A smart copper(II)-responsive binuclear gadolinium(III) complex-based magnetic resonance imaging contrast agent

A novel Gd-DO3A-type bismacrocyclic complex, [Gd-2(DO(3)A)(2)BMPNA], with a Cu2+-selective binding unit was synthesized as a potential "smart" copper(II)-responsive magnetic resonance imaging (MRI) contrast agent. The relaxivity of the complex was modulated by the presence or absence of Cu2+; in the absence of Cu2+, the complex exhibited a relatively low relaxivity value (6.40 mM(-1) s(-1)), while the addition of Cu2+ triggered an approximately 76% enhancement in relaxivity (11.28 mM(-1) s(-1)). Moreover, this Cu2+-responsive contrast agent was highly selective in its response to Cu2+ over other biologically-relevant metal ions. The influence of some common biological anions on the Cu2+-responsive contrast agent and the luminescence lifetime of the complex were also studied. The results of the luminescence lifetime measurements indicated that the enhancement in relaxivity was mainly ascribed to the increased number of inner-sphere water molecules binding to the paramagnetic Gd3+ core upon the addition of Cu2+. In addition, the visual change associated with the significantly enhanced relaxivity due to the addition of Cu2+ was observed from T-1-weighted phantom images.