Lecture 2: Lipid Bilayer Stabilization Forces and Liposome Technology

Lipid Bilayer Fundamentals and Molecular Composition

Primary Membrane Constituents - Phosphatidylcholine (PC) is identified as the most common phospholipid found in cell membranes. - Biological membranes are fundamentally composed of phospholipids, creating a specialized structure known as the lipid bilayer. - Phospholipids are amphiphathic molecules, meaning they possess both a hydrophilic (water-attracting) head and a hydrophobic (water-avoiding) tail.
Head Group Composition in Mammalian Cell Membranes - The composition of head groups in mammalian cell membranes is measured in weight percent. Key types include: - PC: Phosphatidylcholine. - PE: Phosphatidylethanolamine. - PS: Phosphatidylserine. - PI: Phosphatidylinositole. - SM: Sphingomyeline. - CL: Cardiolipin.
Hydrophilicity vs. Hydrophobicity - Hydrophilic molecules: Attracted to water. An example provided is acetone ( $CH_3-C=O-CH_3$ ). In a water environment, the polar nature of acetone allows for favorable interactions with water molecules. - Hydrophobic molecules: Tend to avoid water. An example provided is 2-methylpropane ( $HC(CH_3)_3$ ). When 2-methylpropane is in water, it does not form favorable interactions, leading to its avoidance of the aqueous phase.

Physical Properties and Membrane Fluidity

Basic Bilayer Structure - The basic structure of biological membranes consists of a phospholipid bilayer. - Hydrophobic tails are sequestered in the center of the bilayer. - Hydrophilic heads are positioned at the surfaces, interacting with the aqueous environment.
Two-Dimensional Fluidity and Lipid Motion - The bilayer acts as a two-dimensional fluid where membrane lipids are constantly in motion. - Lateral Diffusion: Phospholipids move rapidly within the plane of the bilayer. - Trans-bilayer movement (Flip-flop): Movement from one leaflet of the bilayer to the other. - Rotation: Rapid rotation around a central axis. - Flexion: Constant bending and movement of the fatty acyl tails.
Factors Affecting Fluidity - Fluidity is heavily dependent on lipid composition. - Saturated lipids: These pack much closer together, reducing fluidity. - Unsaturated lipids (or mixtures with saturated lipids): The presence of "kinks" in unsaturated chains prevents close packing, increasing fluidity.
The Role of Cholesterol - Cholesterol consists of a Head, a Steroid ring structure, and a Chain. - Movement Restriction: It restricts the movement of phospholipids, thereby reducing membrane fluidity. - Packing Disruption: It disrupts the regular packing of hydrocarbon chains, which actually increases flexibility. - Permeability Reduction: It reduces the permeability of the membrane to hydrophilic, water-soluble molecules and ions like sodium ( $Na^+$ ) and hydrogen ( $H^+$ ). - Heterogeneity: It reduces the segregation of different phospholipids, making the membrane lipid composition more heterogeneous.

Thermodynamics of Self-Assembly and Phase Transitions

Micelle vs. Liposome Formation - Micelles: Formed by single-chain phospholipids. Self-assembly into micelles starts at a specific concentration known as the Critical Micellar Concentration (CMC). - Liposomes: Formed by two-chain phospholipids. These molecules prefer the liposome configuration over micelles because the energy cost is lower. In a micelle configuration, two-chain lipids cannot pack as closely as single-chain lipids.
Crystalline Phases and Order - Gel Phase ( $L_{\beta'}$ ): A crystalline lipid phase with three-dimensional order. Chains are tilted, and the angle depends on the polar head group. - Ripple Phase ( $P_{\beta'}$ ): A partially melted phase representing a pre-transition state. - Fluid Phase ( $L_{\alpha}$ ): Chains are disordered and the ordering of the head groups is also lost.
Lipid Melting and Calorimetry - Melting involves a transition from solid-ordered to liquid-disordered states. - From the side: A loss in chain order occurs. - From the top: A loss in lattice order occurs. - Thermodynamic Parameters: Melting is characterized by enthalpy ( $\Delta H$ ), entropy ( $\Delta S$ ), and the melting temperature ( $T_m$ ). - Calorimetric Profiles: Heat capacity ( $\Delta c_p$ in $J/mol \cdot K$ ) is plotted against temperature to identify transitions. - DMPC (dimyristoyl phosphatidylcholine): Shows a pre-transition (ripple phase) and a main transition (fluid phase). - Comparative profiles for diacylphosphatidylcholines include DMPC, DPPC, and DSPC, showing distinct melting peaks at different temperatures. - Biological Example: E. coli membranes grown at different temperatures (e.g., $15^{\circ}C$ and $37^{\circ}C$ ) show shifted phase transition profiles to maintain functional fluidity.

Intermolecular Forces and Chemical Bonding

Atomic Interactions - Chemical bonds involve outer valence electrons. - Types include Covalent bonds (Polar and Nonpolar) and Ionic bonds.
Charge-Charge Interactions - All interactions between molecules are ultimately charge-charge interactions. - Coulomb’s Law Force: $F = k \frac{q_1 q_2}{r^2}$ where $F$ is the force, $q_1$ and $q_2$ are charges, $r$ is distance, and $k ≈ 8.98755 \times 10^9 \, Nm^2/C^2$ . - Potential Energy: $U = \frac{q_1 q_2}{\epsilon r}$ where $U$ is potential energy and $\epsilon$ is permittivity or the dielectric constant.
Hydrogen Bonding in Water - Water molecules ( $H_2O$ ) exhibit a specific structure with a bond angle of approximately $104.45^{\circ}$ . - Hydrogen bonds form between the partial negative charge ( $\delta^-$ ) of oxygen and the partial positive charge ( $\delta^+$ ) of hydrogen on adjacent molecules.
Dipole and Van der Waals Forces - Force Types: Charge-Dipole, Dipole-Dipole, and Induced Dipoles (Charge-induced or Dipole-induced). - Thermal Averaging: The potential energy of charge-dipole interactions is influenced by the Boltzmann constant and Absolute temperature. - Dispersion Forces: Also known as stacking forces, these are caused by electron cloud fluctuations, especially in aromatic rings or alkanes like Octane ( $C_8H_{18}$ ) and Hexane ( $C_6H_{14}$ ). - Steric Forces: Result from molecules "bumping" into each other (steric repulsion). This is modeled by the Lennard-Jones potential.
Hydrophobic Interaction Thermodynamics - Summarized by the phrase "Oil and water don't mix." - Gibbs Free Energy equation: $\Delta G = \Delta H - T \Delta S$ - In hydrophobic interactions, $\Delta G$ increases, making mixing thermodynamically unfavorable.

Liposome Structure, Composition, and Classification

Etymology and Basic Definition - Derived from Greek: "Lipo" (fat) and "soma" (body). - A liposome is a vesicle where an aqueous medium is encapsulated by a lipid bilayer. - It contains a hydrophobic region (within the membrane) and a hydrophilic region (the aqueous core and exterior).
Lipid Components - Natural Phospholipids: Phosphatidylethanolamine, Phosphatidylcholine, Phosphatidylserine. - Synthetic Phospholipids: Dioleoyl phosphatidylcholine, Disteroyl phosphatidylcholine, Dioleoyl phosphatidylethanolamine. - PEGylated Lipids: Polyethylene glycol (PEG) chains attached to a lipid base. These create a "stealth" effect, forming a stabilizing "cloak" that prevents immune system detection and prevents nanoparticle clumping. - Cholesterol: Included to modulate membrane properties.
Structural Classification - Unilamellar Vesicles: Single bilayer. - Small (SUV), Medium (MUV), Large (LUV), Giant (GUV). - Oligolamellar Vesicles: A few bilayers. - Multilamellar Vesicles (MLV): Multiple concentric bilayers. - Multivesicular Vesicles: Vesicles within vesicles.
Functional/Compositional Classification - Conventional Liposomes: Lecithin mixtures or synthetic identical chains. - Specialty Liposomes: - pH-sensitive, Cationic, Fusogenic. - Long-circulating (PEGylated). - Immuno-liposomes (antibody-directed). - Coated liposomes (carbohydrate or lipoprotein coated).
Mechanisms of Action/Formation - Endocytosis: Phagocytic cells (like neutrophils) take up liposomes. - Adsorption: Non-specific electrostatic forces or interaction with cell surface components. - Fusion: Insertion of the liposomal bilayer into the plasma membrane, releasing content into the cytoplasm. - Lipid Exchange: Transfer of liposomal lipids to the cell membrane without releasing encapsulated contents.

Manufacturing and Preparation Methods

Thin-Film Hydration Method 1. Dissolve lipids in organic solvent (chloroform or methanol). 2. Evaporate solvent under reduced pressure to form a thin lipid film. 3. Hydrate the film with an aqueous buffer and vortex/sonicate to form MLVs. 4. Optional: Reduce size to SUVs via extrusion or sonication.
Reverse Phase Evaporation Method - Used for high encapsulation efficiency. - Creates a water-in-oil emulsion by evaporating organic solvent from a lipid-aqueous mixture.
Sonication Method - Application of high-frequency ultrasound to downsize MLVs into SUVs.
Extrusion Method - Passes MLV suspension through polycarbonate filters with specific pore sizes to achieve uniform size and narrow distribution.
Detergent Removal Method - Used for hydrophobic substances. Lipids and drugs are dissolved in detergent, which is then removed via dialysis or chromatography.
Freeze-Thawing Method - Repeated cycles of freezing (below transition temperature) and thawing to enhance stability and entrapment while reducing leakage.

Characterization and Evaluation of Liposomes

Morphology and Structural Analysis - TEM (Transmission Electron Microscopy): High-resolution images of size, shape, and lamellarity. - SEM (Scanning Electron Microscopy): Surface information/morphology.
Size and Surface Charge - DLS (Dynamic Light Scattering): Measures particle size and polydispersity. - NTA (Nanoparticle Tracking Analysis): Tracks and sizes individual liposomes. - Zeta Potential: Determined via Electrophoretic Light Scattering to assess surface charge and colloidal stability.
Chemical and Biological Evaluation - HPLC (High-Performance Liquid Chromatography): Identifies and quantifies lipid composition. - Encapsulation Efficiency: Measured using UV-Visible or Fluorescence Spectroscopy. - Stability: Monitoring changes in size and zeta potential over time and under various pH/temperature conditions. - Drug Release Kinetics: In vitro studies (e.g., Dialysis Method or Franz Diffusion Cell for transdermal modeling). - Cell Studies: Cytotoxicity assays (MTT, Alamar Blue) and uptake studies. - In vivo Studies: Animal models for pharmacokinetics and biodistribution. - Hemolysis Assay: Checks for red blood cell damage. - Surface Characterization: XPS and FTIR spectroscopy to verify surface modifications.

Advantages, Disadvantages, and Applications

Advantages - Targeted drug delivery (reduces side effects). - Improved bioavailability for poorly soluble drugs. - Sustained release and protection of sensitive compounds from enzymes or pH changes. - High biocompatibility and low toxicity. - Versatility in size and surface modification.
Disadvantages - Storage instability (leakage, aggregation). - Scaling up production is difficult and costly. - Short circulation half-life (can be cleared quickly by the immune system). - Limited drug loading capacity. - Regulatory approval is complex and time-consuming.
Diverse Applications - Cancer Therapy: Formulations like Doxil (liposomal doxorubicin). - Vaccines: Used as adjuvants to enhance immune response. - Cosmetics: Controlled release of vitamins and antioxidants in skincare. - Gene Delivery: Transporting DNA/RNA into cells. - Food Technology: Protecting flavors and vitamins to enhance shelf life. - Diagnostics: Carriers for MRI or ultrasound contrast agents. - Other: Transdermal delivery, veterinary medicine, environmental remediation, and nutraceuticals.