W-Band¹⁷O Pulsed Electron Nuclear Double Resonance Study of Gadolinium Complexes with Water

In this work we have studied pulsed 17 O electron nuclear double resonance (ENDOR) spectra of the Gd 3+ aquo ion and the magnetic resonance imaging (MRI) contrast agent MS-325 in an 17 O-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 ρ O ≈ −4 × 10 -3 . The analysis of the anisotropic hfi constant (0.69 ± 0.05 MHz) yields Gd−O distances of about 2.4−2.5 Å. Simultaneous analysis of these distances and the Gd−H distances found earlier allows one to elucidate the details of the Gd−OH 2 coordination geometry.