NMR Studies of ¹H NOEs in Glycogen

We have examined the cross-relaxation behavior among the protons of oyster glycogen using nuclear Overhauser enhancement (NOE). Steady-state and transient NOEs were generated using low-power CW irradiation and frequency-selective inversions. In D₂O, saturation of glycogen H2 and H4′ at 3.64 ppm gave a strong negative NOE (77 = −0.74) at H1. The NOE was similar to the value predicted by the correlation time (τ_c) calculated from the T₁ and T₂ of glycogen H1 in D₂O assuming an isotropic rigid motor dipole-dipole model. Selective inversion of H2 and H4′ gave a transient NOE at H1. In D₂O, selective inversion of H1 also led to negative transient NOEs in the H2+H4′, H3, and H5 resonances. The magnitude and rates of appearance of the NOEs in H3 and H5 were too large to arise from direct H1-H3 and H1-H5 dipolar interactions, but were consistent with very efficient cross-relaxation leading to large second-order NOEs. The glycogen H1 NOE in H₂O was also studied. Replacement of D₂O with H₂O as solvent significantly reduced the steady-state NOE at H1 following saturation of H2+H4′. Saturation of the water resonance caused a large negative NOE at H1 (η = −0.55) consistent with our earlier study which indicated that there was no direct dipolar interaction between H1 and free H₂O.