Centrifugation Divides a Sample into Two Fractions

Differential Centrifugation Divides a Sample into Two Fractions 

0 A typical crude broken-cell preparation contains disrupted cell membranes, cellular organelles, and a large number of soluble proteins, all dispersed in an aqueous buffered solution. The membranes and the organelles can usually be separated from one another and from the soluble proteins by differential centrifugation. Differential centrifugation divides a sample into two fractions the pelleted fraction, or sediment, and the supernatant fraction, that is, the fraction that is not sedimented. The two fractions may then be separated by decantation. 

Once a crude extract of protein has been made, it is common to separate this mixture into different fractions by a precipitation step. The solubility of a protein reflects a delicate balance between different energetic interactions—both internally within the protein and between the protein and the surrounding solvent. Consequently, the choice of solvent can affect both the solubility and the structure of a protein. 

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Proteins tend to show a minimum solubility at their isoelectric pH—a fact that is apparent for /3-lactoglobulin (fig. 6.1). The decrease in solubility at the isoelectric pH reflects the fact that the individual protein molecules, which all have similar charges at pH values away from their isoelectric points, cease to repel each other. Instead, they coalesce into insoluble aggregates. 

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Methods of Protein Fractionation and Characterization Differential Centrifugation Divides a Sample into Two Fractions... 

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