Amphipathicity

Amphipathicity is the property of a molecule to contain both hydrophilic (polar, water-loving) and hydrophobic (nonpolar, water-fearing) regions. This dual character enables the molecule to interact with both aqueous and nonpolar environments, and often drives its placement at interfaces or its self-assembly into organized structures. The term derives from Greek elements meaning “both” and “feeling” or “affinity.”

In chemistry and biology, amphipathic molecules include phospholipids, glycolipids, and many detergents. Phospholipids, for example, have hydrophilic head groups and hydrophobic tails, causing them to form lipid bilayers in water. Detergents form micelles with a hydrophobic core. Through amphipathicity, these molecules reduce unfavorable interactions between water and nonpolar regions, promoting the formation of membranes, vesicles, and stable emulsions.

In proteins and peptides, amphipathicity arises from the uneven distribution of amino acid properties. An alpha helix can be amphipathic, presenting a hydrophobic face and a polar or charged face, which facilitates interactions with membranes or other macromolecules. The hydrophobic moment is a common metric used to quantify this directional distribution of hydrophobicity along a sequence.

Applications and significance of amphipathicity include its central role in membrane structure and protein–lipid interactions, as well as in the mechanism of action of surfactants and in the design of liposomes for drug delivery. It also influences solubility, emulsification, and the development of biomimetic materials.