The thulium complex of 1,4,7,10-tetrakis {[N-(1H-imidazol-2-yl)-carbamoyl]methyl]-1,4,7,10-tetraazacyclododecane (dotami) as a paraCEST contrast agent

1,4,7,10-Tetrakis{[N-(1H-imidazol-2-yl)carbamoyl]methyl}-1,4,7, 10-tetraazacyclododecane (dotami), a tetra(1H-imidazol-2-yl) derivative of the well-studied octadentate 1,4,7,10-tetrakis[(carbamoyl)methyl]- 1,4,7,10-tetraazacyclododecane (dotam) ligand, was synthesized by reaction of 1,4,7,10-tetraazacyclododecane with N-(1H-imidazol-2-yl) chloroacetamide in high yield. Its tricationic thulium complex was isolated as a water-soluble chloride salt. The detection of the mildly acidic amide and amine protons by direct proton NMR in aqueous solution was unsuccessful, but such exchangeable protons could be detected via their chemical exchange-dependent saturation transfer (CEST) effect. The observed CEST effect was distinctly different from that found for respective dotam complexes and is, therefore, ascribed to exchangeable protons associated with the imidazole substituent.