It contain more information about Amino acids and their structure. Then , contain both physical and chemical properties. Next Classification of amino acids based on nutritional requirements, based on metabolic fate, Position of NH2 group, etc.,

1. AMINO ACIDS
STRUCTURE OF AMINO ACIDS
Amino Acids are the organic compounds that combine to form proteins; hence they are
referred to as the building components of proteins. These biomolecules are involved in several
biological and chemical functions in the human body and are the necessary ingredients for the
growth and development of human beings.
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 are called non-protein amino acids.
While amino acids are necessary for life, not all of them can be produced naturally in the
body. The 20 amino acids, 11 can be produced naturally. These nonessential amino acids are
Alanine, Arginine, Asparagine, Aspartic acid, Cysteine, Glutamic acid, Glutamine, Glycine,
Proline, Serine, and Tyrosine.
Amino acids that cannot be produced naturally are called essential amino acids. They
are Histidine, Isoleucine, Leucine, lysine, Methionine, phenylalanine, Threonine, Tryptophan,
and Valine. Essential amino acids must be acquired through diet. Common food sources for these
amino acids include eggs, soy protein, and whitefish. Unlike humans, plants are capable of
synthesizing all 20 amino acids.
Generally, amino acids have the following structural properties:
1. Carbon atom (alpha carbon)
2. Hydrogen atom (H)
3. A Carboxyl group (-COOH)
4. Amino group (-NH2)
5. Variable group or "R" group

2. Amino Acid Groups:
Amino acids can be classified into four general groups based on the properties of the "R"
group in each amino acid.
1. Polar Amino Acid – Ex: Cysteine, Glutamine, Serine, Proline, Threonine, Asparagine
Polar amino acids have "R" groups that are hydrophilic, meaning that they seek contact
with aqueous solutions.
2. Nonpolar Aliphatic Amino Acid– Ex: Alanine, Glycine, Isoleucine, Leucine,
Methionine, Valine
Nonpolar amino acids are the opposite (hydrophobic) in that they avoid contact
with liquid. These interactions play a major role in protein folding and give proteins their 3-D
structure
3. Positively charged Amino Acid– Ex: Histidine, Lysine, Arginine
4. Negatively charged Amino Acid– Ex: Aspartic acid, Glutamic acid
5. Aromatic Amino Acid – Ex: Tryptophan, Phenylalanine, Tyrosine
PROPERTIES OF AMINO ACIDS
Physical Properties:
1. Amino acids are colorless, crystalline and solid.
2. All amino acids have a high melting point greater than 200o.
3. Solubility: They are soluble in water, slightly soluble in alcohol, and dissolve with
difficulty in methanol, ethanol, and Propanol. R-group of amino acids and pH of the
solvent play important role in solubility.
4. On heating to high temperatures, they decompose.
5. All amino acids (except Glycine) are all have asymmetric carbon, which causes plane
polarised light to rotate. Optically active.
6. Peptide bond formation: Amino acids can connect with a peptide bond involving their
amino and carboxylate groups. A covalent bond formed between the alpha-amino group
of one amino acid and an alpha-carboxyl group of other forming -CO-NH-linkage.
Peptide bonds are planar and partially ionic.
7. They are Amphoteric, which means they react with acids and bases.

3. Chemical Properties:
1. Zwitterionic property
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. Amino
acids are the best-known examples of zwitterions. 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 the solution.
2. Amphoteric property
Amino acids are Amphoteric in nature that is they act as both acids and base due to the
two amine and carboxylic groups present.
3. Ninhydrin test
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.
4. Xanthoproteic test
The xanthoproteic test is performed for the detection of aromatic amino acids in a protein
solution. Ex: Tyrosine, tryptophan, and phenylalanine.
The nitration of benzoid radicals present in the amino acid chain occurs due to a reaction
with nitric acid, giving the solution yellow coloration.
5. Reaction with Sanger’s reagent
Sanger’s reagent (1-fluoro-2, 4-dinitrobenzene)
Sanger’s reagent reacts with a free amino group in the peptide chain in a mild alkaline
medium under cold conditions.
6. Reaction with nitrous acid
Nitrous acid reacts with the amino group to liberate nitrogen and form the corresponding
hydroxyl.
CLASSIFICATION OF AMINO ACIDS
1. Classification Based on the “R” Group
2. Classification Based on the Nutritional Requirement of the Body
3. Classification Based on the Position of NH2 Group
4. Classification Based on the Metabolic Fate
Classification Based on “R” Group:
Amino acids can be classified into three groups: Aliphatic, Aromatic, and Heterocyclic
amino acids.

4. 1. Aliphatic Amino Acids:
Simple / Neutral amino acids: these have no functional group in their side chain. Their
molecules have an equal number of carboxylic and amino groups. Ex: Glycine, Valine, Alanine,
Leucine, Isoleucine
(Glycine)
Hydroxyl Group containing amino acids: these have a hydroxyl group in their side chain. Ex:
Serine, Threonine
Acidic amino acids: have carboxyl group in their side chain. In their molecules, they have a
higher proportion of carboxylic groups than amino groups. This contains two –COOH groups
and one –NH2 group. Ex: Aspartic acid and Glutamic acid
(Aspartic acid)
Basic amino acids: contain amino group in their side chain. This contains two –NH2 groups and
–COOH groups. Ex: Lysine, Arginine
(Lysine)
Imino acid (NH): contain NH group instead of NH2 group. Ex: Proline
(Proline)
Sulfur containing amino acids: have sulfur in their side chain. Ex: Cysteine, methionine

5. (Methionine)
2. Aromatic amino acids: have benzene ring in their side chain. Ex: phenylalanine, tyrosine
(Phenylalanine)
3. Heterocyclic amino acids: having side chain rings which possess atleast on atom other than
carbon. Ex: Tryptophan, histidine, Proline
(Proline)
Classification Based on the Nutritional Requirement of Our Body:
A. Non-essential amino acids:
These amino acids are produced by the body and do not need to be consumed. Out of
the 20 amino acids, 10 are non-essential. Glycine, Alanine, serine, Cysteine, glutamine,
tyrosine, Proline, aspartic acid, Asparagine, and glutamic acid are amino acid.
B. Essential Amino Acids:
These amino acids are not synthesized by the body and must be obtained from food.
Out of the twenty amino acids, ten are non-essential. Valine, Leucine, Isoleucine, Arginine,
lysine, Threonine, phenylalanine, tryptophan, and histidine are the amino acids that make
up the human body. These essential amino acids are necessary for our bodies to grow, and a
lack of them in our diet can lead to disorders like kwashiorkor.

6. Classification Based on the Position of NH2 Group:
A. α Amino Acids: The α-amino acids are the amino acids whose one carbon contains both
the amino group (NH2) and carboxyl group (COOH). NH2 group attached to the next “C”
of the carboxyl group.
Ex: Glycine, Alanine
B. β Amino Acids: NH2 group attached to the third “C” of the carboxyl group.
Ex: β Alanine
C. Gama Amino Acids: NH2 group attached to the fourth “C” of the carboxyl group.
Ex: GABA (Gama Amino Butyric Acid)
Classification based on the Metabolic Fate:
A. Purely Ketogenic: Ketone bodies are formed when these amino acids are broken down.
Ex: Leucine and Lysine are two amino acids.

7. B. Purely Glucogenic: Some amino acids are converted to carbohydrates and are called as
glucogenic amino acids. Ex: Alanine, Arginine, asparagine, aspartic acid, Cysteine,
glutamic acid, glutamine, Glycine, histidine, methionine, Proline, serine and Valine are
some of the amino acids found in the human body
.
Ketogenic & Glucogenic: Amino acids that are both glucogenic and Ketogenic break
down to form precursors for both ketone bodies and glucose. Ex: Isoleucine, tryptophan,
phenylalanine and tyrosine are all essential amino acids.
Common Structure of Amino Acids:
REFERENCE:
1. https://unacademy.com/content/cbse-class-11/study-material/chemistry/classification-of-
amino-acids/
2. https://soe.unipune.ac.in/studymaterial/swapnaGaikwadOnline/aminoacids-
171113130407.pdf
3. https://www.toppr.com/guides/chemistry/biomolecule/amino-acids/
4. https://www.onlinebiologynotes.com/amino-acids-characteristics-classification-amino-
acids/
5. https://www.google.com/search?client=firefox-b-
d&q=classification+of+amino+acids#ip=1