Ammonium persulfate is added to an SDS-PAGE setup because:

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Question restatement: Ammonium persulfate is added to an SDS-PAGE setup because:

Option 1: it denatures the protein.
This is not correct. Denaturation of proteins in SDS-PAGE is achieved mainly by SDS detergent and reducing agents, not by ammonium persulfate. Ammonium persulfate acts in the gel preparation rather than directly affecting protein structure.

Option 2: it breaks disulfide bonds.
This is incorrect. Breaking disulfide bonds is accomplished by reducing agents such as beta-mercaptoethanol or dithiothreitol (DTT). Ammonium persulfate does not function as a reducing agent for disulfide bonds.

Option 3: it prevents the proteins from exiting the bottom of the gel.
This statement is inaccurate. The placement of protein bands within the gel is determined by their size and the gel’s sieving properties, as well as the electrophoretic force. Ammonium persulfate does not act as a barrier to migration.

Option 4: it promotes the cross-linking of the polyacrylamide.
This is the correct description. Ammonium persulfate is an initiator used in combination with TEMED to generate free radicals that initiate polymerization of acrylamide, leading to the formation and cross-linking of the polyacrylamide gel matrix. This cross-linked network provides the sieving medium through which proteins migrate during SDS-PAGE.

Additional context for understanding: In SDS-PAGE, the gel polymerization step relies on free-radical polymerization chemistry. APS decomposes to produce sulfate radicals in the presence of TEMED, which then polymerize acrylamide monomers into a cross-linked network. The protein components are denatured and coated with SDS to impart uniform charge, but APS’s role is specifically in gel formation, not in protein denaturation or disulfide bond reduction, nor in trapping proteins at the bottom.

Ammonium persulfate is added to an SDS-PAGE setup because:

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