Define peptide linkage.

Hint: A chemical bond which is formed between two molecules when the carboxyl group of one molecule reacts with the amino group of the other molecule is called as a peptide linkage. This usually occurs between amino acids and is a dehydration synthesis reaction or a condensation reaction.

Complete answer:
- A peptide linkage is a sort of amide bond which is formed between two molecules where an α-amino group of one molecule reacts with the α- carboxyl group of another molecule and a water molecule is being eliminated.
- The peptide linkage is also mentioned to as the isopeptide bond where the amide bond forms between the amino group of one amino acid and the carboxyl group of another amino acid at positions other than the alpha.
- As we know, amino acids are made up of basic or amino groups ($-N{{H}_{2}}$) and the acid group ($-COOH$) along with an alkyl chain which is unique to each amino acids. A peptide linkage will form when the carboxyl group one of one molecule reacts with the amino group of another molecule and the $CO-NH$ or peptide bond is formed.
-The above mentioned method of formation of the peptide bond is an example of a reaction that results in the removal of water or in other words condensation reaction.
- The Peptide linkages have a planar configuration and will experience a very tiny movement around the C-N bond but the other single bonds on either side of the C-N bond will exhibit a high degree of rotational motion.

Note: The difference between the terms glycosidic linkage and peptide linkage should be noted. A glycosidic linkage is made when two carbon atoms of two different monosaccharides are bonded together whereas a peptide linkage occurs when a carbon atom of one amino acid is bonded to a nitrogen atom of a different amino acid.