Question 25

[mvenus9292913]

How are we supposed to know the isoelectric point (within 1 unit of pH) without being given the pKa values? I know that glutamatic acid has a relatively acidic pI, but without knowing the exact values, how are we supposed to guess?

[mcat_premed3832]

Here is what you are expected to know for the MCAT: usually, amino acids in solution are in equilibrium with their dipolar form (zwitterion), anionic form and cationic form. The pH at which the amino acid is neutral (no net charge) is the isoelectric point: pI. You are expected to know how to calculate pI: usually it is simply the average of the pKa values of the amino and carboxylic acid groups. Those values were provided in the passage.

This problem is one level more difficult than what you should know but helps to deepen your understanding of pI. When calculating the pI of an amino acid that has a titratable group on the R side chain (ie, glutamic acid), it is useful to start by writing the structure of the amino acid at physiological pH (around 7) at which point you would have one amino group protonated and 2 carboxylate anions (overall negative). To be neutral, you would have to lower the pH. Thus pI is actually the average of the pKa of the 2 carboxylic acid groups.

[mvenus9292913]

Ok, I see the pKa value for one of the carboxylic acid groups, but only one of them. As I said, I know how to calculate pI, but we weren't given all the values.

The formula of glycine is as follows: H2N-CH2-COOH. The pKa of the -COOH group is 2.3 and the pKa of the -NH3 group is 9.6. The isoelectric point is at pH 6.0.

...

25. The structure of glutamic acid is shown below.

The isoelectric point of glutamic acid is:

If the answers were different, like, say, an acidic group, a basic group, a neutral group, and a slightly acidic group, I could understand where the answer would come from. But the answer choices weren't that spread out. They were all acidic.

If you consider the isoelectric point of glycine as the time when the other carboxylic acid group gets deprotonated, then you'd choose C as the answer, not B.

[admin]

Sorry, there was one piece of information you must incorporate: The pKa of -CH2CH2COOH is 4.7.

The answer is the average of the 2 lower pKa values for the reason previously explained.

[nsross]

why wouldn't the amino group in glutamic acid be taken into consideration when calculating the IP? i understand it's a rather acidic molecule and it has a titratable R group, but how does the basic character of the amino have NO effect on the overall IP?

[admin]

First some perspective: as long as you understand the true meaning of amphoteric, zwitterion, pI is the average of the pKa of -NH2 and -COOH, pH above which the molecule is overall negative and below overall positive then you will get 99% of MCAT problems on this topic correct!

Now this question is one level more difficult than what they would normally ask but it is based on the same principles. At what pH would the molecule be electronically neutral? Because that is the definition of ISOelectric point, iso meaning the same.

So, let's not worry about calculating yet. Let's reason where the pH should be. So here you start by writing the structure of the amino acid at physiological pH (around 7) at which point you would have one amino group protonated and 2 carboxylate anions (overall negative). To be neutral, you would have to lower the pH. In fact, even if you lowered the pH to 4.8 (way below the amino group but still just above both carboxylic acid groups), still the overall charge would be negative. So clearly pI must be below 4.8 and thus it must be between the pKa values of the carboxylic acid groups rendering the amino group (already protonated and "neutralized" by the carboxylate anion with the lowest pKa) essentially irrelevant.

Thus pI is actually the average of the pKa of the 2 carboxylic acid groups.

I hope that seems clearer. If not, here are some references:

http://www.bio.mtu.edu/campbell/Hispi1.htm

http://chemed.chem.purdue.edu/genchem/topicreview/bp/1biochem/amino2.html

PS you do not need to memorize the names of amino acids, nor their side groups (-R) nor any of their related constants for the new MCAT.

[jgeng03]

If the pH was 4.7 wouldn't the NCC part of the AA have a + and - charge and wouldn't half the R groupes be protonated and half deprotonated? Why do we have to take the average of the R group and the COOH?

[admin]

This is a special case. Please refer to the links above to get better insight.

[asfino12217]

okay i understand the solution theoretically, when the ip isn't being calculated, but based solely on the calculations...why isn't the pKa of the amine group taken into consideration? doesn't the ip calculation take all functional groups into consideration?

[nedaa.asba6809]

Most MCAT problems have only 2 groups that can dissociate on the amino acid (so the simplest case is taken) in which case, you would be correct to include the amine. But here we have 3 groups which can ionize and the impact immediately changes. So in this case, the pKa of the amine group is not included in the Isoelectric Point calculations because of the following:
The Isoelectric point (pI) is the pH at which a particular molecule or surface carries no net electrical charge.
Amphoteric molecules called zwitterions contain both positive and negative charges depending on the functional groups present in the molecule. The pI is pH value at which the molecule carries no electrical charge or the negative and positive charges are equal.
Surfaces naturally charge to form a double layer. In the common case when the surface charge-determining ions are H+/OH-, the net surface charge is affected by the pH of the liquid in which the solid is submerged. Again, the pI is the pH value of the solution at which the surfaces carries no net charge.
The pI value can affect the solubility of a molecule at a given pH. Such molecules have minimum solubility in water or salt solutions at the pH which corresponds to their pI and often precipitate out of solution. Biological amphoteric molecules such as proteins contain both acidic and basic functional groups. Amino acids which make up proteins may be positive, negative, neutral or polar in nature, and together give a protein its overall charge. At a pH below their pI, proteins carry a net positive charge; above their pI they carry a net negative charge.
http://butane.chem.uiuc.edu/cyerkes/Chem204sp06/Lecture_Notes/lect27c.html

[kpbakshi5858]

Won't it migrate to the anode? I know that the AA will have a net positive charge, so it should migrate to the negatively charged electrode... but I thought the anode was negatively charged?

This is a confusing topic, because I know in glavanic cells, the anode is negative, while in electrolytic cells, the anode is positive? Are we suppose to assume that because the electric field would need a power source (i.e. battery), that we should refer to the anode as the positively charged electrode?

[mcat_premed3832]

Your question is referring to Q24 and it is answered in detail in that thread: www.mcat-prep.com/forum/viewtopic.php?t=723

[GraemeMatt8033]