Tris amine complexes, optimization

Clinical CAs have one inner-sphere water molecule, and thus an increase in q will directly increase the inner-sphere relaxivity. However, this leads to a trade-off in stability by decreasing the coordination number. Desirable modifications of Gd-DTPA or Gd-DOTA to optimize the relaxivity would be to use octadentate or heptadentate chelates. Scientists have designed many different chelates to increase q [19-21]. Raymond and coworkers reported a novel class of chelate, HOPO (tris[(3-hydroxy-l-methyl-2-oxo-l,2-didehydropyridine-4-carboxamido)-ethyl] amine), which can accommodate two or more inner-sphere water molecules and have a good thermodynamic stability and relaxivity [22-25]. The observed r of some HOPO-Gd complexes can be as high as 11.1 mM s at 298 K and 20 MHz [26]. The g value of Gd complexes in solution can be estimated by luminescence lifetime studies of Eu or Tb analogs, according to the Horrocks equation. 

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