Morphology study of DMPC/DHPC mixtures by solution-state 1H, 31P NMR, and NOE measurements

Abstract

The temperature and mixing-ratio dependencies are studied by solution-state NMR for the morphology of aggregates composed of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC). The 1H and 31P NMR and nuclear Overhauser effect (NOE) measurements were performed at 25–60°C for molar ratio of q=1–5 (q=[DMPC]/[DHPC]). For the q=1 and 2 systems, the turbidity and viscosity of the sample are unchanged and the 1H and 31P NMR spectra are sharp over the whole temperature range examined. Combined with the NOE measurement and compared to the results for small unilamellar vesicle system, the q=1 and 2 aggregates are found to form the spherical mixed micelle and/or small bicelle over 25–60°C. When q is larger and is 2.5 or 3, the lipid-mixture system exhibits two morphological changes with temperature variation. At low temperature, the DMPC/DHPC system is of spherical mixed micelle and/or small bicelle, as evidenced from the visual observation, the 1H and 31P NMR spectra, and the NOE measurement. In the intermediate temperature range, the aggregate is the magnetically oriented bicelle since the solution is observed to be opaque and viscous with the 1H signal broadening and the 31P signal splitting into two peaks. When the temperature is high and above ~50°C, the sample becomes milky with sharp 1H and singlet 31P signals. The NOE measurement shows that the lipid dynamics of the aggregate is faster than of the large unilamellar vesicle with low curvature. These show the formation of cylindrical mixed micelle.