leucine electropherosis

[goodtime]

Electrophoretic separation of leucine from a protein sample would be least effective at which of the following pH values?

• 1.4
• 7.4
• 2.4
• 0.4

Answer is C. 7.4 because it has no net charge. Electrophoretic separation depends on the existence of a negative net charge.
None of these answer choices can represent leucine having a negative charge.... does it imply positive charges work?
Leucine will have a positive charge at phs of 0.4 and 1.4 ( b/c CA is not deprotonated yet) and a neutral charge at 2.4 and 7.4. It would be least effective in all cases.

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[Quinoline]

Have you read that electrophoresis can only be done on molecules with a negative charge? There are many types of electrophoresis. It would seem to me that you can just switch the power terminals of the 'negative molecule electrophoresis'. if you have positively charge molecules, you can set up the gel so that you insert the sample at the end of the gel with the positively charged anode and the other end being the negatively charged cathode, so the proteins will migrate.

pH 2.4 is very close to leucine's COOH pka, so around 1/2 of the molecules will still be charged. At pH 7.4, more will be neutral.

 

[jd989898]

Electrophoretic separation of amino acids usually drags basic AAs to the cathode, acidic AAs to the anode, and leaves non-charged AAs (like leucine) in the middle.

 

[RH8448]

In this case, they must have loaded the amino acids in the middle of the gel. The leucines wouldn't move which means that cannot be differentiated with other uncharged molecules/AA.

 

[desertrat12]

pH 2.4 is very close to leucine's COOH pka, so around 1/2 of the molecules will still be charged. At pH 7.4, more will be neutral.

I think this is a very important point when thinking about this question. There are different types of electrophoresis, but even if you didn't know this you would be able to reason through this question to get the right answer by thinking about what leucine would look like under each given condition. pH of 1.4 and .4 would basically be the same, net positive charge. pH of 2.4 would be (as quoted) some positive and some neutral. pH of 7 it would be all neutral. It is pretty clearly the answer. This isn't to say that the odd one out is always the correct answer but with some basics knowledge about electrophoresis it can be reasoned that the most neutral molecule will have a hard time being separated.

Also, you said that electrophoretic separation depends on the existence of a net negative charge. The general electrophoresis that I think we learn about is a system that will pull a negative charge toward the bottom of the gel thus pulling the most negative the farthest and thus is capable of separating amongst a sample of negative amino acids, however the question stem doesn't tell us what type of amino acids are in the protein sample. If we placed leucine and, say, aspartic acid together in a gel at a pH that leaves aspartic acid negative and leucine positive, it will pull the aspartic acid toward to the bottom of the gel, but it still successfully separated leucine from aspartic acid despite the fact that leucine was positive. A more correct way to describe the type of electrophoresis you are talking about it that migration to the bottom of the gel depends on the existence of a net negative charge. So, because we don't know what is in the random protein sample we shouldn't assume that a negative charge would be required for separation and we can be content with the knowledge that regardless of what is in the protein sample, a neutral amino acid would be least effectively separated.

 

[goodtime]

Have you read that electrophoresis can only be done on molecules with a negative charge? There are many types of electrophoresis. It would seem to me that you can just switch the power terminals of the 'negative molecule electrophoresis'. if you have positively charge molecules, you can set up the gel so that you insert the sample at the end of the gel with the positively charged anode and the other end being the negatively charged cathode, so the proteins will migrate.

pH 2.4 is very close to leucine's COOH pka, so around 1/2 of the molecules will still be charged. At pH 7.4, more will be neutral.

yeah i was thinking of the main electrophoresis done with DNA and the explanation also said electrophoresis separation is based on the existence of negative charge in the next question. But it makes sense you would just put the positive charged molecules at the anode so it travels to cathode