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Topic: When will an amino acid exists as anion?  (Read 188 times)

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Offline JasonTee

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When will an amino acid exists as anion?
« on: February 23, 2021, 12:07:09 AM »
Hi, Im a college student and came across this question recently:

The pH value of Cysteine, Isoleucine, Histidine and Lysine are 5.0, 6.0, 7.6 and 9.7 respectively. Which of the amino acid mentioned exists as anion at pH 6.0?


The answer given states that Lysine (with the pI of 9.7) will exist as anion at pH 6.0. However, my concept is amino acid will only exist as anion when the surrounding pH is greater than pI. So if my concept is correct, I suppose that the correct answer will be Cysteine, as it exist as Zwitterion at pH 5.0 and the surrounding pH 6 is higher than the isoelectric point.
Anyone can point out the flaws in my concept? Thanks in advance.


Offline JasonTee

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Re: When will an amino acid exists as anion?
« Reply #2 on: February 23, 2021, 01:41:39 AM »
https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book%3A_Basic_Principles_of_Organic_Chemistry_(Roberts_and_Caserio)/25%3A_Amino_Acids_Peptides_and_Proteins/25.02%3A_Acid-Base_Properties_of_(alpha)-Amino_Acids

So after reading the text, it seems that anion forms when the surrounding pH is at the extreme basic end (towards the direction of pH12) and the formation of cation happens at the extreme acid end (towards the direction of pH 1) while for the pH in between the amino acid tends to be in the form of dipolar ion? However, how do I use this info to tackle the question?

Online AWK

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Re: When will an amino acid exists as anion?
« Reply #3 on: February 23, 2021, 02:03:13 AM »
https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book%3A_Basic_Principles_of_Organic_Chemistry_(Roberts_and_Caserio)/25%3A_Amino_Acids_Peptides_and_Proteins/25.02%3A_Acid-Base_Properties_of_(alpha)-Amino_Acids

So after reading the text, it seems that anion forms when the surrounding pH is at the extreme basic end (towards the direction of pH12) and the formation of cation happens at the extreme acid end (towards the direction of pH 1) while for the pH in between the amino acid tends to be in the form of dipolar ion? However, how do I use this info to tackle the question?
A poor question is sometimes difficult to answer. The attaching or cleavage of a proton is an equilibrium process and some of the information we can get without computation is only qualitative. Point pI will not answer your question. This corresponds approximately to the pKa point of the amino group.
AWK

Offline Babcock_Hall

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Re: When will an amino acid exists as anion?
« Reply #4 on: February 23, 2021, 10:35:01 AM »

So after reading the text, it seems that anion forms when the surrounding pH is at the extreme basic end (towards the direction of pH12) and the formation of cation happens at the extreme acid end (towards the direction of pH 1) while for the pH in between the amino acid tends to be in the form of dipolar ion? However, how do I use this info to tackle the question?
http://tools.thermofisher.com/content/sfs/appendix/elec_blotting/overview.pdf
I would have said, "Which amino acid bears some negative charge?"  A useful example from biochemistry is the stacking gel of SDS PAGE of proteins is glycine at pH 6.8 (Tris•HCl), which bears less of a negative charge than chloride ion in the buffer.  The proteins are sandwiched in between the chloride ion, which move quickl, and the glycine species, which move more slowly (see link).

I don't agree with what you have written above.  I do think that pI values are useful in this problem, but I have seen slight differences in pKa values for the amino acids in various tables.  This would lead to slight differences in the pI values calculated from them.
EDT
You wrote, "The pH value of Cysteine, Isoleucine, Histidine and Lysine are 5.0, 6.0, 7.6 and 9.7 respectively..."  Do you mean pH or pI values?
If the answer key said lysine is an anion at pH 6, then the answer key is wrong.  It happens.
« Last Edit: February 23, 2021, 02:18:12 PM by Babcock_Hall »

Offline JasonTee

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Re: When will an amino acid exists as anion?
« Reply #5 on: February 23, 2021, 07:44:20 PM »

So after reading the text, it seems that anion forms when the surrounding pH is at the extreme basic end (towards the direction of pH12) and the formation of cation happens at the extreme acid end (towards the direction of pH 1) while for the pH in between the amino acid tends to be in the form of dipolar ion? However, how do I use this info to tackle the question?
http://tools.thermofisher.com/content/sfs/appendix/elec_blotting/overview.pdf
I would have said, "Which amino acid bears some negative charge?"  A useful example from biochemistry is the stacking gel of SDS PAGE of proteins is glycine at pH 6.8 (Tris•HCl), which bears less of a negative charge than chloride ion in the buffer.  The proteins are sandwiched in between the chloride ion, which move quickl, and the glycine species, which move more slowly (see link).

I don't agree with what you have written above.  I do think that pI values are useful in this problem, but I have seen slight differences in pKa values for the amino acids in various tables.  This would lead to slight differences in the pI values calculated from them.
EDT
You wrote, "The pH value of Cysteine, Isoleucine, Histidine and Lysine are 5.0, 6.0, 7.6 and 9.7 respectively..."  Do you mean pH or pI values?
If the answer key said lysine is an anion at pH 6, then the answer key is wrong.  It happens.

Yes I made a mistake when typing the question, the pH value of the amino acids mentioned is the pH value that the amino acid exists as Zwitterion, which I think means the pI point of the amino acids as well. I do think the question mean to ask "which amino acid bears the most negative charge" in this case. If this is the case, is lysine the amino acid that bears the most negative charge?
My first instinct told me that the one that bears the most negative charge will be cysteine in this case because the surrounding pH value is greater than the pI point of it, which makes it act as an acid and therefore donate proton and form COO- anion. Isoleucine have the same pI point as the surrounding pH so it will remain as neutral dipolar ion, while Histidine and Lysine will act as base and receive proton to form NH3 + cation, where Lysine bears more positive charge than Histidine. Sorry that I did not make my question and doubts clear enough, it's the first time I post in this forum :'(.

Offline Babcock_Hall

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Re: When will an amino acid exists as anion?
« Reply #6 on: February 23, 2021, 09:03:05 PM »
At pH 6 the predominant form of lysine will have a negatively charged carboxylate group, a positively charged alpha-ammonium group, and a positively charged epsilon-ammonium group.  Its net charge will be close to +1.  One can use pKa values to find the net charge at any pH.

The isoelectric point is the pH at which an amino acid or protein has no net charge.  At other pH values, the net charge on an amino acid or protein will be positive or negative.  I agree with your answer, cysteine, but I would also consider the behavior of the R-SH group, which can lose a proton.
« Last Edit: February 23, 2021, 09:37:34 PM by Babcock_Hall »

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