New insight into the organization of myelin water using deuterium NMR

Purpose: Myelin water is commonly characterized by its short proton T₂ relaxation time, suggesting strong association with the polar head groups of the bilayer constituents. Deuterium NMR of water in ordered structures exhibits splittings as a result of quadrupolar interactions that are observable using the double-quantum filter. The purpose of the current study was to identify and characterize the water populations. Methods: The ²H double-quantum-filtered spectroscopic experiments were conducted at 62 MHz (9.4 T) on a sample of reconstituted myelin from ovine spinal cord after exchange of native water with D₂O. Results: Signals passing the double-quantum filter were attributed to 2 water pools: 1 consisting of a doublet of 650-Hz splitting, and a second unsplit signal. Similar signals were observed in the sciatic and optic nerves and in the spinal cord. Further, data suggest that diffusion of water molecules in these 2 pools (D_app ≤ 5 × 10⁻⁷ cm²/s) is either hindered or restricted. An estimate of exchange lifetime of 10-15 ms between water pertaining to the single peak and that of the split peaks suggests exchange occurs in a slow-intermediate rate regime. Further distinction between the 2 pools was obtained from T₁ measurements. Deuterons belonging to the doublet resonance were found to have short T₁, estimated to be on the order of 10-20 ms, whereas those corresponding to the single peak were close to that of bulk D₂O. Conclusion: The results suggest that myelin extract water consists of 2 hindered populations with distinct degrees of anisotropic motion that can be studied by ²H double-quantum-filtered NMR.