Liquid ordered phase explained

Regarding biological membranes, the liquid ordered phase is a liquid crystalline phase of a lipid bilayer, and is of significant biological importance. It occurs in many lipid mixtures combining cholesterol with a phospholipid and/or sphingolipids e.g. sphingomyelin. This phase has been related to lipid rafts that may exist in plasma membranes.

Definition

The liquid ordered phase can be defined as:

• fluid and lamellar phase, including the Wide angle X-ray scattering pattern centered by broad diffraction peak at 4.2Å
• acyl hydrocarbon chains are in the all-trans state
• rapid lateral diffusion
• ²H-NMR quadrupolar splitting is ca. 50 kHz

History

This was first called the liquid ordered phase by Ipsen et al. (1987). However, it has also been called the LG_I subgel phase by Huang et al. (1993) and the β phase by Vist and Davis (1990).

References

• Ipsen, J. H., G. Karlstrom, O. G. Mouritsen, H. Wennerstrom, and M. J. Zuckermann. 1987. Phase equilibria in the phosphatidylcholine-cholesterol system. Biochim. Biophys. Acta. 905:162–172.
• Huang TH, Lee CWB, Dasgupta SK, Blume A, Griffin RG. 1993. "A C-13 and H-2 Nuclear-Magnetic-Resonance Study of Phosphatidylcholine Cholesterol Interactions - Characterization of Liquid-Gel Phases." Biochemistry 32(48):13277-13287
• Vist MR, Davis JH. 1990. "Phase-Equilibria of Cholesterol Dipalmitoylphosphatidylcholine Mixtures - H-2 Nuclear Magnetic-Resonance and Differential Scanning Calorimetry." Biochemistry 29(2):451-464.