Diffusion in Oriented Lamellar Phases by Pulsed NMR
The bilayer structure (lamellar phase) of lipid-water and soap-water systems is the subject of considerable research because of the known occurrence of bilayer structures in biological membranes. (l) The nuclear magnetic resonance (NMR) line widths in this phase are very broad due to the incomplete motional averaging of the static dipolar interactions. This has until recently precluded the use of NMR spin echo techniques to measure diffusion because spin echo occurrence requires the removal of the static dipolar interactions. DeVries and Berendsen (2) have shown by optical and NMR measurements that the potassium oleate (KO)-water lamellar phase can be oriented between glass plates with the optical axis (along the hydrocarbon chains) perpendicular to the glass plates. They demonstrated that the line width varied as (3 cos2 Ω - 1) where Ω is the angle between the optical axis and the magnetic field. This provided a conclusive demonstration that the broad lines arise from incompletely averaged dipolar interactions which have residual static interactions along the chain direction. In particular, they noted that “the lines got very narrow at the so-called “magic angle” where Ω = 54o44’ and the term (3 cos2 Ω - 1) (and hence the dipolar interactions) becomes very small.
KeywordsNuclear Magnetic Resonance Free Induction Decay Magic Angle Nuclear Magnetic Resonance Measurement Gradient Pulse
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