The cell must be broken open and the protein must be released into a solution called a crude extract. If necessary, differential centrifugation can be used to prepare subcellular fractions or to isolate organelles. Once the extract or organelle preparation is ready, a variety are available for separation of proteins. Other chromatographic methods take advantage of differences in size, binding affinity, and solubility. Nonchromatographic methods include the selective precipitation of proteins with salt, acid, or high temperatures.
Gel electrophoresis is especially useful as an analytical method.
Its advantage is that proteins can be visualized as well as separated, permitting a researcher to estimate quickly the number of proteins in a mixture or the degree of purity of a particular protein preparation. Also, gel electrophoresis allows determination of crucial properties of a protein such as its isoelectric point and approximate molecular weight. Amino acids differ not only in R-group characteristics but also in molecular weight. Different amino acids are linked together in a linear chain by peptide bonds in various combinations and sequences to form specific proteins. A protien may be comprised of amino acids from all of the categories.