Towards the rational design of MRI contrast agents: δ-Substitution of lanthanide(III) NB-DOTA-tetraamide chelates influences but does not control coordination geometry

LnDOTA-tetraamide chelates (DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10- tetraacetic acid) have received considerable recent attention as a result of their potential to act as PARACEST contrast agents for magnetic resonance imaging (MRI). Although PARACEST agents afford several advantages over conventional contrast agents they suffer from substantially higher detection limits; thus, improving the effectiveness of LnDOTA-tetraamide chelates is an important goal. In this study we investigate the potential to extend conformational control of LnDOTA-type ligands to those applicable to PARACEST. Furthermore, the question of whether δ- rather than α-substitution of the pendant arms could be used to control the chelate coordination geometry is addressed. Although δ-substitution does influence coordination geometry it does not afford control. However, it can play an important role in governing the conformation of the amide substituent relative to the chelate in such as way that suggests a PARACEST agent could be designed that has detection limits at least as low as a conventional MRI contrast agent.