W-band 17O pulsed electron nuclear double resonance study of gadolinium complexes with water

In this work we have studied pulsed O-17 electron nuclear double resonance (ENDOR) spectra of the Gd3+ aquo ion and the magnetic resonance imaging (MRI) contrast agent MS-325 in an O-17-enriched frozen glassy water/methanol solution. The isotropic hyperfine interaction (hfi) constant of the water ligand 17(O) was found to be about 0.75 MHz, which corresponds to a spin density delocalized to the ligand of rho(O) approximate to -4 x 10(-3). The analysis of the anisotropic hfi constant (0.69 +/- 0.05 MHz) yields Gd-O distances of about 2.4-2.5 (A) over circle. Simultaneous analysis of these distances and the Gd-H distances found earlier allows one to elucidate the details of the Gd-OH2 coordination geometry.