Chapter 3. Polymorphism of lipid-water systems

J.M. Seddon and R.H. Templer
Department of Chemistry, Imperial College,
Exhibition Road, London SW7 2AY, U.K.

1. Introduction

This chapter will describe the types of liquid-crystalline phases adopted by lipids in water, and the factors which control phase stability. Even single lipid systems can display a quite extraordinarily rich variety of liquid-crystalline phase structures upon varying the water content and/or the temperature. These different phases result from an optimization of the hydrophobic effect with a variety of intra- and intermolecular interactions, in combination with a number of geometric packing constraints. Examples of lyotropic phase structures are the fluid lamellar L_a phase, fig. 1a, the inverse hexagonal H_II phase, fig. 1b, and the inverse bicontinuous cubic phase of crystallographic spacegroup Pn3m, fig. 1c.

In this chapter we will deal primarily with the fluid lyotropic phases, since these are likely to be of the most direct relevance to the structure and function of biomem-branes. The properties of lamellar phases (those based on lipid bilayers) are discussed extensively in various other chapters in this volume; therefore, in this chapter the emphasis will rather be on the various non-lamellar phases, whose roles in biomembrane structure and function are still controversial and poorly understood. Furthermore we will focus attention on biological lipids such as phospholipids, rather than dealing with all surfactant systems. However, much of the behaviour described here is of quite general relevance to lyotropic systems: most if not all of the structures formed by biological lipids can also be observed in simpler surfactants, under appropriate
conditions.

Although biopolymers such as DNA also form lyotropic liquid crystalline phases, the mechanisms are quite distinct from those applying to amphiphiles, and such systems will not be described here. Much of the experimental work on lipid systems has been performed on pure, well-defined synthetic lipids, although natural lipid extracts from membranes often exhibit the same phase structures, notwithstanding the fact that they usually consist of very complex mixtures of different lipids.

The structures of the translationally ordered lipid phases have been reviewed a number of times [1-10], and these articles should be consulted for further details. A recent issue of Chemistry and Physics of Lipids [11] was devoted to the subject of lipid polymorphism, and an issue of Journal de Physique [12] to geometry and interfaces, relating mainly to lyotropic liquid crystals. Books have appeared on the subject of phospholipid bilayers [13] and the physics of amphiphilic layers [14, 15], and theoretical approaches to membrane conformations have been recently reviewed [16, 17].
 

Fig. 1a. Examples of lyotropic structures: La fluid lamellar phase.
Fig. 1b. Examples of lyotropic structures: inverse hexagonal H II phase (from [1]).
Fig. 1c. Examples of lyotropic structures: inverse bicontinuous cubic phase Pn3m (from [2]).

 
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