Question

3. (a) What is SDS (sodium dodecylsulfate)? Draw the structure

(b) How would you separate a mixture of polypeptides according to their lengths using SDS –

polyacrylamide gel electrophoresis? Describe in a few sentences.

Answer 1

(a) SDS (sodium dodecyl sulphate) is an anionic detergent that unfolds proteins and provides them with extra negative charges.

(b) The SDS gel separates individual polypeptide chains (monomeric proteins and subunits of multimeric proteins) according to their size. The velocity of the proteins is an inverse linear function of the logarithm of their molecular mass. Proteins of known molecular mass can be used to establish a calibration curve (a descending line) along which the unknown molecular mass of other proteins can be estimated. the migration velocity of proteins is a function of their size, shape and the number of electric charges they carry. In SDS-PAGE (PolyAcrylamide Gel Electrophoresis) An electric field is applied across the gel, causing the negatively charged proteins or nucleic acids to migrate across the gel away from the negative electrode (which is the cathode being that this is an electrolytic rather than galvanic cell) and towards the positive electrode (the anode). Depending on their size, each biomolecule moves differently through the gel matrix: small molecules more easily fit through the pores in the gel, while larger ones have more difficulty.SDS-treated proteins—just like denatured linear DNA molecules—will be separated solely based on their size. As size is a linear function of mass, SDS-PAGE ultimately separates proteins based on their molecular mass.