Amino acids are organic compounds found in all living systems, they contain one -COOH (Carboxyl) group and one -NH2 (amino) group attached to an alpha (𝜶) carbon atom, hence also called “𝜶 Amino Acids”.
Twenty amino acids are found in proteins they are called “Standard Amino Acids”.
They are classified as follows,
Aliphatic Amino Acids:
The R groups in this class of amino acids are nonpolar and hydrophobic.
e.g. Glycine, Alanine, Valine, leucine, Isoleucine, Methionine, Proline.
Aromatic amino acids:
They contain aromatic rings in R.
Phenylalanine, tyrosine, and tryptophan, with their aromatic side chains, are relatively nonpolar (hydrophobic).
Acidic amino acids:
Amino acids in which the R-group is acidic or negatively charged.
e.g.; Glutamic acid and Aspartic acid.
Basic amino acids:
Amino acids in which R-group is basic or positively charged.
e.g. Lysine, Arginine, Histidine.
Sulfur Containing Amino Acids.
These amino acids contain sulphur in their R group.
e.g Cysteine, Methionine.
Hydroxyl Group Containing Amino Acids.
These amino acids contain Hydroxyl Group in their R group.
e.g Serine , Threonine.
Imino Amino Acids:
They contain the Imino group (=NH) instead of -NH2.
The nitrogen of the -NH2 group is part of the ring structure.
e.g. Proline
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 e.g. Creatinine, ornithine, B Arginine.
Amino acids are obtained by hydrolysis of proteins
Essential & Non essential Amino Acids:
Essential Amino Acids:
These amino acids are not synthesized by the body and hence have to be taken from external sources like food, for smooth functioning of the body.
Non essential Amino Acids:
These amino acids are synthesized by the body.
Biological Functions of Amino acids:
Building blocks for all living things.
Determines the three-dimensional configuration of a protein, and the structure of a protein determines its function.
They largely promote the:
Production of hormones
Structure of muscles
Human nervous system’s healthy functioning
The health of vital organs
Normal cellular structure.
Physical Properties of Amino Acids:
Appearance:
Amino acids are colorless, crystalline solid.
All amino acids have a high melting point greater than 200℃.
Solubility:
They are soluble in water, slightly soluble in alcohol and dissolve with difficulty in methanol, ethanol, and propanol.
Their solubility depends largely upon the nature of the R group.
Amino acids with polar R group are soluble in water and alcohol while amino acids with nonpolar groups are soluble in organic solvents like ether, chloroform etc.
Optical Activity:
All standard amino acids except glycine contain an asymmetrical carbon atom.
Hence all standard amino acids except glycine are optically active.
All standard amino acids are L rotatory while some D rotatory are also reported like D Alanine from bacterial cell wall.
Acid Base Behavior:
Amino acids in their structure contain an acidic carboxyl group (-COOH) and a basic amino group (-NH2).
As they possess both acidic and basic groups they are Amphoteric in nature.
Zwitterion Formation:
A zwitterion is a molecule with functional groups, of which at least one has a positive and one has a negative electrical charge.
The net charge of the entire molecule is zero.
They contain an amine group (basic) and a carboxylic group (acidic).
The -NH2 group is the stronger base, and so it picks up H+ from the -COOH group to leave a zwitterion.
The (neutral) zwitterion is the usual form of amino acids that exist in solution.
Isoelectric pH:
The pH at which the amino acids in a solution exist as Zwitterion is called “Isoelectric pH”.
As zwitterion the amino acids are insoluble and hence precipitate out of the solution.
Each amino acid has a specific isoelectric pH which can be used for their isolation.
Chemical Properties of Amino Acids:
Ninhydrin Reaction:
When 1 ml of Ninhydrin solution is added to a 1 ml protein solution and heated, the formation of a violet color indicates the presence of α-amino acids.
This test is specific for alpha amino acids and used for their identification and estimation.
Reaction with Sanger’s reagent:
Sanger’s reagent (1-fluoro-2, 4-dinitrobenzene (FDNB)) reacts with a free amino group in the peptide chain in a mild alkaline medium at room temperature.
The reaction forms a Dinitro phenyl product of the amino acid called DNP amino acid.
Reaction with Edman’s Reagent:
Edman’s Reagent is Phenylisothiocyanate.
It reacts specifically with the free amino group of the amino acid.
It is used to find out the terminal amino acid of a protein
Reaction with Dansyl Chloride:
This reaction is the same as Edmans Reagents reaction.
This reaction is also used to detect the last amino acid of the polypeptide chain i.e. N terminal amino acid.
The formed product is called “Dansylated amino acid.”
Amide Formation:
Carboxyl group of amino acids react with ammonia to form amides.
The carboxyl group of Aspartic acid and Glutamic acid reacts with ammonia to form Asparagine and Glutamine respectively.;
Decarboxylation:
When an amino acid undergoes decarboxylation reaction it liberates CO2 and forms the corresponding amine as follows,
Esterification:
Carboxyl group of an amino acid reacts with the hydroxyl group of an alcohol to form an ester.
Xanthoprotic Test:
The xanthoproteic test is performed for the detection of aromatic amino acids (tyrosine, tryptophan, and phenylalanine) in a protein solution.
The nitration of the aromatic amino acid chain occurs due to reaction with nitric acid, giving the solution yellow coloration.
Sakaguchi Reaction:
The Sakaguchi test is a chemical test used for detecting the presence of arginine in proteins.
The Sakaguchi reagent used in the test consists of 1-Naphthol and a drop of sodium hypobromite.
The guanidine group in arginine reacts with the Sakaguchi reagent to form a red-coloured complex.
Nitroprusside Test:
The sulfur containing amino acids like cysteine reacts with nitro;ruside to produce red color.
Millon’s Test:
Millon’s test is an analytical test used for the detection of the amino acid tyrosine.;
Millon’s test is based on the principle of nitrification of the phenol group in tyrosine, which then forms complexes with heavy metals like mercury.
The reagent used for the test is called Millon’s reagent, and it consists of mercuric nitrate and mercurous nitrate that is dissolved in concentrated nitric acid.
Peptide Bond Formation:
The amino acids in the proteins are bound together by peptide bonds.
The carboxyl group and amino group of neighboring amino acids react with each other to form a peptide bond as follows,
