Separation by Precipitation

Abstract

In the early days of protein chemistry the only practical way of separating different types of proteins was by causing part of a mixture to precipitate through alteration of some property of the solvent. Precipitates could be filtered off and redissolved in the original solvent. These procedures remain a vitally important method of protein purification, except that filtration has mostly been replaced by centrifugation. Protein precipitates are aggregates of protein molecules large enough to be visible and to be centrifuged at reasonably low g forces. In some cases the aggregation continues and the precipitate flocculates, but usually the motions and collisions of the particles in suspension keep their size small. This results in clogging of filter papers and necessitates centrifugation at considerably more than 1 g. The various methods of obtaining a precipitate are described in the separate sections below.