# Isoelectric point

Discussion in 'General Science' started by boks, Nov 23, 2008.

1. ### boks Thread Starter Active Member

Oct 10, 2008
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The isoelectric point is define as the pH t which there is no net charge.

If the carbonyl group has pKa = 2 and the protonated amino group pKa = 9 the amino acid will exist as a zwitteron in solutions with pH between 2 and 9, right? A zwitteron has no net charge, right? So why is it called the isoelectric point, and not the isoelectric region?

2. ### jpanhalt Expert

Jan 18, 2008
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How did you do with last week's assignment? It would help us to know at what level of training you are, as some questions may be answered at different levels. I for one would appreciate some response from our questions to you.

As for your current question, you recognize there is a typo. It is not the carbonyl group, but rather the carboxyl group.

You are correct that in the pH range between the two pKa's some of the amino and carboxylic acid groups will be ionized. Given a single pKa, you should be able to calculate the percentage of either group that is ionized at any pH. Obviously, near pH 2, most of the amino group will be ionized (protonated) and only half of the carboxyl groups will be, so there is a net positive charge. Conversely, at pH 9, the carboxylic acid will be essentially completely ionized and the amino group will be half ionized. Somewhere in between those extremes, the extent of ionization of each groups will be equal, which results in no net charge (isoelectric point).

Are you aware of the Henderson-Hasselbalch equation, which can be of great help?

John

3. ### boks Thread Starter Active Member

Oct 10, 2008
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Danke schön.

Last edited: Nov 23, 2008
4. ### jpanhalt Expert

Jan 18, 2008
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Yes, but a mental picture of what's going on, either from the equilibrium equations or HH , would help one understand there is an isoelectric point, not range. John

Edit: A simple solution for pI like you suggest is fine for simple amino acids. However, have you considered dicarboxylic and dibasic amino acids, like aspartic and lysine? That's why in answering your questions it would help to know whether you are in the equivalent of high school (generally ages 13-18), college, university, or post garduate.

Last edited: Nov 23, 2008
5. ### studiot AAC Fanatic!

Nov 9, 2007
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This stuff is well above high school level.

Yes we are happy to help and offer encouragement and full recognition of your efforts.

But it is not a trick question to wonder what you are studying, especially taken with your earlier organic chemistry and wave mechanics questions.

This is meaty stuff, keep it up.

However I for one am not happy to spend time and effort replying to questions at this level without knowing if the answers ever reach their target. So please complete your side of the bargain.

6. ### boks Thread Starter Active Member

Oct 10, 2008
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I'm studying nanotechnology. Chemistry, physics, electricity and biology melt together in the nano world. That's why I have questions from all these subjects.

OK, as an example:

Histidine:

pK1 (-COOH) = 1.82

pK2 (imidazolium) = 6.0

pK3 (-NH3+) = 9.17

How to calculate isoelectric point of histidine?

My book tells me to take the average of the pKa values of the similarly ionizing groups. The similarly ionizing groups is -COOH and imidazolium since these two are uncharged in their acidic form and negatively charged in their basic form, right? The answer I get is 3.91. The correct answer is 7.59 though, which is the average of 9.17 and 6.0. What's wrong with my thinking here?

7. ### jpanhalt Expert

Jan 18, 2008
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Did you read the edit to my most recent post on this matter? Check Wikipedia for aspartic. It has the answer, and you can work backwards from that.

John

8. ### boks Thread Starter Active Member

Oct 10, 2008
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http://en.wikipedia.org/wiki/Aspartate

I can't see the answer here.

Just to check that I've understood the basics; at low pH (< 1.8 ) the groups are present as COOH (neutral), imidazolium (neutral) and NH3 (+). At pH = 1.8, the COOH loses a proton and becomes negatively charged. At pH = 6 imidazolium becomes negatively charged and at pH = 9.2 NH3 becomes neutral. Correct?

Edit: Oh, I didn't realize that imidazolium is positively charged at low pH. The table I have is for pH = 7.3. Then I get the correct answer.

Last edited: Nov 23, 2008

Jan 18, 2008
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10. ### boks Thread Starter Active Member

Oct 10, 2008
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Arginin has pKa values 2.17, 9.04 and 12.48. Am I then right to say that the net charge of arginin at pH 9.04 and 12.48 is +0.5 and 0 respectively?

Last edited: Nov 24, 2008
11. ### jpanhalt Expert

Jan 18, 2008
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1,393
The pI is 10.76 (net charge = 0). At pH 12.48, the guanido group is still 50% protonated, the alpha amino group is mostly not protonated, and the carboxylic acid group is virtually 100% ionized. Thus, the net charge is about -0.5.

How close do you need to get to the actual calculated pI?

I think the best approach, if you are studying complex amino acids, is to attempt to solve the simultaneous equilibrium equations or HH equations for a compound like arginine or aspartic for which you know what the pI is supposed to be. You cannot simply average the logarithms. The simple alpha amino acids you asked about earlier were a special case.

John

12. ### boks Thread Starter Active Member

Oct 10, 2008
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I'm simply asked to find the net charges at the different pH values. Isn't this the simplest and best way to do it?

I see that I forgot to cancel the +0.5 charge from the alpha amino group before calculating the net charge at pH 12.48.

OK, then I move to the next chapter. Thanks a lot!

13. ### boks Thread Starter Active Member

Oct 10, 2008
218
0
How can I find the pI of a tetrapeptide like Lys-Ser-Asp-Ala?

There are four ionizable groups that I have to take into account:

Alpha amino of Lys: 8.95
Amino side chain of Lys: 10.79
Carboxyl side chain of Asp: 3.86
Alpha carboxyl of Ala: 2.34

The two first are positively charged ionizing to uncharged groups. The two last are uncharged groups ionizing to negatively charged groups.

pI is somewhere between 3.76 and 8.95 of course, but thats not a very accurate answer. Is it simply the average (6.46)?

When the net charge is positive, will it then move toward the anode or a cathode in an electric field? I'm confused which electrode is negative.

How can I know how many percent of the epsilon amino groups are protonated at pH 9.5?

Last edited: Nov 26, 2008