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The purity of protein samples can be assessed and the progress of a fractionation or purification procedure can be followed. Protein separation by SDS-PAGE can be used to estimate relative molecular mass, to determine the relative abundance of major proteins in a sample, and to determine the distribution of proteins among fractions. If proteins of known mass are run simultaneously with the unknowns, the relationship between Rf and mass can be plotted, and the masses of unknown proteins estimated. In a gel of uniform density the relative migration distance of a protein (Rf, the f as a subscript) is negatively proportional to the log of its mass. Because the charge-to-mass ratio is nearly the same among SDS-denatured polypeptides, the final separation of proteins is dependent almost entirely on the differences in relative molecular mass of polypeptides. Polyacrylamide gels restrain larger molecules from migrating as fast as smaller molecules. The negative charges on SDS destroy most of the complex structure of proteins, and are strongly attracted toward an anode (positively-charged electrode) in an electric field. A polypeptide chain binds amounts of SDS in proportion to its relative molecular mass. SDS (also called lauryl sulfate) is an anionic detergent, meaning that when dissolved its molecules have a net negative charge within a wide pH range. Laemmli, who was the first to publish a paper employing SDS-PAGE in a scientific study. The most commonly used system is also called the Laemmli method after U.K. The method is called sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). A very common method for separating proteins by electrophoresis uses a discontinuous polyacrylamide gel as a support medium and sodium dodecyl sulfate (SDS) to denature the proteins. Introduction to SDS-PAGE The separation of macromolecules in an electric field is called electrophoresis.