Biophysics and Biomechanics - V. S. Antoniuk - 2012

Chapter 3. MOLECULAR BIOPHYSICS

3.4. Biological membranes

3.4.3. Physical state and phase transitions of lipids in membranes

Lipid-Water mixtures exhibit polymorphic properties. These can include the lamellar gel phase, lamellar liquid-crystalline phase, and hexagonal type II phase, each depending on lipid concentration, Temperature, pressure, Lipid Composition, Ionic strength, and acidity (Fig. 3.12).

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Fig. 3.12. Schematic representation of various phases of water-lipid systems: a – lamellar gel phase; b – lamellar liquid-crystalline phase; c – hexagonal type II phase; d – hexagonal type I phase

Under normal physiological conditions, membranes exist in a fluid state; however, unlike ordinary liquids, they possess an ordered Spatial Structure. Due to these properties, the membrane state is referred to as liquid-crystalline.

1. Lamellar liquid-crystalline phase L_α. This phase is characterized by an ordered arrangement of layered structures alongside a high degree of disorder in the acyl chains of the molecules. The bulk of Lipids in Biomembranes resides precisely in this phase.

2. Lamellar gel phase Lβ. As the temperature decreases, membranes transition from the liquid-crystalline state to the solid-crystalline (gel) state. This transition preserves the overall Membrane Structure, while increasing order within the system even further. Whereas in the fluid state the membrane area per lipid molecule is 0.58 nm , in the gel state this value drops to 0.48 nm . The membrane thickness increases upon transitioning to the solid-crystalline state, but due to the reduction in surface area, the overall volume of the membrane decreases. The molecules are packed more tightly (occupying less surface area per molecule), and the acyl chains are more ordered and exist entirely in the trans-conformation. The density and thickness of the bilayer in the gel phase are greater than in the liquid-crystalline phase (with the molecules maximally extended).

3. Hexagonal phase I (H_II). Lipid molecules form cylindrical structures whose surfaces are constituted by polar HEAD groups in contact with water. These cylinders are packed to form a hexagonal lattice.

4. Hexagonal phase II (H_II). Lipids also form cylinders, but the polar head groups of the lipids are oriented toward the interior of the cylinders, where water is located. The packing of the cylinders is likewise hexagonal.

The phase state, and most importantly the viscosity of Cell/29.html">The Lipid Bilayer of membranes, affects the catalytic activity of membrane Enzymes, membrane permeability, and consequently Cellular metabolic processes. The greater the mobility of phospholipid molecules, the higher the membrane viscosity and the better the permeability for diffusing substances. When the membrane bilayer transitions to the gel state, the lateral diffusion rate of Phospholipids decreases by more than two orders of magnitude (D < 10⁴ cm²/s).

The mobility of lipid molecules differs significantly between the two phase states. In the gel state, lipids are capable of performing only concerted vibrational or rotational movements. In the liquid state, lipid tails possess significantly greater freedom of movement, which is especially high within the interior of the membrane.

During a phase transition, membrane permeability increases for ions and low-molecular-weight compounds capable of passing through such pores. The phase transition in membranes does not occur instantaneously, but rather over a certain temperature range. The phase transition temperature is defined as the temperature at which one half of the Membrane Lipids are in the liquid-crystalline state and the other half are in the solid-crystalline state.

Thus, for normal functioning, a membrane must be in the liquid-crystalline state. At sufficiently low temperatures, the lipid bilayer exists in a quasi-crystalline state (gel state). As the temperature rises, a "gel–liquid crystal" transition is observed. The change in lipid properties occurs within a narrow temperature range characteristic of the phase transition seen during the melting of a solid. The phase transition temperature decreases as the degree of unsaturation of the bonds between carbon atoms in the hydrocarbon tails of the lipid molecules increases. For instance, the "gel–liquid crystal" phase transition temperature for membranes composed of unsaturated lipids can be as low as minus 20 °C, whereas for saturated lipids it can reach plus 60 °C. Therefore, prolonged exposure to low temperatures in microorganisms, PLANT AND ANIMAL Cells triggers an adaptive shift in membrane chemical composition to lower the phase transition temperature. It is hypothesized that the primary mechanism of cryogenic damage to biomembranes is influenced by their phase transition into the gel state. Studies on The Role of Cholesterol in biomembranes have shown that in the liquid-crystalline state of lipid mixtures, cholesterol restricts the conformational mobility of phospholipid chains. In the gel state, it hinders the optimal packing of chains into the full trans-conformation, reducing the attractive forces between the hydrocarbon chains of lipids. As a result, phospholipid–cholesterol mixtures occupy an intermediate position in terms of orderliness between the gel and liquid-crystalline states of pure phospholipids. Thus, the presence of cholesterol can dampen the membrane changes that accompany phase transitions.

"Gel–liquid crystal" type phase transitions occur at a temperature T_ph.t (Fig. 3.13), the magnitude of which depends on the water content of the system.

Fig. 3.13. Phase diagram of the egg lecithin mixture: II — two-phase system: water, bilayer; III — region

of existence of hexagonal structures; IV — gel; T_ph.t — phase transition temperature curve

Specifically, T_ph.t reaches a minimum when the total water content exceeds the amount that can be bound by the lipids (i.e., when lipid concentration is at a minimum).

However, at temperatures above T_ph.t and in conditions of water deficiency, lipids can remain in an ordered state.

In addition to "gel–liquid crystal" transitions, certain lipids (such as phosphatidic acid and phosphatidylserine) can undergo transformations leading to The formation of hexagonal structures. Elevated temperatures, reduced bilayer Hydration, increased unsaturation of fatty acid chains, and high ionic strength under alkaline pH conditions all promote the formation of hexagonal structures within the bilayer. The transition of specific regions into the hexagonal phase leads to a disruption of membrane integrity, the formation of permeability channels, etc.

Living organisms inhabiting different climatic environments feature varying ratios of saturated to unsaturated bonds in their lipid molecules, ensuring adaptation to cold or heat depending on their habitat. Moreover, this ratio varies across different body PARTS OF THE same Organism. For example [21], the temperature of a reindeer’s leg near the hoof can drop to minus 20 °C, while the temperature near the body reaches +30 °C. The cellular membranes do not undergo a phase transition because the membranes of cells near the hoof contain more unsaturated lipids, whereas those near the body contain more saturated lipids. It is well known that animal fats are typically solid at room temperature, whereas plant fats are liquid. This is because animal cells contain fewer unsaturated lipids than plant cells do.

One of the most common Methods FOR STUDYING phase transitions in membranes is microcalorimetry, which makes it possible to determine The amount of heat Q absorbed during the melting of a substance containing ν moles of lipids. Knowing Q, one can calculate the specific enthalpy of melting: ΔΗ = Q/v.

Thus, the melting of a substance occurs at a temperature T_m at which the Gibbs energy in the solid state (G_s = H_s - T_mS_s) is equal to the Gibbs energy in the liquid state (G_l = H_l - T_mS_l).