You try to set up an SDS-PAGE but forget to add the SDS to the sample or the gel; you don't realize this until the experiment is completed. At the end of the run, you see that protein A has traveled farther that protein B. What can you definitively say about the two proteins?

I. protein B is larger than protein A

II. protein A has a more compact shape than protein B

III. protein B has a greater negative charge than protein A

Question

You try to set up an SDS-PAGE but forget to add the SDS to the sample or the gel; you don't realize this until the experiment is completed. At the end of the run, you see that protein A has traveled farther that protein B. What can you definitively say about the two proteins?

I. protein B is larger than protein A

II. protein A has a more compact shape than protein B

III. protein B has a greater negative charge than protein A

BlackTom AI · Accepted Answer

The scenario describes an SDS-PAGE run where SDS was omitted. That means proteins were not denatured and coated with the anionic detergent, so their migration now reflects native properties such as size, shape, and charge, rather than a uniform separation by molecular weight.
Option I: 'protein B is larger than protein A.' In a native gel context, larger size tends to slow migration, but migration is also influenced by shape and net charge. Since the absence of SDS removes the consistent size-to-charge normalization, seeing A migrate farther than B does not definitively indicate that B is larger; it could be that B is larger yet more extended or carries a different charge that impedes movement differently. Therefore this statement cannot be concluded with certainty.
Option II: 'protein A has a more compact shape than protein B.' A more compact shape can contribute to faster migration in a native PAGE, but it is not the sole determinant; an elongated but highly charged protein might still migrate quicker under certain conditions, and conversely, a compact protein with a unfavorable charge could lag. Given only the observation that A moved farther, we cannot definitively attribute this to shape alone without considering charge and overall native properties. So this option is not definitively true.
Option III: 'protein B has a greater negative charge than protein A.' Net charge affects the direction and rate of migration in a native context, but various factors (size, shape, charge density, hydration) interplay to determine movement. The fact that A traveled farther does not allow us to conclude B has more negative charge; A could be moving faster due to a combination of smaller size, favorable shape, or higher net charge, and we lack specifics to isolate charge as the cause. Hence this statement cannot be concluded.
Thus, no single statement among I–III can be definitively inferred from the given information. The only accurate option given is that none of the above statements can be asserted with certainty under these SDS-less, native conditions.

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