A Quadrupole-Central-Transition ⁵⁹ Co NMR Study of Cobalamins in Solution

We report the first observation of quadrupole-central-transition (QCT) ⁵⁹ Co (I=7/2) NMR signals from three cobalamin (Cbl) compounds (CNCbl, MeCbl, and AdoCbl) dissolved in glycerol/water. Measurements were performed at four magnetic fields ranging from 11.7 to 21.1 T. We found that the ⁵⁹ Co QCT signals observed for cobalamin compounds in the slow motion regime (ω₀ τ_C ≫1) are significantly narrower than those observed from their aqueous solutions where the molecular tumbling is near the condition of ω₀ τ_C ≈1. We demonstrated that an analysis of ⁵⁹ Co QCT signals recorded over different temperatures and at multiple magnetic fields allowed determination of both the ⁵⁹ Co quadrupole coupling constant and chemical shift anisotropy for each of the three cobalamins. We successfully applied the ⁵⁹ Co QCT NMR approach to monitor in situ the transformation of CNCbl to its "base off" form in the presence of KCN. We further discovered that, to obtain the maximum QCT signal intensity with the Hahn-echo sequence, a strong B₁ field should be used for the first 90° pulse, but a weak B₁ field for the second 180° pulse. The reported ⁵⁹ Co QCT NMR methodology opens up a new direction for studying structure and function of cobalamin compounds and their roles in biological processes.