Paper 4. Biomolecules and their interactions Module 22: Aggregates of lipids: micelles, liposomes and their applications OBJECTIVE

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1 Paper 4. Biomolecules and their interactions Module 22: Aggregates of lipids: micelles, liposomes and their applications OBJECTIVE The main aim of this module is to introduce the students to the types of aggregates that lipids can attain when mixed with water. The ability of lipids to form micelles has implications in the health and washing industry, which will also be discussed here. The applications of one such lipid aggregate called liposomes will be discussed. 1. INTRODUCTION All lipid derivatives such as the phospholipids, sphingolipids, sterols etc are hydrophobic, with virtually zero solubility in water. Therefore, when these lipids are placed in an aqueous solvent, they cluster together forming microscopic lipid aggregates. In a lipid aggregate, the hydrophobic component of lipids are in contact with each other, while the polar head groups are in contact with the aqueous solvent. This type of clustering results in an increase in the entropy by minimizing the number of molecules exposed to water at the lipid-water interface. The thermodynamic driving force for the formation of these clusters comes from the hydrophobic interactions between the lipids. Lipids can form three types of lipid aggregates, depending on the nature of the lipids and the solvent conditions (Figure 1). Figure 1 2. LIPID AGGREGATES 2.1. Micelles Micelles are a type of lipid aggregate, which may contain a few dozen to a few thousand amphiphilic molecules. 1

2 The hydrophobic regions of the lipid molecules aggregate towards the interior excluding water while their hydrophilic head groups are on the surface, in contact with water. Micelles are formed when the cross sectional area of the head group is greater than the acyl side chains as in free fatty acids or in lysophospholipids or in detergents such as sodium dodecyl sulfate (SDS). Ionized fatty acids form micelles. Micelles form only after a critical micelle concentration is reached (dependent on the amphiphile). For short tails (dodecyl sulfate) need higher conc. (1 mm) whereas for longer hydrophobic tails in biological lipids need a lower concentration (<10-6 ) (Figure 2). Evidence for the presence of a liquid-like interior in the micelles exist because of their ability to dissolve some hydrophobic substances in them. Two or three amphipathic molecules cannot form micelles, instead micelle formation is considered to be a cooperative process with simultaneous involvement of both the amphipathic molecules/ions and water. Figure Surfactant in medicines Surfactants have a head region that is water loving and a oily tail region that is water-hating. In order to accommodate these two regions, these molecules tend to sit at interfaces between water and another phase such as oil or air! The tendency of amphipathic molecules to form micelles lends them the ability to function as surfactants. Therefore, micelles are of interest to scientists. Surfactant molecules will expand the surface and lower the surface tension (contracting forces). 2

3 Surfactant is the natural detergent in the lungs that reduces alveolar surface tension and prevents atelectasis. Atelectasis is defined as the collapse of lung tissue affecting part or all of one lung, preventing normal oxygen absorption to healthy tissues. Dipalmitoyl phosphatidylcholine (DPPC) serves as the natural surfactant in lung alveoli. It is the protein-lipid mixture that is a requisite for pulmonary function. The inside of the lung alveoli is a thin film of liquid, which is highly curved as individual alveoli are very small, so alveoli have a tendency to collapse about the high surface tension % of the surfactant in the lung is the dipalmitoyl species of phosphatidylcholine. The saturation in the palmitoyl chains of the DPPC allows them to extend in a straight form, which facilitates a very tight packing of these lipids in the lung alveoli. The DPPC molecules are arranged in such a way that their non-polar tails are facing the air, while the polar heads are facing the alveolar cells. During expiration, the surface area and the volume of the alveoli decreases, due to which the lungs can easily collapse. This is prevented by DPPC molecules that have the ability to withstand the compression, caused during expiration. This film is rich in PC and maintains the surface tension at very low levels, preventing the alveoli from collapsing! The synthesis of the surfactant is a continuous process, which are used upon synthesis and recycled by the alveolar cells. Respiratory distress syndrome is a condition that is seen in premature infants due to lack of enough surfactant in their lungs. This condition is treated by giving the infants exogenous surfactants Surfactant in washing industry All soaps and detergents contain a surfactant as their active ingredient. Surfactants are either derived from petrochemicals, vegetable oils or animal fats, or combinations of these sources. There are three main types of surfactants used in laundry detergents: anionic, non- ionic and cationic. What do the surfactants do? Surfactants are the active cleaning agents that perform three major roles: penetrating and wetting fabric loosening soils (assisted by the mechanical action of the washing machine) emulsifying soils and keeping them suspended in the wash solution 3

4 How do the surfactants work? Surfactants consist of two domains within the one molecule: (a) a polar, or hydrophilic ( water-loving ) head group, and (b) a non-polar, fatty or hydrophobic ( water-hating ) tail. The basic principle behind the ability of the surfactant to work during cleaning of fabrics is that polar substances interact well with other polar substances, and non-polar substances interact well with other non-polar substances. Water has a high surface tension, that is, it resists distortion at its surfaces (water-air, water-oil, water-solid). Upon addition of a detergent, the surfactant molecules accumulate near the surface of the water because the non-polar (hydrophobic) tail of the surfactant wants to get away from the water. The water distorts as the surfactant disrupts the bonding of water molecules due to which more surfactant molecules fit near the surface. The reduced surface tension of the water then allows it to permeate the otherwise non-wettable surfaces, such as fabrics (Figure 3). Surfactant molecules on surface More number of surfactant molecules on surface Water surface Surfactant molecules in water Distorted water surface Figure 3 Loosening and emulsifying dirt (Figure 4) Since oil is a non-polar substance and water is a polar substance, oil and water do not usually mix. However, oil can effectively be evenly suspended/emulsified in water in the presence of a high enough concentration of surfactant molecules. In this situation, the hydrophilic part of the surfactant molecule interacts with water, while the non-polar part surrounds the oil. Micelle formation occurs around the oil, once there is enough accumulation of the surfactant around the oil. As a consequence, oil is evenly suspended in water. 4

5 Figure Phospholipid- bilayers Hydrophobic intermolecular interactions can also lead to the formation of socalled bilayers, instead of forming sphere-like micelles. These are formed by tail-to-tail interactions of the hydrophobic chains between two different layers, so that water-insoluble films are spread out within the liquid phase. In case of glycerophospholipids and sphingolipids, where the cross-sectional area of the head group and the acyl side chain are similar, bilayer formation is favored (Figure 5). Bilayer formation is the structural basis for biological membranes, wherein proteins are associated with the lipid bilayer. The biological membranes are impermeable to polar substances. Figure Lipid distribution in cell membrane Lipids on the plasma membrane are unequally distributed within the two monolayers. 5

6 Phosphatidylcholine (PC), sphingomyelin are predominant in the outer leaflet of the plasma membrane of the erythrocytes whereas PE, PS and PI are predominant in their inner leaflet. Changes in membrane asymmetry are perceived as primary signals for initiating some cellular processes. For example, exposure of PS on the outer surface is a signal for cells to undergo programmed cell death OR a platelet is enabled to participate in the clotting of blood only when PS is exposed on its outer surface. Movement of lipids within the monolayer is termed as lateral diffusion, which is a rapid process. Transverse diffusion is movement of lipids between the two monolayers, a slow process facilitated by specific proteins called phospholipid translocators. Membrane asymmetry is maintained by these phospholipid translocators Lipid fluidity in the membranes Plasma membranes are fluid in nature. Fluidity is maintained by change in temperature. Each lipid bilayer has a (Phase) Transition temperature T m (melting temperature). T m increases with increasing chain length (butter vs. cooking oil). T m decreases with increasing degree of unsaturation (poor packing of bent tails). T m modulated by cholesterol: broadens effective T m : Lowers T m at high temp., increases T m at low temp. Most organisms can regulate fluidity by modulating their lipid composition. For example by changing the ratio of 16C to 18C chain length or by changing the degree of unstauration. Lipids in a bacterial membrane can diffuse the 1μm length of their membranes in approximately 1s. Since the lipids are mobile, the lipid bilayer is also referred to as a two-dimensional fluid. Due to the rotations around the C-C bonds of the fatty acyl chains of the lipids, the inside of the lipid bilayer is in constant motion. The viscosity of the inside of the bilayer is similar to that of a light machine oil! Molecular dynamics simulations can predict the nature of the bilayer core by calculating the forces acting on the atoms with time. The viscosity of the bilayer, near the polar head groups increases due to restricted rotational movements around the head groups. The lipids within the bilayer bounce to some extent in natural biological membranes. 6

7 The polar nature of the outer surface of the bilayer begins from the head groups and continues to the carbonyl groups of the ester and amide bonds that link the fatty acyl chains. As a result, the water molecules can penetrate only 15 Å deep within the lipid bilayer, thus evading the approximately 30 Å deep hydrophobic core Cholesterol lends rigidity to the cell membrane Cholesterol intercalates between both the monolayers of the membrane (Figure 6). Figure 6 The cholesterol molecule orients itself within the monolayers in such a way that a. that its single hydroxyl group is close to the polar head group of a neighboring phospholipid molecule AND b. This hydroxyl group forms a hydrogen bond with the oxygen of the ester bond between the glycerol c. backbone and a fatty acid. 7

8 2.3. Vesicles In the bilayer sheet, the hydrophobic regions at the edges are in contact with water, which makes these sheets relatively unstable. Hence, the bilayer sheet spontaneously folds back on itself to form hollow spheres with an aqueous interior. These spheres are called vesicles. The formation of vesicles minimizes the exposure of the hydrophobic regions to the aqueous environment within the bilayer and maximizes the stability of the bilayer Liposomes- bilayered vesicles Liposomes are bilayered vesicles that contain an aqueous interior (Figure 7) and were discovered by Alec Bangham and his group. They are generated artificially in the laboratory using pure lipid solutions. Figure 7 The membrane bilayer of the liposomes may contain natural or synthetic lipids. Liposomes can encapsulate water-soluble drugs in their interior and lipid soluble drugs in their membrane. The membrane bilayer of the liposomes resembles the biological membranes and hence they are considered ideal for drug delivery in humans Advantages/Disadvantages of liposomes Advantages 8

9 The toxicity of the drug/encapsulated agent is reduced, which increases the therapeutic index of the drug. Selective targeting to tumor tissues can be done. They facilitate slow and sustained delivery of the drugs. They can be made into a variety of sizes Disadvantages The encapsulated agent may leak from the liposomes. The liposomes can be cleared from the human body by the reticuloendothelial system. There could be batch-to-batch variation in making the liposomes. Large scale synthesis and sterilization of the liposome preparation is difficult. MLV SUV MVV LUV Therapeutic applications of liposomes in drug formulations Many drugs that are marketed are available as liposomal formulations. One such example is the antifungal drug, amphotericin B that is marketed for treatment of fungal infections. Table 1 lists all the marketed drugs that are sold as liposome formulations. This reinforces the importance of liposomes as drug delivery vehicals. Table 1 9

10 Summary Lipids form aggregates in aqueous solvents. Micelles, bilayer sheets and vesicles are different types of lipid aggregates. Micelle forming ability of lipids has application in medicine and in the washing industry. Bilayer formation ability of lipids allow them to serve as constituents of all biological membranes. Vesicles are bilayer sheets folded back on itself to form a hollow bilayered sphere. Liposomes are artificially generated vesicles with an aqueous interior and with therapeutic implications. 10