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Hydrogen bonding or H-bonding or H-bond is a dipole-dipole interaction between “hydrogen” attached to a highly electronegative atom through a covalent bond and a highly electronegative atom. There are three elements with higher electronegativity than the rest of the elements. These elements are N, O, and F capable of forming a hydrogen bond.
Hydrogen atoms attached to these elements are able to form hydrogen bonding with these elements. Examples of the molecule capable of forming H-bonding are HF, NH3, H2O, organic acids like CH3COOH, alcohol, aldehyde, and biomolecules such as carbohydrates, proteins, nucleic acids and some lipids like phospholipids, etc.
There are two types of hydrogen bonding; intermolecular H-bonding and intramolecular H-bonding. H-bonding involved in between two molecules is called intermolecular H-bonding while H-bonding between hydrogen and electronegative atoms within the single-molecule is called as intramolecular H-bonding.
Hydrogen bonding among water molecules or among water molecules and alcohol/organic acids/ or ammonia is called as intermolecular H-bonding. However, H-bonding between the hydroxy group and Nitro-group in O-Nitro phenol, or H-bonding within the nucleic acids, proteins are called as intramolecular H-bonding.
Hydrogen bonding has great importance in the solubility of compounds and is one of the major non-covalent interactions between the biomolecules inside the cellular environment. Biomolecules like proteins, nucleic acids and carbohydrates are folded and organized in the three-dimensional structures due to intermolecular as well as intramolecular H-bonding along with the other non-covalent interactions. DNA is in a double stranded form, this is due to the H-bonding between the bases of complementary strands.
Sugar like monosaccharide, oligosaccharide, and some polysaccharides like starch and glycogen are soluble in water due to their ability to form hydrogen bonding with water molecules. Similarly, amino acids and some proteins like globular proteins are also soluble in water due to the H-bonding.
Based on the ability to form hydrogen bonds, solvents are classified as a polar solvent and non-polar solvent. Solvents with the ability to form hydrogen bonding with solute molecules are called polar solvents while rests are called non-polar solvents.
There are some polar solvents like water, liquid ammonia, hydrogen fluoride, acetic acid, ethanol, etc. However, based on their degree of polarity they can also be categorized as less polar and highly polar solvents. Liquid ammonia is the least polar solvent while water is highly polar solvent.
H-bonding also plays an important role in biochemical reactions. Macromolecules in the form of nutrients are digested in the gastrointestinal tract by different enzymes involve H-bonding between the substrate and a key amino acid residue present in the active site of the enzyme.
The key amino acid residue in the active site of an enzyme interacts with the substrate in many types of reactions to perform catalysis leading to the formation of products. The cytosol is also an aqueous environment and different proteins and biomolecules are dissolved in the cytosol are only possible due to the formation of the hydrogen bond with water molecules of the cytosol.
The same is true with membrane-bound vesicles like mitochondria, nucleus, Golgi complex and endoplasmic reticulum. Therefore, H-bonding is one of the most important non-covalent interactions, necessary for living beings.