Determination of the hydration number of gadolinium(III) complexes by high-field pulsed 17O ENDOR spectroscopy

Pulsed O-17 Mims electron-nuclear double resonance (ENDOR) spectroscopy at the W band (95GHz) and D band (130 GHz) is used for the direct determination of the water coordination number (q) of gadolinium-based magnetic resonance imaging (MRI) contrast agents. Spectra of metal complexes in frozen aqueous solutions at approximately physiological concentrations can be obtained either in the presence or absence of protein targets. This method is an improvement over the H-1 ENDOR method described previously (Zech et aL, ChemPhysChem 2005, 6, 2570), which involved the difference ENDOR spectrum of exchangeable I protons from spectra taken in H2O and D2O. In addition to exchangeable water protons, the H-1 ENDOR method is also sensitive to other exchangeable protons, and it is shown here that this method can overestimate hydration numbers for complexes with exchangeable protons at (GdH)-H-... distances similar to that of the coordinated water, for example, from NH groups. The O-17 method does not suffer from this limitation. O-17 ENDOR spectroscopy is applied to Gd(III) complexes containing zero, one, or two inner-sphere water molecules. In addition, C-13 and H-1 ENDOR studies were performed to assess the extent of methanol coordination, since methanol is used to produce a gloss in these experiments. Under the experimental conditions used for the hydration number determination (30 mol% methanol), fewer than 75% of the coordination sites were found to be occupied by methanol.