Amphipathic
 
Compounds that contain both highly polar and nonpolar, hydrophobic moieties are amphipathic. In the presence of water, these compounds tend to aggregate, forming structures that are strongly influenced by the relative extents of the polar and hydrophobic characters of the molecule, with the polar groups acting to shield the hydrophobic moieties from the solvent. In lipids, the hydrophobic group is the hydrocarbon domain of the aliphatic chain (see Lipids and Fatty Acids). Typical examples are (a) the sodium salts of fatty acids, which form micelles in water, and (b) phospholipids, which form bilayers in membranes. The presence of significant amounts of nonpolar amino acids, such as alanine, valine, leucine, isoleucine, proline, methionine, phenylalanine, and tryptophan, make certain proteins amphipathic; these amino acids are believed to increase the accessibility of the proteins to the hydrophobic domain of the membrane bilayer.a