Industrial Biotransformations

5.1 Definitions
In iso-electric precipitation, advantage is taken of the fact that at the pH at which the
protein or a zwitter-ion bears zero net charge, its solubility is at a minimum. The value of
the pH at which this occurs is called the iso-electric point. Most proteins have an iso-electric
point of less than 7. Iso-electric focusing is an electrophoretic separation method by
which amphoteric compounds are fractionated according to their iso-electric point along
a continuous pH gradient.
5.1.2.4 Dialysis and Electrodialysis
In dialysis, one or more solutes are transferred from one solution (“feed”) to another
(“dialysate”) through a membrane down their concentration gradient. When pressure is
employed in the separation, in addition to the concentration gradient, the process is
called pervaporation.
In electrodialysis, the separation of components of an ionic solution occurs in a cell consisting
of a series of anion- and cation-exchange membranes arranged alternately between
an anode and a cathode, to form individual electrodialysis cells. During the process
of electrodialysis, there is an increase in the ion concentration of one type of ion in one
type of compartment and is accompanied by a simultaneous decrease in the concentration
in the other type of compartment.
5.1.2.5 Extraction
Liquid–liquid extraction is a classical process used to purify bioproducts. Both aqueous
and organic extraction media are employed. Extraction using aqueous two-phase polymer
systems (PEG, dextran) provides an excellent method of protein separation [11, 12].
5.1.2.6 Adsorption
Adsorption is a common operation employed for the purification of biological products.
Activated carbon or ion-exchange resins can be used. In adsorption, material accumulates
on the surface of a solid adsorbent that has a multiplicity of pores of different sizes.
In affinity adsorption, proteins can be absorbed biospecifically on the basis of their interaction
with a complimentary tertiary structure. In ion-exchange adsorption, the adsorbents
have ionic groups with easily dissociable counter ions.
5.1.2.7 Chromatography
Chromatography covers a wide variety of methods of separation and purification of molecules.
It is based on the differences in equilibrium constants for the components of a
mixture placed in a biphasic system. Ion-exchange chromatography, affinity chromatography,
gel filtration chromatography and hydrophobic interaction chromatography (HIC)
are used in protein purification in decreasing order of importance. Reversed phase chromatography
is primarily of interest in the separation of peptides and small molecules.
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