Amino Acids- Definition, Classification, Preparation, Uses

Amino acids are the building block of the protein and also serve as the nitrogenous backbone for the compounds like neurotransmitters and hormones. Chemically they are the organic compounds containing both amino (-NH2 ) and carboxylic (-COOH) groups. So, they are called amino acids.

Amino Acids
Amino Acids

Classification of Amino Acids on the basis of nutritional requirements:

On the basis of nutrition, amino acids are classified into two groups, they are:

Essential amino acid

These are the amino acids that cannot be synthesized by the human body. These must be supplied by the exogenous diet. Essential amino acids are important for the growth and development of human beings. There are nine essential amino acids including phenylalanine, valine, tryptophan, threonine, isoleucine, methionine, histidine, leucine, and lysine. The acronym PVT TIM HaLL is commonly used to remember the essential amino acids, which include the first letter of these amino acids. Protein-energy malnutrition caused by a lack of these amino acids can prevent growth and even death.

Non-essential amino acids:

Amino acids, which are synthesized by our body, are called non-essential amino acids. These are also called dispensable amino acids, as they can be excluded from the diet. E.g., alanine, arginine, asparagine, cysteine.

Conditional amino acids: These aren’t usually necessary in everyday life, but they actually are useful when you’re sick, injured, or stressed, e.g., glutamine, tyrosine, arginine,

Classification of Amino Acids on the basis of distribution in protein:

Standard amino acids: 

These are the amino acids that are used to form protein Histidine Isoleucine, Lysine

Non-standard protein amino acids

These amino acids are not required to build the proteins but play a crucial role as a metabolic intermediate. E.g., Hydroxy proline, Hydroxylysine.

Non-standard non-protein amino acids

These amino acids are the derivatives of amino acids and plays important role in metabolism. E.g, ornithine, citrulline, alpha amino butyrate.

Classification of Amino Acids on the basis of Chemical nature

On the basis of the number of carboxylic and amino groups present, amino acids are classified as

Neutral amino acids

Amino acids have one amino and one carboxylic group. E.g., Glycine, alanine, proline.

Acidic amino acids

Amino acids containing one amino (or imino) and two carboxylic groups are called acidic amino acids like aspartic acid, and glutamic acid.

Basic amino acids

Amino acids containing two amino (or imino) and one carboxylic are called basic amino acids. E.g., Lysine and arginine.

Classification of Amino Acids on the basis of polarity

Amino acid with non-polar side chain 

These amino acids are hydrophobic in nature and cannot participate in hydrogen bonding and ionic bonding. E.g. alanine, valine, leucine.

Amino acids with uncharged polar side chains

These amino acids are polar in nature but possess a neutral pH value. They are soluble in water and hydrophilic in nature. The functional group present makes them able to form hydrogen bonds. E.g, glycine , serine threonine

Amino acids with polar negatively charged side chain

These amino acids have an extra carboxylic group in the side chain with a dissociable proton. These amino acids have a net negative charge at neutral pH. E.g.,   aspartic acid, glutamic acid

Amino acids with polar positively charged side chain

These amino acids consist of the extra amino acid on their side chain rendering basic nature to protein. They will have a net positive charge at pH 7. E.g., Lysine arginine histidine.

Classification of Amino Acids
Classification of Amino Acids

Amino acids as dipolar ions

An amino acid consists of both an amino group and a carboxylic group with the general formula H2NCHRCOOH, but some chemical and physical properties are not always behaving like this structure. It has been found that the amino acid acts as the dipole, one part of it carrying a positive charge and the second part carrying a negative charge. The dipolar ionic structure of amino acids can be represented as:

This is also called Zwitter ion. There is no free amino or carboxylic group present in the molecule. The evidence are:

  • Spectroscopic analysis of the amino acid does not show the characteristic bonds or -NH2 and -COOH groups.
  • Amino acids are insoluble in non-polar solvents and soluble in polar solvents.
  • Amino acids are non-volatile crystalline solids having a high melting point. This property is similar to ionic substances but not to amines and carboxylic acids.
  • The polar nature of amino acids is due to the high dipole moment.

The isoelectric point of amino acids

Amino acids have polar characteristics and electrical properties. When an electric field is applied to an amino acid solution, the amino acids migrate from one electrode to another. If the solution is acidic, the equilibrium lies towards positively charged amino acid i.e. (+NH3CHRCOOH), so it migrates towards the cathode on passing the electricity through it. While if the solution is basic, the equilibrium lies toward negatively charged amino acid (NH2CHRCOO), so it migrates towards the anode on passing electricity on it.

Configuration of natural amino acids

All amino acids except glycine have four different units attached to the alpha carbon. So they are optically active except for glycine. As they belong to the same stereochemical series, they have a similar configuration as that of L (-) glyceraldehyde.

Sign ‘L’ refers to the configuration, whereas the (+) or (-) sign refers to the direction of rotation of plane-polarised light.

Amino acid isolated from the proteins rotates the plane of polarized light, but the same acids that are chemically synthesized, are optically inactive. During the synthesis of amino acids, both dextro and laevo forms are obtained in equal amounts.

Preparation of alpha amino acids

  1. From alpha halo acids: Alpha halo acids on treatment with ammonia or ammonium hydroxide give alpha amino acids.
  1. Gabriel phthalimide synthesis: Potassium salt of the phthalimide on reaction with alpha halo ester gives the intermediate product, which on hydrolysis produces amino acids.

Physical properties of amino acids

  • Amino acids are colorless crystalline solids.
  • Most amino acids are tasteless, some are sweet (E.g., Glycine Alanine) in taste, and some are bitter (E.g., Arginine).
  • They have a high melting point, i.e., above 200°C, due to their ionic property.
  • Due to the presence of asymmetric carbon, amino acids are optically active ( except glycine)
  • Amino acids can act as acids and bases due to their dipolar ion nature, i.e., Zwitter ion.
  • They are soluble in water and slightly soluble in alcohol. R group, present in the side chain, and the pH of the solvent medium affect the solubility of amino acids.

Some general reactions of amino acids

The chemical reactions of amino acids are generally those of the amino group and carboxylic group. There are some properties that are due to the presence of both amino group and carboxylic group in same molecules are also responded by the amino acid.

Reaction of amino group:

  1. Reaction with mineral acid: The dipolar ionic structure of amino caid exists in equilibrium in acidic medium.
  1. Alkylation: In an alkaline medium amino acid reacts with the alkyl halide to form N- alkyl substituted amino acid.
  1. Reaction with nitrous acids: Amino acids on reaction with the nitrous acid liberate nitrogen gas. This method is used to analyze the amino acids, known as Van Slyke’smethod.

Reactions of carboxylic group

1.  Reaction with alkali: The dipolar ionic structure exists in the alkaline medium as follows:

2. Esterification: In acidic medium amino acids gives the reaction of carboxy groups like formation ester, acid chlorides, and acid anhydrides.

3. Decarboxylation reaction: Amino acids undergo decarboxylation by the action of heat, acids, bases, or specific enzymes to form primary amines.

Some decarboxylation reactions of amino acids are helpful for the human body such as the decarboxylation of histidine to the histamine

The reaction of both carboxyl and amino groups

A. Action of heat: Applying the heat to amino acids helps to distinguish different amino acids like (alpha, beta, gamma, and delta) since these amino acids give different products on heating.

B. Methylation: Amino acids on reaction with methyl iodide give trimethyl derivatives which are known as betaines. Betaines exist as dipolar.

C. Reaction with ninhydrin. Amino acids on heating with alcoholic solution of the triketone ‘ninhydrin’, a complex reaction occurs, which produces the brilliant blue red color. This reaction is useful for the qualitative and quantitative analysis of amino acids.

Benifits of amino acids

  • Amino acids provide an energy source.
  • Amino acids help in growth and development.
  • They help to boost the immune system.
  • It helps to regulate the digestive system and enhance the musculoskeletal system.
  • Amino acids help to maintain body weight and balance our mood.
  • Amino acid acts as a natural anti-inflammatory and anti-aging agent.

Harmful effect of excessive amino acids

Excessive amino acid levels in our bodies have the following negative effects on our bodies:

  • Change in eating habits
  • Abdominal pain
  • Gastrointestinal discomfort
  • Causes abnormal drop of blood pressure
  • Increase the chance of getting gout



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