Purity is assessed by SDS-PAGE and/or nondenaturing electrophoresis for molecular weight and disulfide polymerization. Isoelectric focusing can be performed for identifying the isoelectric point (pI) of a protein. To determine homogeneous pure proteins, N-terminal amino acid sequencing is used. Other methods can be developed upon request.
Electrophoresis describes the migration of charged particles under the influence of an electric field. It can be used to evaluate protein purity and provide an estimation of characteristics such as isoelectric point, charge, and subunit composition. Gel electrophoresis is the technique in which molecules are forced across a span of gel by an electrical current. Proteins, peptides, amino acids, nucleotides etc. contain groups that can ionize and at any given pH, exist in solution as electrically charged species either as cations (+) or anions (-). Separation of large (macro) molecules depends upon two forces: charge and mass. During electrophoresis, the rate of migration in the electric field depends on the strength of the field, relative hydrophobicity of the samples, size and shape of the molecules, and on the ionic strength and temperature of the buffer in which the molecules are moving. The most common one-dimensional gel methods use sodium dodecyl sulfate (SDS), which binds to proteins in a uniform amount per microgram of protein. This results in a uniform charge density per unit mass, providing a separation based on the mass of the polypeptide chain.
Proteins can be separated for further purification by isoelectric focusing (IEF) in slab gels in which the movement of proteins through pores in a polyacrylamide gel matrix is controlled by a pH gradient created by soluble ampholytes.

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