Biological membrane phase transition

Hydrated bilayers containing one or more lipid components are commonly employed as models for biological membranes. These model systems exhibit a multiplicity of structural phases that are not observed in biological membranes. In the state that is analogous to fluid biological membranes, the liquid crystal or La bilayer phase present above the main bilayer phase transition temperature, Ta, the lipid hydrocarbon chains are conforma-tionally disordered and fluid ( melted ), and the lipids diffuse in the plane of the bilayer. At temperatures well below Ta, hydrated bilayers exist in the gel, or Lp, state in which the mostly all-trans chains are collectively tilted and pack in a regular two-dimensional... 

Native biological membranes also display characteristic phase transitions, but these are broad and strongly dependent on the lipid and protein composition of the membrane. 

Fourier Transform Infra Red Spectroscopy, Arrhenius plots of rate vs. temperature of a membrane-linked phenomenon) that biological membranes from nonhibemat-ing or cold acclimated animals show a phase transition around 12 °C to 17 °C. Thus, at useful cold storage temperatures, it is expected that the plasma membrane and membranes of the cellular organelles will be mostly in a gel or solid state. 

Crowe, L.M. Crowe, J.H. (1986). Hydration-dependent phase transitions and permeability properties of biological membranes. In Membranes, Metabolism and Dry Organisms, ed. A.C. Leopold, pp. 210-30. Ithaca, N.Y. Comstock Publishing Associates. 

This chapter describes some of the properties of solids that affect transport across phases and membranes, with an emphasis on biological membranes. Four aspects are addressed. They include a comparison of crystalline and amorphous forms of the drug, transitions between phases, polymorphism, and hydration. With respect to transport, the major effect of each of these properties is on the apparent solubility, which then affects dissolution and consequently transport. There is often an opposite effect on the stability of the material. Generally, highly crystalline substances are more stable but have lower free energy, solubility, and dissolution characteristics than less crystalline substances. In some situations, this lower solubility and consequent dissolution rate will result in reduced bioavailability. 

Biological membranes Fluidity and order parameters Determination of the phase transition temperature Effect of additives (e.g. cholesterol)... 

The vast majority of biological membranes are in the liquid-crystalline phase. There are many experimental studies on model bilayer phase behavior [3]. Briefly, at low temperatures lipid bilayers form a gel phase, characterized by high order and rigidity and slow lateral diffusion. There is a main phase transition, as the temperature is increased, to the liquid-crystalline phase. The liquid-crystalline phase has more fluidity and fast lateral diffusion. 

Closed bilayer aggregates, formed from phospholipids (liposomes) or from surfactants (vesicles), represent one of the most sophisticated models of the biological membrane [55-58, 69, 72, 293]. Swelling of thin lipid (or surfactant) films in water results in the formation of onion-like, 1000- to 8000-A-diameter multilamellar vesicles (MLVs). Sonication of MLVs above the temperature at which they are transformed from a gel into a liquid (phase-transition temperature) leads to the formation of fairly uniform, small (300- to 600-A-diameter) unilamellar vesicles (SUVs Fig. 34). Surfactant vesicles can be considered to be spherical bags with diameters of a few hundred A and thickness of about 50 A. Typically, each vesicle contains 80,000-100,000 surfactant molecules. 

Natural biological membranes were studied from 0 to 50 C 334). The plasma membrane of the microorganism Acholeplasma laidlawii was compared with a model lipid (l,2-diperdeuteropalmitoyl-sn-glycero-3-phosphocholine). The phase transition... 

Since phase transitions take place in biological systems, as for example in membranes, it may be quite fruitful to investigate precipitation bands and their electrical manifestations in biological systems, especially the ability of electrical fields, possibly generated by the organism itself, to regularize a precipitation pattern. 

Emphasis is placed here on features of the biological membranes which are implicated in substrate transport. The lipid bilayer in the "gel" state, in the absence of additives, forms an effective barrier against polar ions and water soluble substrates. Changing the fluidity, by phase transition (induced by temperature changes and/or by the addition of foreign ions or molecules) or by the incorporation of additives (cholesterol, for example), profoundly influences the structure and, hence, the transport properties of membranes. This, and the presence of channel or pore forming peptides or proteins, opens the door to a number of transport mechanisms which will be summarized in the following section. 

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