# Electrophoresis/Isoelectric Point

Discussion in 'MCAT Study Question Q&A' started by Revilla, Jan 3, 2009.

1. ### Revilla New Member 5+ Year Member

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I thought the anode in electrophoresis is negatively charged. If that's true, can someone please explain this answer to me? It's from Kaplan FL 2.

An amino acid is subjected to electrophoresis at pH 8.5 and is observed to migrate to the anode. The IP point of this amino acid is:

The answer is less than 8.5 because at 8.5, it has a net negative charge and is migrating toward the anode.

But if amino acid is migrating toward the anode, shouldn't it have a net positive charge since the anode is negative? And if it has a net positive charge, would the IP be more than 8.5 or less?

Can someone please help? I guess I don't understand isoelectric point as well as I thought I did.

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3. ### rocuronium 7+ Year Member

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In electrophoresis the anode is positively charged. Take a read through this post by BerkReviewTeach.

http://forums.studentdoctor.net/showpost.php?p=7201963&postcount=2

As for the isoelectric point, it is the pH at which a protein carries no net electrical charge. However, since there is migration during electrophoresis, we know there is a charge on the protein. Whether the protein carries a positive or negative charge is dependent on the difference between the pI of the protein and the pH of the solution it is in.

So let's look at what we have so far. We know that the anode is positively charged and that the protein is moving toward it. This means that the protein must hold a negative charge.

So then what we need to know to answer this question is whether a protein has a negative charge in a solution more acidic or more basic than its pI.

You have the answer. Can you figure out why it's right?

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