Principle of SDS–PAGE — On what fundamental property does SDS–PAGE separate protein species?

Introduction:
SDS–PAGE is designed to compare polypeptides primarily by size. SDS binds and unfolds proteins, minimizing effects of native charge and shape so that separation reflects length (mass) of the polypeptide chain.

Given Data / Assumptions:

• SDS confers a near-constant negative charge-to-mass ratio.
• Reducing agents may be included to break disulfide bonds.
• Electrophoretic mobility in the gel is inversely related to log(molecular mass).

Concept / Approach:
Because charge is standardized by SDS binding, migration differences are predominantly due to size. Smaller proteins travel farther through the polyacrylamide matrix within a fixed time frame.

Step-by-Step Solution:

Verification / Alternative check:
Plotting migration distance against log(MW) for standards produces a calibration curve used to estimate unknowns.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming glycosylation or extreme hydrophobicity never affect mobility; unusual modifications can cause anomalies, but the core principle remains mass-based separation.

Final Answer:
Molecular weight (apparent mass).