A new metallostar complex based on an aluminum(III) 8-hydroxyquinoline core as a potential bimodal contrast agent.

A ditopic DTPA monoamide derivative containing an 8-hydroxyquinoline moiety was synthesized and the corresponding gadolinium(III) complex ([Gd(H5)(H(2)O)](-)) was prepared. After adding aluminum(III), the 8-hydroxyquinoline part self-assembled into a heteropolymetallic triscomplex [(Gd5)(3)Al(H(2)O)(3)](3-). The magnetic and optical properties of this metallostar compound were investigated in order to classify it as a potential in vitro bimodal contrast agent. The proton nuclear magnetic relaxation dispersion measurements indicated that the relaxivity r(1) of [Gd(H5)(H(2)O)](-) and [(Gd5)(3)Al(H(2)O)(3)](3-) at 20 MHz and 310 K equaled 6.17 s(-1) mM(-1) and 10.9 s(-1) mM(-1) per Gd(III) ion respectively. This corresponds to a relaxivity value of 32.7 s(-1) mM(-1) for the supramolecular complex containing three Gd(III) ions. The high relaxivity value is prominently caused by an increase of the rotational tumbling time τ(R) by a factor of 2.7 and 5.5 respectively, in comparison with the commercially used MRI contrast agent Gd(III)-DTPA (Magnevist®). Furthermore, upon UV irradiation, [(Gd5)(3)Al(H(2)O)(3)](3-) exposes green broad-band emission with a maximum at 543 nm. Regarding the high relaxivity and the photophysical properties of the [(Gd5)(3)Al(H(2)O)(3)](3-) metallostar compound, it can be considered as a lead compound for in vitro bimodal applications.