Protein affinity chromatography molecular properties

The acid-base properties, and hence ionic character, of peptides and proteins also can be used to achieve separations. Ion-exchange chromatography, similar to that described for amino acids (Section 25-4C), is an important separation method. Another method based on acid-base character and molecular size depends on differential rates of migration of the ionized forms of a protein in an electric field (electrophoresis). Proteins, like amino acids, have isoelectric points, which are the pH values at which the molecules have no net charge. At all other pH values there will be some degree of net ionic charge. Because different proteins have different ionic properties, they frequently can be separated by electrophoresis in buffered solutions. Another method, which is used for the separation and purification of enzymes, is affinity chromatography, which was described briefly in Section 9-2B. 

Originally devised to separate low-molecular-weight substances such as sugars and amino acids, chromatography has become an invaluable tool in protein purification. There are a wide variety of chromatographic techniques. They can be used to separate protein mixtures on the basis of molecular properties such as size, shape, and weight, or certain binding affinities. Often several techniques must be used sequentially to obtain a demonstrably pure protein. 

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