CHAPTER 2 - POLYPEPTIDES AND PROTEINS

Abstract

This chapter discusses the extraction, purification, and fractionation techniques of polypeptides and proteins. It also discusses the structure of polypeptides and proteins. In plants and animals, polypeptides and proteins generally occur as components of complex mixtures and only rarely are in the free state or in a readily extractable form. The extract contains various contaminants such as cell debris, various colloidal constituents, lipids and low molecular weight impurities typified by salts and sugars. The coarse impurities can conveniently be removed by centrifugation and lipids by extraction with water-immiscible solvents. The removal of salts and low molecular weight impurities is usually accomplished by dialysis or the more efficient technique of electro dialysis in which abstraction of ions is facilitated by an applied electric field. Ultra-filtration through cellophane or collodion is another method, whereby the proteins can be freed from low molecular weight contaminants and also solvent. The variation in the chemical and physical properties of poly-peptides and proteins is reflected by the differences in their solubility, and these differences form the basis of protein separation techniques. The solubility of proteins is greatly dependent on the pH of the solution being least at the isoelectric point. The dielectric constant, temperature, and the ionic composition and concentration also exert profound effects on the solubility behavior of these materials.