Journal of Lipid Research, Vol 36, 2478-2492, Copyright © 1995 by Lipid Research, Inc.
Physicochemical characterization of a model digestive mixture by 2H NMR
PW Westerman
Department of Biochemistry and Molecular Pathology, Northeastern Ohio Universities College of Medicine, Rootstown 44272, USA.
2H nuclear magnetic resonance (NMR) spectra were obtained at 30.87 MHz for
8% (w/v) aqueous dispersions of mixtures of bile salts (MBS), mixed
intestinal lipids (MIL; myristic acid, monomyristoylglycerol,
dimyristoylphosphatidylcholine = 5:1:1), and cholesterol, in which a single
lipid component is selectively 2H-labeled. Using the observation that the
time-averaged quadrupole splitting of a C2H3 group varies according to
whether it exists in a micellar, multilamellar or solid phase, one-, two-,
and three-phase regions in the equilibrium phase diagram have been
identified. From the intensities of the singlets and powder patterns in the
wide-line 2H NMR spectra, the relative amounts of these organized molecular
assemblies were determined. With different C2H3-labeled components in
samples of identical total composition, the chemical composition of each
phase was calculated for one point (20 mol % cholesterol; 50 mol % MIL, and
30 mol % MBS) in a two-phase region of the phase diagram where the 2H NMR
spectrum displayed both a sharp spectral component and a broad uniaxial
powder pattern. X-ray diffraction measurements on this sample confirmed
that the uniaxial powder pattern in the NMR spectra can be assigned to
multilamellar vesicles. At this same point in the phase diagram with the 2H
label on the alpha-methylene site of myristic acid, both narrow and broad
(delta v = 37 kHz) spectral components were again observed. Relaxation time
(T1 and T2) measurements of the sharp spectral component indicate that this
peak arises from rapidly tumbling aggregates which, at a total lipid
concentration of 8% (w/v), are micellar particles and not unilamellar
vesicles. These experiments demonstrate the feasibility of structural
investigations of model digestive mixtures by 2H NMR.
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