SDS-PAGE . BACKGROUND Anionic surfactant compounds such as sodium dodecyl sulfate(SDS) are composed of a negatively charged ionic“ head group” and a hydrophobic hydrocarbon“tai” CH3 CH2 CH2 CHCHCHCHCHCHCHCHCH OSO3- Nat The high solubility in water imparted by the ionic head group and the high solubility in le hydrocarbon tail result in a compromise in aqueous solution. Surfactant molecules form aggregates called micelles in aqueous solution. These ggregates satisfy the solubility characteristics of both the head and tail regions of the surfactant; ionic groups are exposed to water on the surface of the aggregate while the hydrophobic tails associate with each other within the interior of a roughly spherical aggregate of 60 to 100 molecules. Hydrophobic guest molecules can be taken into the interior of the micelle. This includes the hydrophobic amino acids normally confined to the interior ative protein. The association between hydrophobic amino acid residues and the interior is strong enough to denature most proteins, turning them inside-out so that effectively coated with anionic surfactant molecules. Reductive cleavage of all -S-S-bonds followed by treatment with the anionic surfactant sodium dodecyl sulfate(sDs) will disrupt the native tertiary structure of most proteins, causing them to adopt rodlike structures. It has been established that this binding occurs with a constant surfactant-totprotein weight ratio and with enough anionic surfactants to totally dominate the native charge of the protein. Therefore, the charge per unit protein weight is nearly constant and the electrophoretic mobility of sds denatured proteins is a function of size alone. The technique of SDS-polyacrylamide gel electrophoresis is widely used to determine the molecular weight of unknown proteins by comparing their relative electrophoretic mobility to standard proteins of known molecular weight. a direct comparison of the mobilities of known and unknown proteins run under identical conditions (in the same cell at the same time) is recommended. The mobilities of known proteins can be plotted as a function of log(molecular weight)and the mobilities of known proteins used to estimate molecular weights by extrapolation A necessary first step in the protein sample preparation for SDs electrophoresis is treatment with an excess of 2-mercaptoethanol, which reduces all disulfide(-s-S-)bonds in the protein. This permits total disruption of the protein native structure, which is usually stabilized by disulfide linkages. Some proteins(e. g, chymotrypsin) contain polypeptides194 SDS-PAGE I. BACKGROUND: Anionic surfactant compounds such as sodium dodecyl sulfate(SDS) are composed of a negatively charged ionic “head group” and a hydrophobic hydrocarbon “tail”. CH3CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 OSO3 – Na+ The high solubility in water imparted by the ionic head group and the high solubility in nonpolar solvents implied by the hydrocarbon tail result in a compromise in aqueous solution. Surfactant molecules form aggregates called micelles in aqueous solution. These aggregates satisfy the solubility characteristics of both the head and tail regions of the surfactant; ionic groups are exposed to water on the surface of the aggregate while the hydrophobic tails associate with each other within the interior of a roughly spherical aggregate of 60 to 100 molecules. Hydrophobic guest molecules can be taken into the interior of the micelle. This includes the hydrophobic amino acids normally confined to the interior of a native protein. The association between hydrophobic amino acid residues and the micelle interior is strong enough to denature most proteins, turning them inside-out so that become effectively coated with anionic surfactant molecules. Reductive cleavage of all –S-S- bonds followed by treatment with the anionic surfactant sodium dodecyl sulfate(SDS) will disrupt the native tertiary structure of most proteins, causing them to adopt rodlike structures. It has been established that this binding occurs with a constant surfactant-totprotein weight ratio and with enough anionic surfactants to totally dominate the native charge of the protein. Therefore, the charge per unit protein weight is nearly constant and the electrophoretic mobility of SDS denatured proteins is a function of size alone. The technique of SDS-polyacrylamide gel electrophoresis is widely used to determine the molecular weight of unknown proteins by comparing their relative electrophoretic mobility to standard proteins of known molecular weight. A direct comparison of the mobilities of known and unknown proteins run under identical conditions (in the same cell at the same time) is recommended. The mobilities of known proteins can be plotted as a function of log(molecular weight) and the mobilities of known proteins used to estimate molecular weights by extrapolation. A necessary first step in the protein sample preparation for SDS electrophoresis is treatment with an excess of 2-mercaptoethanol, which reduces all disulfide(-S-S-) bonds in the protein. This permits total disruption of the protein native structure, which is usually stabilized by disulfide linkages. Some proteins(e.g., chymotrypsin) contain polypeptides