optically-active bis(amino acid) ligands and their complexation with gadolinium
Abstract
A condensation reaction of glyoxal with (S)-histidine and (S)-aspartic acid yielded a new optically-active bis(amino acid) ligand, N-[(S)-1-carboxy-2-(imidazol-4-yl)ethyl]-N′-[(S)-1,2-dicarboxyethyl]ethylenediamine. Two bis(amino acid) ligands with picolyl (pyridylmethyl) groups were synthesized by condensation reactions of glyoxal with the picolyl derivative of (S)-histidine and that of (S)-aspartic acid: the obtained ligands are N,N′-bis(2-pyridylmethyl)-N-[(S)-1-carboxy-2-(imidazol-4-yl)ethyl]-N′-[(S)-1,2-dicarboxyethyl]ethylenediamine and N,N′-bis(2-pyridylmethyl)-N,N′-bis[(S)-1-carboxy-2-(imidazol-4-yl)ethyl]ethylenediamine. The protonation constants of these ligands were determined by potentiometry, and the corresponding protonation sites were located on the basis of 1H NMR spectra obtained at different pD values. The formation constants of the Gd3+ complexes were determined by potentiometric titrations, and the NMR relaxivities r1 and r2 by the measurements of the NMR relaxation times.