Biochemistry

1. Properties of Amino Acids
By
Mrs Sanchita Choubey
(M.Sc., PGDCR, Pursuing Ph. D)
Assistant Professor of Microbiology
Dr. D Y Patil Arts Commerce and Science College Pimpri, Pune

2. Structure of Amino acid

4. Physical Properties
 Amino acids are colorless, crystalline solid.
 All amino acids have a high melting point greater than 200o
 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.
 On heating to high temperatures, they decompose.
 All amino acids (except glycine) are optically active.
 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.

5. Optical rotation
 Optical rotation is the rotation of the plane of polarized light, which is
composed of light waves that vibrate in one plane, or direction, only.
Solutions of substances that rotate the plane of polarization are said to
be optically active, and the degree of rotation is called the optical
rotation of the solution. The direction in which the light is rotated is
generally designed as plus, or d, for dextrorotatory (to the right), or as
minus, or l, for levorotatory (to the left). Some amino acids are
dextrorotatory, others are levorotatory. With the exception of a few
small proteins (peptides) that occur in bacteria, the amino acids that
occur in proteins are L-amino acids.

7. Acid-Base properties of amino acids
 All amino acids contain at least two ionizable groups
the α-amino group and the α-carboxyllic group ,some
contain an additional acidic or basic group in their
side chain ,which are responsible for the amino acids
, acid- base behaviour.
 As a result of their ionizability the following
equilibrium reaction can be written;
 R-COOH R-COO- + H+
 R-NH2 R-NH3 +

8. Ultraviolet Absorption Spectrum of
Aromatic amino acids
The aromatic amino acids
tryptophan , tyrosine ,
histidine and phenyl alanine
absorb ultraviolet light at
280nm ,which explains the
absorption of proteins at
280nm.

9. Absorption in the Ultraviolet Region
 Amino acids present an appreciable absorption at
wavelengths below 230 nm (2 300 Å); furthermore,
some of them absorb between 250 and 300 nm, due to
the presence of chromophores like the phenyl radical
(Tyr) or the indole ring (Trp) in their R chain, thus
permitting the spectrophotometric titration of proteins.

10. Ionization
 All amino acids contain at least two ionizable groups, the
carboxyl group and the amino group; they are amphoteric;
the carboxyl group of an amino acid can yield one proton,
and an anion appears:
The amino group can fix one proton and form a cation.
These two dissociation reactions correspond to equilibrium
governed by the law of mass action; the proportions of ionized
and unionized amino acid present in solution will therefore
depend on the concentration of H+ ions.

11.  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 amino acids exist in solution.

12.  Amphoteric property
 Amino acids are amphoteric in nature that is they act as
both acids and base since due to the two amine and
carboxylic group present.
 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.

13.  Xanthoproteic test
 The xanthoproteic test is performed for the detection of
aromatic amino acids (tyrosine, tryptophan, and
phenylalanine) in a protein solution. The nitration of
benzoid radicals present in the amino acid chain occurs
due to reaction with nitric acid, giving the solution
yellow coloration.

14.  Reaction with Sanger’s reagent
 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.
 Reaction with nitrous acid
 Nitrous acid reacts with the amino group to liberate
nitrogen and form the corresponding hydroxyl.