Why does COOH lower the isoelectric point ?
An isoelectric point means more H+ ions are needed to cause NH2 to accept a H+ .
If COOH is present, then it isn't really going to compete with NH2 for Hydrogen atoms. It will, however , release H+ ions into solution, potentially increasing the pH that is the isoelectric point.
With this in mind, why does COOH lower the isoelectric point ?
Thanks
If COOH is present, then it isn't really going to compete with NH2 for Hydrogen atoms. It will, however , release H+ ions into solution, potentially increasing the pH that is the isoelectric point.
With this in mind, why does COOH lower the isoelectric point ?
Thanks
The isoelectric point is the pH at which a zwitterion is formed. A zwitterion is formed when the amine group accepts the H+ from the carboxylic acid group in an amino acid. This gives the overall ion both a positive charge (at the amine side, NH3+) and a negative charge (at the carboxylic acid side, COO-).
So if the zwitterion is added to a solution with a pH greater than it’s isoelectric point (aka more basic) then it will act as an acid and lose a proton. Losing a proton will increase it’s pH / isoelectric point.
If the zwitterion is added to a solution with a pH lower than its isoelectric point (aka more acidic) then it will act as a base and gain a proton. Gaining a proton will decrease it’s pH / isoelectric point.
COOH is essentially an acid, so it will have a lower pH than the isoelectric point of the zwitterion that it reacts with. So the COO- part of the zwitterion will gain a proton and hence make the overall pH more acidic and hence lowering the isoelectric point.
Not sure if any of that made sense?? I hope it did though!
So if the zwitterion is added to a solution with a pH greater than it’s isoelectric point (aka more basic) then it will act as an acid and lose a proton. Losing a proton will increase it’s pH / isoelectric point.
If the zwitterion is added to a solution with a pH lower than its isoelectric point (aka more acidic) then it will act as a base and gain a proton. Gaining a proton will decrease it’s pH / isoelectric point.
COOH is essentially an acid, so it will have a lower pH than the isoelectric point of the zwitterion that it reacts with. So the COO- part of the zwitterion will gain a proton and hence make the overall pH more acidic and hence lowering the isoelectric point.
Not sure if any of that made sense?? I hope it did though!
(edited 3 years ago)
Original post by ramzinoo
The isoelectric point is the pH at which a zwitterion is formed. A zwitterion is formed when the amine group accepts the H+ from the carboxylic acid group in an amino acid. This gives the overall ion both a positive charge (at the amine side, NH3+) and a negative charge (at the carboxylic acid side, COO-).
So if the zwitterion is added to a solution with a pH greater than it’s isoelectric point (aka more basic) then it will act as an acid and lose a proton. Losing a proton will increase it’s pH / isoelectric point.
If the zwitterion is added to a solution with a pH lower than its isoelectric point (aka more acidic) then it will act as a base and gain a proton. Gaining a proton will decrease it’s pH / isoelectric point.
COOH is essentially an acid, so it will have a lower pH than the isoelectric point of the zwitterion that it reacts with. So the COO- part of the zwitterion will gain a proton and hence make the overall pH more acidic and hence lowering the isoelectric point.
Not sure if any of that made sense?? I hope it did though!
So if the zwitterion is added to a solution with a pH greater than it’s isoelectric point (aka more basic) then it will act as an acid and lose a proton. Losing a proton will increase it’s pH / isoelectric point.
If the zwitterion is added to a solution with a pH lower than its isoelectric point (aka more acidic) then it will act as a base and gain a proton. Gaining a proton will decrease it’s pH / isoelectric point.
COOH is essentially an acid, so it will have a lower pH than the isoelectric point of the zwitterion that it reacts with. So the COO- part of the zwitterion will gain a proton and hence make the overall pH more acidic and hence lowering the isoelectric point.
Not sure if any of that made sense?? I hope it did though!
Im still a bit confused ; I understand if the Amino Acid is in basic conditions it acts as an acid and vice versa, but this still doesn't address the question. You are very generous for your time and explanation though , thank you
Original post by seals2001
An isoelectric point means more H+ ions are needed to cause NH2 to accept a H+ .
If COOH is present, then it isn't really going to compete with NH2 for Hydrogen atoms. It will, however , release H+ ions into solution, potentially increasing the pH that is the isoelectric point.
With this in mind, why does COOH lower the isoelectric point ?
Thanks
If COOH is present, then it isn't really going to compete with NH2 for Hydrogen atoms. It will, however , release H+ ions into solution, potentially increasing the pH that is the isoelectric point.
With this in mind, why does COOH lower the isoelectric point ?
Thanks
The COOH part of the molecule loses its H+ - it acts as an acid. COO- and H+ form.
The NH2 part of the molecule accepts a H+ - it acts as a base. NH3+ forms.
It isn't competition. Both get to do what they want to do.
Original post by Pigster
The COOH part of the molecule loses its H+ - it acts as an acid. COO- and H+ form.
The NH2 part of the molecule accepts a H+ - it acts as a base. NH3+ forms.
It isn't competition. Both get to do what they want to do.
The NH2 part of the molecule accepts a H+ - it acts as a base. NH3+ forms.
It isn't competition. Both get to do what they want to do.
I understand what they do, but I don't understand why they individually lower or increase the isoelectric point.
Original post by seals2001
I understand what they do, but I don't understand why they individually lower or increase the isoelectric point.
First of all, an amino acid at its isoelectric point does not contain all molecules with an NH3+ group AND a COO- group. On average, there are as many NH3+ as COO- groups on the various molecules present.
It is all down to interplay between multiple equilibria.
Using simple numbers:
At pH 8 both COOH and NH2 are deprotonated, i.e. COO- and NH2
At pH 7 both are 50% deprotonated, i.e. there are as many COO- as there are COOH, likewise NH2 and NH3+
At pH 6 both are protonated, i.e. COOH and NH3+
The isoelectric point here is 7. On average, there are as many COO- ions as there are NH3+.
Now, imagine a molecule with 2x COOH and 1xNH2, e.g. aspartic acid.
At pH 7, there will be twice as many COO- as there are NH3+, i.e. it isn't the zwitterion.
At pH 6.7 (made up number somewhere lower than 7) both equilibria are shifted towards protonation. If we get to the point of 2/3 protonation we have the situation where 1/3 of the 2x COO- groups are unprotonated and 1/3 of the NH2 are unprotonated.
If your math is any good, there are now as many COO- as there are NH3+, i.e. you have yourself a zwitterion at a pH value less than when you only had one COOH group.
w00t w00t
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