Physical and Chemical Properties of proteins

Properties of proteins

Physical properties

1. Colour and taste

Proteins are colourless and usually tasteless. These are homogeneous and crystalline.

2. Solubility

Solubility of proteins is influenced by pH. Solubility is lowest at isoelectric point and increased with increasing acidity of alkalinity.

3. Optical activity

All protein solutions rotate the plane polarised light to the left i.e. these are levorotatory.

4. Colloidal nature

Because of their giant size, the proteins exhibit many colloidal properties are:

·           Their diffusion rate is extermely low.

·           They may produce considerable light-scattering in solution, thus resulting in visible turbidity (Tyndall effect).

5. The comparatively week forces responsible for maintaining secondary, tertiary and quarternary structure of proteins are readily distrupted with resulting loss of biologic activity. This distruption of native structure is termed denaturation. Physically, denaturation may be viewed as randomizing the conformation of a polypeptide chain without affecting its primary structure (Fig.5.3).

The biological activity of most proteins is destroyed by exposure to strong mineral acids or bases, heat, urea, acetone, alcohol and ionic detergents etc. Denatured proteins are less soluble in water.

Chemical properties

1. Hydrolysis

i. By acidic agents

Proteins upon hydrolysis with concentrated mineral acids such as, HCl yield amino acids in the form of their hydrochlorides.

ii. By proteolytic enzymes

Under relatively mild conditions of temperature and acidity, certain proteolytic enzymes like pepsin and trypsin hydrolyse the proteins. Enzyme hydrolysis is used for the isolation of certain amino acids like tryptophan. Two important drawbacks with this type of hydrolysis are:

It requires prolonged incubation and

Hydrolysis may be incomplete

2. Colour reaction with Biuret reagent

When a protein solution is treated with alkaline CuSO₄ reagent, the peptide bonds present in the protein interact with copper ions and forms violet coloured Biuret complex (Fig.5.4). The colour deepens which depend on the number of peptide bond present in the protein. The sturcture of the voilet complex is

All proteins except dipeptides react with Biuret reagent because a minimum of two peptide linkages are involved in this reaction.

This reaction is widely used both as a qualitative test for the detection of proteins and also as a quantitative test for the estimation of protein in biological materials.