Noncovalent bonding

A noncovalent bond is a type of chemical bond, typically between macromolecules, that does not involve the sharing of pairs of electrons, but rather involves more dispersed variations of electromagnetic interactions. The noncovalent bond is the dominant type of bond between supermolecules in supermolecular chemistry. Noncovalent bonds are critical in maintaining the three-dimensional structure of large molecules, such as proteins and nucleic acids, and are involved in many biological processes in which large molecules bind specifically but transiently to one another. The energy released in the formation of noncovalent bonds is on the order of 1-5 kcal per mol. There are four commonly mentioned types of non-covalent interactions: hydrogen bonds, ionic bonds, van der Waals forces, and hydrophobic interactions. The noncovalent interactions hold together the two strands of DNA in the double helix, stabilize secondary and tertiary structures of proteins, and enable enzyme-substrate binding and antibody-antigen association.

Overview
In general, noncovalent bonding refers to attractive intermolecular forces that are not covalent in nature. Noncovalent forces are dominant in supramolecular chemistry. These noncovalent interactions may include ionic bonds, hydrophobic interactions, hydrogen bonds and van der Waals forces (dispersion attractions, dipole-dipole and dipole-induced dipole interactions). Noncovalent bonds are weak by nature and must therefore work together to have a significant effect. In addition, the combined bond strength is greater than the sum of the individual bonds. This is because the free energy of multiple bonds between two molecules is greater than the sum of the enthalpies of each bond due to entropic effects.

Protein structure
Main article: Protein structure

Intramolecular noncovalent interactions are largely responsible for the secondary and tertiary structure of proteins and therefore the protein's function in the mechanisms of life. Intermolecular noncovalent interactions are responsible for protein complexes (quaternary structure) where two or more proteins function in a coherent mechanism.

Pharmaceuticals
Most drugs work by noncovalently interacting with biomolecules such as proteins or RNA. Relatively few drugs actually form covalent bonds with the biomolecules they interact with; instead, they interfere with or activate some biological mechanism through noncovalently interacting in very specific locations on specific biomolecules which present the perfect combination of noncovalent binding partners in just the right geometry.