HYDROGEN BOND
Hydrogen bond (hydrogen bridge, protonic bond, H-bond) is a weak bond. It appears when a hydrogen atom, which is chemically bonded with one electronegative atom, keeps near itself another electronegative atom. Hydrogen bonds appear both on the intramolecular level (interatomic interactions) and on the level of formation of intermolecular complexes (intermolecular interactions).
Hydrogen bond is of electrostatic nature. Hydrogen bond X—H···Y is formed as the result of attraction between a hydrogen atom H, which is covalently bound with X atom and possesses some positive charge (a donor), and a negatively charged covalently bound acceptor atom Y. In biological most important hydrogen bonds (between peptide groups, between hydroxyl groups, between a carboxyl group and a tyrosine hydroxyl, between a charged amino group and a charged carboxyl group) oxygen and nitrogen most often appear as atom X. Nitrogen and oxygen atoms also usually act as negatively charged acceptors. These are examples of hydrogen bonds, which are found in biological systems: O—H···N, O—H···O, N—H···O, N—H···N.
Unlike van der Waalse interactions, the hydrogen bond is sensitive to directions, especially to the direction of the donor group. Usually the valence bond of the donor faces the acceptor of the hydrogen bond. In the most firm hydrogen bonds the hydrogen atom is situated along a straight line connecting the donor and acceptor atoms. Hydrogen bonds can twist or expand easier, and as the result of this geometry of the system X—H···Y is changeable. The energy of a hydrogen bond, which is placed at an angle with the covalent bond, is expressed by a far less value.
Hydrogen bonds are characterized by low energy, and their firmness is 20-30 times weaker than that of covalent bonds, but mush higher than firmness of van der Waalse bonds. Owing to hydrogen bonds, secondary and tertiary protein structures, the bonds in the DNA double helix, etc., are formed.