SDS–PAGE workflow in protein biochemistry — What is the correct initial sample treatment before electrophoretic separation on a polyacrylamide gel containing the anionic detergent SDS?

Introduction:SDS–PAGE is a foundational technique to separate proteins primarily by molecular mass. The preparative chemistry before loading is critical: proteins are denatured and, if required, disulfide bonds are reduced to ensure subunits migrate according to size rather than shape or native charge.

Given Data / Assumptions:

• SDS is an anionic detergent that binds proteins roughly in proportion to length.
• Reducing agents such as β-mercaptoethanol or DTT cleave disulfide bonds.
• Electrophoresis is performed in a polyacrylamide matrix under an electric field.

Concept / Approach:SDS unfolds proteins and confers a near-constant negative charge-to-mass ratio. Reducing agents remove disulfide constraints so each polypeptide chain migrates independently. Correct sample preparation precedes electrophoresis to standardize migration behavior across diverse proteins.

Step-by-Step Solution:

Verification / Alternative check:When reduction is omitted, covalently linked subunits co-migrate as larger species. Including reducing agent yields band patterns that match subunit molecular masses expected from known sequences.

Why Other Options Are Wrong:

Common Pitfalls:Confusing native PAGE with SDS–PAGE; neglecting reduction when analyzing multimeric proteins; underheating samples leading to incomplete denaturation.

Final Answer:Treat with a reducing agent and SDS, then perform electrophoresis.