What is the purpose of SDS in SDS-PAGE?

Breaks disulfide linkages
Unfolds proteins to secondary structure
Coats proteins in a negative charge

Question

What is the purpose of SDS in SDS-PAGE?

Breaks disulfide linkages 
Unfolds proteins to secondary structure
Coats proteins in a negative charge

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Question: What is the purpose of SDS in SDS-PAGE?

Option 1: '1 and 2' — This would imply that the purpose includes both 'Breaks disulfide linkages' and 'Unfolds proteins to secondary structure.' While SDS-PAGE sample preparation can involve reducing agents that break disulfide bonds, and SDS itself helps to denature proteins, the claim that SDS unfolds proteins specifically to a secondary structure is scientifically inaccurate. Denaturation generally leads to unfolded or linearized polypeptides (reducing or uncoiling beyond native tertiary structure), not a defined secondary structure state. Thus this combination mixes an element that can be true under reducing conditions with an incorrect description of the unfolding outcome.

Option 2: '1 only' — This option asserts only that SDS breaks disulfide linkages. As a standalone claim about SDS, this is incomplete. SDS is primarily known for imparting a uniform negative charge and helping denature proteins, whereas breaking disulfide bonds is typically achieved by a reducing agent (e.g., DTT or beta-mercaptoethanol) in the sample buffer, not by SDS alone. Therefore, this option omits a major, correct aspect and overstates the role of SDS in isolation.

Option 3: '3 only' — This statement claims that SDS coats proteins in a negative charge. This is a central and defining feature of SDS in SDS-PAGE: it binds to proteins and confers a roughly uniform negative charge-to-mass ratio, enabling separation by size during electrophoresis. However, claiming this option as the entire purpose ignores the denaturing effect of SDS as well as the role of reducing agents in removing disulfide bonds when present. But as a description of one key effect, it captures a core function of SDS.

Option 4: '2 and 3' — This option combines two claims: (a) SDS causes unfolding of proteins to a (misstated) secondary structure and (b) SDS coats proteins with a negative charge. The second half (coating with negative charge) correctly describes SDS’s major role. The first half, as discussed, misstates the typical outcome of unfolding (proteins are denatured toward a linear form, not specifically to a defined secondary structure). If we interpret the intention as acknowledging both denaturation and charge impartation, the second component is correct while the first is problematic if read as stating the precise structural outcome.

Option 5: '2 only' — This would indicate that SDS only unfolds proteins to secondary structure. As noted, SDS does contribute to denaturation, but the claim that it produces a defined secondary structure is inaccurate; furthermore, SDS’s most distinctive, consistently cited role is to impart negative charge for electrophoretic separation, which this option omits.

Across these choices, the reasoning hinges on understanding what SDS does in the context of SDS-PAGE: it denatures proteins to facilitate linearized polypeptides for size-based separation, and it binds to these polypeptides to confer a fairly uniform negative charge, allowing them to migrate according to length. The disulfide bond-breaking aspect is typically a function of reducing agents used in conjunction with SDS in the sample buffer, not an inherent sole property of SDS itself. The notion of unfolding to a specific secondary structure is not aligned with the standard description of SDS-induced denaturation.

CHEM108_WI25_A00 Final Exam

What is the purpose of SDS in SDS-PAGE? Breaks disulfide linkages Unfolds proteins to secondary structure Coats proteins in a negative charge

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