LASER-INDUCED LUMINESCENCE STUDIES OF EU(HAM)3+ - AN EVALUATION OF COMPLEX STABILITY AND THE USE OF THE GADOLINIUM ANALOG AS A RELAXATION AGENT IN Y-89 NMR

The hexaazamacrocyclic (HAM) complex Eu(C22H26N6)3+ has been characterized via laser-induced Eu3+ luminescence in an effort to identify the potential biological probe function of the Eu3+ and Gd3+ macrocyclic species. Examination of the Eu(HAM)3+ F-7(0) --> 5D0 excitation spectra (578-581 nm) and excited state lifetime values as a function of pH reveals a species conversion which occurs on the time scale of the luminescence lifetime (0.063 ms). The sensitivity of the Eu3+ luminescence technique is exploited in a thorough evaluation of the stability of the Eu(HAM)3+ complex. After 48 h in solution at pH 7.0 and 25-degrees-C, approximately 4% of the macrocycle decomposes. The presence of an added excess of the strong chelating agent diethylenetriaminepentaacetic acid (dtpa) or a competing metal ion (Y3+) does not increase the rate of decomposition. However, strong pH and temperature dependencies are observed for the rate of Eu(HAM)3+ degradation, with the elevation of the temperature to 37-degrees-C causing the largest increase in the decomposition rate (12% in 48h). The Gd(HAM)3+ complex is an effective H-1 NMR relaxation agent that has been proposed for use in magnetic resonance imaging (MRI) (Smith, P. H.; Brainard, J. R.; Morris, D. E.; Jarvinen, G. D.; Ryan, R. R. J. Am. Chem. Soc. 1989, 111, 7437-7443). Currently, Gd(dtpa)2- is the most commonly used contrast agent for MRI. In a comparison of the ability of Gd(HAM)3+ and Gd(dtpa)2- to relax the Y-89 NMR nucleus, the concentration of Gd(HAM)3+ required to achieve maximum signal enhancement of the single Y-89 NMR resonance of Y(dtpa)2- is one-tenth of that necessary for Gd(dtpa)2-.