Secrets of protiens

vi. Nitroprusside Reaction:
To a solution of protein, add sodium nitroprusside in ammoniacal solution. A reddish colour is produced. This is due to the presence of sulphydryl group in the protein molecule. (Disulphide groups are to be reduced first to sulphydryls by suitable reducing agent to give this reaction.)
2. Coagulation Reaction:
Albumin and globulin solution when heated, fine flocculi appear. They are denatured.
3. Precipitation Reaction:
In a protein solution the proteins may be precipitated by;
(a) Acids, e.g. tannic acid, picric acid, trichloro-acetic acid and phosphotungstic acid,
(b) Salts of heavy metals, and
(c) Alcohol.
Formation of the Protein Molecule:
The synthesis of protein molecule takes place by the union of the -NH₂ group of one amino acid with the -COOH group of another.


For instance:
In this way infinite number of amino acids may join up in straight chains. This junction, CO-NH, through which the ami­no acids become joined together, is called the peptide linkage.
It is obvious that in such a simple chain there will be one free -NH₂ group at one end and another free -COOH group at the other. It sometimes so happens that such a straight chain bends upon itself and the basic radicle at one end combines with the acid radicle on the other by forming the same peptide linkage.
In this way a ring structure is produced. These compounds are called diketopiperazines. The following scheme – shows the formation of diketopiperazine by the condensation of two molecules of glycine.
Amino Acids:
The amino acid is an organic acid in which one or more hydrogen atoms are replaced by -NH, group. Thus it contains at least a free amino group (-NH₂) and a carboxyl group (-COOH). The empirical formula is R-CH.NH₂, COOH. Amino acids are amphoteric in reaction and form salts with both acids and bases.
The amino acids in the body are almost all a-amino acids. They should be regarded as derivatives of saturated fatty acids in which the amino group is attached to that carbon atom which is situated in the a-position, i.e. next to the COOH group. Amino acids are colurless, crystalline substances, soluble in water, easily diffusible and except glycine all are optically active.