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Amino Acids

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Amino Acids

Amino acids are naturally occurring organic compounds that combine to form proteins. Amino acids and proteins are the building blocks of life. The term amino acid is short for α-amino [alpha-amino] carboxylic acid.

When proteins are digested or broken down, amino acids are left. The human body uses amino acids to make proteins to help the body.

Amino acids are the structural units (monomers) that make up proteins. They join together to form short polymer chains called peptides or longer chains called either polypeptides or proteins. 

Physical Properties

  • colorless, crystalline solids
  • have a high melting point greater than 200o
  • soluble in water, slightly soluble in alcohol and dissolve with difficulty in methanol, ethanol, and propanol
  • On heating to high temperatures, they decompose
  • All amino acids (except glycine) are optically active
  • Amino acids can connect with a peptide bond involving their amino and carboxyl groups

Chemical Properties

  • Amino acids show zwitterionic  properties 
  • Amino acids are amphoteric in nature 
  • The presence of α-amino acids can be detected by adding Ninhydrin solution in it
  • The xanthoproteic test is performed for the detection of aromatic amino acids (tyrosine, tryptophan, and phenylalanine) in a protein solution
  • Sanger’s reagent (1-fluoro-2, 4-dinitrobenzene) reacts with a free amino group in the peptide chain in a mild alkaline medium under cold conditions
  • Nitrous acid reacts with the amino group to generate nitrogen and form the corresponding hydroxyl


  • In particular, 20 very important amino acids are crucial for life as they contain peptides and proteins and are known to be the building blocks for all living things.
  • determines the three-dimensional configuration of a protein
  • Production of hormones 
  • Involved in structure of muscles 
  • Human nervous system’s healthy functioning 
  • The health of vital organs 
  • Normal cellular structure
  • Energy production
  • Helps to bind cells together into tissues
  • Involved in transport and the storage of nutrients.

Basic Structure 

All amino acids have the same basic structure. At the “center” of each amino acid is a carbon called the α carbon and attached to it are four groups – a hydrogen, an α- carboxyl group, an α-amine group, and an R-group, sometimes known as a side chain. The α carbon, carboxyl, and amino groups (backbone) are common to all amino acids, so the R-group is the only unique feature in each amino acid. With the exception of glycine, which has an R-group consisting of a hydrogen atom, all of the amino acids in proteins have four different groups attached to them and consequently can exist in two mirror image forms, L and D. 

Types/Classification of Amino Acids

More than 300 amino acids are found in nature but only 20 amino acids are standard and present in protein because they are coded by genes. Other amino acids are modified amino acids and called non-protein amino acids. The twenty amino acids each have assigned to them both three-letter codes (can be upper or lower case) and one-letter codes (upper case). These make it quicker and easier for notation purposes and are worth learning.

There are basically different classes of amino acids determined by different side chains;

  • Aliphatic: alanine, glycine, isoleucine, leucine, proline, valine
  • Aromatic: phenylalanine, tryptophan, tyrosine 
  • Acidic: aspartic acid, glutamic acid 
  • Basic: arginine, histidine, lysine
  • Hydroxylic: serine, threonine 
  • Sulphur-containing : cysteine, methionine 
  • Amidic (containing amide group): asparagine, glutamine 

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