Morphological Characterization of DMPC/CHAPSO Bicellar
Mixtures: A Combined
SANS and NMR Study
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Abstract
Spontaneously forming structures
of a system composed of dimyristoyl
phosphatidylcholine (DMPC) and 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate
(CHAPSO) were studied by small-angle neutron scattering (SANS), <sup>31</sup>P NMR, and stimulated echo (STE) pulsed field gradient (PFG) <sup>1</sup>H NMR diffusion measurements. Charged lipid dimyristoyl phosphatidylglycerol
(DMPG) was used to induce different surface charge densities. The
structures adopted were investigated as a function of temperature
and lipid concentration for samples with a constant molar ratio of
long-chain to short-chain lipids (= 3). In the absence of DMPG, zwitterionic
bicellar mixtures exhibited a phase transition from discoidal bicelles,
or ribbons, to multilamellar vesicles either upon dilution or with
increased temperature. CHAPSO-containing mixtures showed a higher
thermal stability in morphology than DHPC-containing mixtures at the
corresponding lipid concentrations. In the presence of DMPG, discoidal
bicelles (or ribbons) were also found at low temperature and lower
lipid concentration mixtures. At high temperature, perforated lamellae
were observed in high-concentration mixtures (≥7.5 wt %) whereas
uniform unilamellar vesicles and bicelles formed in low-concentration
mixtures (≤2.5 wt %), respectively, when the mixtures were
moderately and highly charged. From the results, spontaneous structural
diagrams of the zwitterionic and charged systems were constructed