Properties of Amino Acids

The physical and chemical properties of amino acids vary with their function and influence their participation in various biological reactions.

Physical properties of amino acids

Physical properties are important that decide the function of an amino acid.
Most amino acids are easily soluble in water but insoluble in organic solvents.
Their melting point is higher, close to 200 degrees Celsius.
All amino acids have asymmetric carbon, except glycine.
They show optical isomerism as well.
Amino acids are ampholytes, having an acidic and alkaline nature due to the presence of -COOH and -NH2 groups.
The aromatic amino acids have conjugated rings that show stronger absorption of light in the near-UV region that helps detect the proteins.

Chemical properties of amino acids

The chemical properties of amino acids depend on the presence of carboxyl (-COOH) and amino (-NH2) groups. Generally, all the amino acids are weak polyprotic acids. The carboxyl group enables them to form salts when reacting with bases and form esters by reacting with alcohol.

Zwitterion

Amino acids are zwitterions, molecules with at least one functional group with a positive charge and one group with a negative charge. They contain an amine group (a strong basic) and a carboxylic group (acidic). The -NH2 group takes H+ from the -COOH to form the neutral zwitterion.

Decarboxylation

Amino acids can undergo both transamination and oxidative deamination reactions. They undergo decarboxylation to form their corresponding amines.

Histidine → Histamine + CO2
Lysine →Cadaverine + CO2
Tyrosine →Tyramine + CO2

The carboxyl group of some dicarboxylic amino acids reacts with ammonia and produces amide.

Reaction with Alkalis

The carboxyl group of amino acids releases an H+ ion to form a carboxylate (COO–) ion.

Reaction with Ninhydrin

The α-amino acids when reacting with ninhydrin can form a purple, pink, or blue-coloured complex called Ruhemann’s purple. The exceptions are proline and hydroxyproline which produce a yellow colour). This reaction is used for the quantitative analysis of amino acids.

Reaction with Alcohols (Esterification)

Amino acids react with alcohol to produce volatile esters- a stark contrast to the amino acid form.

Reaction with DANSYL Chloride

DANSYL chloride is Dimethyl Amino Naptha Sulphonyl Chloride. An amino acid treated with this reagent produces a fluorescent DANSYL derivative.

Acylation

In an alkaline medium, an amino acid reacts with acid chloride and acid anhydride to form phthaloyl amino acid

Reaction with Sanger’s reagent

An α-amino acid reacts with Sanger’s reagent- 1-fluoro-2,4-dinitrobenzene, producing a yellow derivative called DNB amino acid.

Reaction with Edmann’s reagent

When amino acids react with Edmann’s reagent – phenyl isothiocyanate it first produces phenylthiohydantoin acid which then turns into the cyclic form Phenyl thiohydantoin. This final product is called Edmann’s derivative.

Biological Role of Amino Acids

Amino acids are mainly involved in the formation of protein, and other metabolic reactions inside the body. Each amino acid will have a specific role and each reaction requires the assistance of a particular amino acid as well. The various biological functions of amino acids are enumerated below.

Building blocks for proteins

The primary function of amino acids is protein formation. Amino acids form peptide bonds to form polypeptides. Protein molecules may have one or more polypeptide chains. Depending on the type of protein, these polypeptides further undergo inter-bonding to form the specific structure and shape of the particular protein. For example,

Proline forms bands in polypeptide chains.
Cysteine joins the polypeptide chains together by forming disulfide bonds.

Though there are L-amino acids and D-amino acids exist in nature, only the L-amino acids participate in protein formation.

Metabolic intermediates and biosynthesis of compounds

Several amino acids are intermediate compounds in the various metabolic reactions. They may be directly involved in the reaction or form a coenzyme that does not directly participate in the reaction. They help in nerve communication, cell growth, and biosynthesis of compounds such as urea, purines, pyrimidines, and urea porphyrin. Examples of such intermediate metabolites and compounds by amino acids are

Ornithine and citrulline in urea synthesis
Homocysteine in amino acid metabolism
Histidine is found in the active site of enzymes that assist in bond formation or breakage.
Aromatic amino acids such as Phenylalanine, Tyrosine, and Tryptophan can help with electron transfer.
Some amino acids can form glucose by losing their amino group.
Tyrosine produces thyroxine, adrenaline, and melanin.
Glycine is an essential component for the production of heme
Tryptophan produces vitamin B3, and IAA (Indole Acetic acid), the plant hormone.
Coenzyme glutathione and alkaloids are other compounds that are produced from amino acids.

Cell communication

Some amino acids can be chemical messengers that send signals between the cells or neurotransmitters, form a part of the reactions or that can trigger other reactions in the cells. Examples are,

Glycine, Gamma-aminobutyric acid (GABA- a glutamate), and Dopamine (a derivative of tyrosine) are neurotransmitters.
Histamine mediates allergic reactions
Thyroxine (derived from tyrosine) stimulates metabolism in vertebrates
S-adenosylmethionine is a biological methylating reagent
Azaserine and Valinomycin produced by Streptomyces, have antibiotic properties.
Creatine is involved in energy metabolism in the muscles.
β-Alanine is a part of Vitamin B5 or pantothenic acid and thereby in coenzyme A.

Biological buffers

Amino acids are amphoteric in nature, which means they have acidic and alkaline properties. This property allows them to act as buffers in solutions. Buffers can resist any change in the pH by donating H+ ions when the pH increases or accepting H+ when the pH decreases.

Nitrogen storage

Amino acids such as asparagine and glutamine are derivatives of aspartic acid and glutamic acid respectively which serve as a temporary storage of nitrogen that needs to be eliminated from the cells.