What Are Amphipathic Molecules? Definition, Properties, and Functions

Amphipathic molecules are chemical compounds that have both polar and nonpolar regions, giving them both hydrophilic (water-loving) and lipophilic (fat-loving) properties. Amphipathic molecules are also known as amphiphilic molecules or amphiphiles. The word amphiphile comes from the Greek words amphis, which means "both," and philia, which means "love." Amphipathic molecules are important in chemistry and biology. Examples of amphipathic molecules include cholesterol, detergents, and phospholipids.

Structure and Properties

An amphipathic molecule has at least one hydrophilic portion and at least one lipophilic section. However, an amphiphile may have several hydrophilic and lipophilic parts.

The lipophilic section is usually a hydrocarbon moiety, consisting of carbon and hydrogen atoms. Lipophilic portions are hydrophobic and nonpolar.

The hydrophilic group can be charged or uncharged. Charged groups may be cationic (positively charged), such as the ammonium group (RNH₃⁺). Other charged groups are anionic, such as carboxylates (RCO₂⁻), phosphates (RPO₄^2-), sulfates (RSO₄⁻), and sulfonates (RSO₃⁻). Examples of polar, uncharged groups include alcohols.

Amphipaths may partially dissolve in both water and non-polar solvents. When placed in a mixture containing water and organic solvents, amphipathic molecules partition the two phases. A familiar is example is the way liquid dishwashing detergent isolates oils from greasy dishes.

In aqueous solutions, amphipathic molecules spontaneously assemble into micelles. A micelle has lower free energy than free-floating amphipaths. The polar portion of the amphipath (the hydrophilic part) forms the outer surface of the micelle and is exposed to water. The lipophilic portion of the molecule (which is hydrophobic) is shielded from the water. Any oils in the mixture are isolated within the interior of the micelle. Hydrogen bonds stabilize the hydrocarbon chains within the micelle. Energy is required to break a micelle apart.

Amphipaths can also form liposomes. Liposomes consist of an enclosed lipid bilayer that forms a sphere. The outer, polar portion of the bilayer faces and encloses an aqueous solution, while the hydrophobic tails face each other.

Examples

Detergents and soaps are familiar examples of amphipathic molecules, but many biochemical molecules are also amphipaths. Examples include phospholipids, which form the basis of cell membranes. Cholesterol, glycolipids, and fatty acids are amphipaths which also incorporate into cell membranes. Bile acids are steroid amphipaths used to digest dietary fats.

There are also categories of amphipaths. Amphipols are amphiphilic polymers that maintain membrane protein solubility in water without the need for detergents. The use of amphipols allows the study of these proteins without denaturing them. Bolaamphipathic molecules are those that have hydrophilic groups at both ends of a ellipsoid-shaped molecule. Compared to amphipaths with a single polar "head," bolaamphipaths are more soluble in water. Fats and oils are a class of amphipaths. They dissolve in organic solvents, but not in water. Hydrocarbon surfactants used for cleaning are amphipaths. Examples include sodium dodecyl sulfate, 1-octanol, cocamidopropyl betaine, and benzalkonium chloride.

Functions

Amphipathic molecules serve several important biological roles. They are the primary component of the lipid bilayers that form membranes. Sometimes there is a need to alter or disrupt a membrane. Here, the cell uses amphipathic compounds called pepducins that push their hydrophobic region into membrane and expose the hydrophilic hydrocarbon tails to the aqueous environment. The body uses amphipathic molecules for digestion. Amphipaths are also important in the immune response. Amphipathic antimicrobial peptides have antifungal and antibacterial properties.

The most common commercial use of amphipaths is for cleaning. Soaps and detergents both isolate fats from water, but customizing detergents with cationic, anionic, or uncharged hydrophobic groups expands the range of conditions under which they function. Liposomes may be used to deliver nutrients or drugs. Amphipaths are also used to make local anaesthetics, foaming agents, and surfactants.

Sources

• Fuhrhop, J-H; Wang, T. (2004). "Bolaamphiphile". Chem. Rev. 104(6), 2901-2937.
• Nagle, J.F.; Tristram-Nagle, S. (November 2000). "Structure of lipid bilayers". Biochim. Biophys. Acta. 1469 (3): 159–95. doi:10.1016/S0304-4157(00)00016-2
• Parker, J.; Madigan, M.T.; Brock, T.D.; Martinko, J.M. (2003). Brock Biology of Microorganisms (10th ed.). Englewood Cliffs, N.J: Prentice Hall. ISBN 978-0-13-049147-3.
• Qiu, Feng; Tang, Chengkang; Chen, Yongzhu (2017). "Amyloid-like aggregation of designer bolaamphiphilic peptides: Effect of hydrophobic section and hydrophilic heads". Journal of Peptide Science. Wiley. doi:10.1002/psc.3062
• Wang, Chien-Kuo; Shih, Ling-Yi; Chang, Kuan Y. (November 22, 2017). "Large-Scale Analysis of Antimicrobial Activities in Relation to Amphipathicity and Charge Reveals Novel Characterization of Antimicrobial Peptides". Molecules 2017, 22(11), 2037. doi:10.3390/molecules22112037