A hydrogen bond can be defined as the polar interaction between an electronegative atom (Nitrogen, oxygen or fluorine) and a hydrogen atom which is covalently bonded to another electronegative atom that is wither on the same molecuke, or on a different molecule. The bond is strongest when all three of these atoms are arranged in a way in which they can be linked along a straight line.
Hydrogen bonding is extremely prevalent throughout nature and can be found in water, DNA base-pair interactions, protein folding, protein structure and protein-ligand binding.
The simplest example of a hydrogen bond can be found in water molecules. A water molecule consists of one oxygen atom attached to two hydrogen atoms. A hydrogen bond can be formed between two molecules of water. In the case of liquid water where there are many water molecules present, each water molecule could potentially hydrogen bond with up to 4 other molecules (2 through its 2 hydrogen atoms with each hydrogen bonding to another oxygen and another 2 through its 2 lone pairs on the oxygen that can hydrogen bond to 2 other hydrogen atoms).
Although water has a low molecular mass, it has an unusually high boiling point. This property can be attributed to the large amount of hydrogen bonds that exists within water. Since these bonds are difficult to break, water’s melting and boiling points are relatively high in comparison to other liquids that are similar but lack the hydrogen bonding.
In the DNA helix,the bases: adenine, cytosine, thymine and guanine are each linked with their complementary base by hydrogen bonding. Adenine pairs with thymine with 2 hydrogen bonds. Guanine pairs with cytosine with 3 hydrogen bonds.