in Amino acids, when pH is less than pKa, why does site remain protonated?

[csx]

and visa versa become deprotonated when pH is higher than pKA??

im having troulbe wrapping my head around this

---

Members don't see this ad.

 

[justadream]

If you have low pH, that means you have a ton of H+ floating around. Some of those are bound to attach to your molecule, giving it a positive charge.

If you have a high pH, that means you have barely any H+ floating around. So it's more likely that when your molecule momentarily loses its proton, it's harder to get another one to attach to it. This may leave your molecule with a negative charge.

 

[blackmarble]

I view it more as an equilibrium between an acid and its conjugate base, so when pH is higher than the pKA, this means H+ levels are declining, and the acid will give up its proton (become deprotonated) in an attempt to bring the H+ levels back to its pKA.

Can someone confirm that my Kaplan Orgo book has it wrong? It states: "When pH>pKa1, the carboxyl group is protonated and the amino acid has a positive charge", which I believe should be the opposite.

 

[popopopop]

An amino acid has an amino side and a carboxylic side on it's backbone, attached to a chiral carbon. The R group will vary and determines what type of amino acid you have.

The amino side has a pH of about 9-10 while the COOH side will stay protonated at a pH of 2. The R groups will each have their own pKa if they are acidic/basic.

Compare the chart above to the pH curve of aspartate (aspartic acid depending on its environment). Pk1 is the half equivalence point, so pk1 = pH. The COOH will stay protonated at that pH of 2.09. You go up the curve and see the pI (isolectric point), where the molecule has no net charge. The pI is about 3, so since it's in a more basic environment (more OH- to deprotonate), COOH gets deprotonated to COO-. The side chain of aspartate has a pK of 3.86, so anything above 3.86 on the curve, it too will get protonated. Lastly, pK3 is the half-equivalence point where pK3 = pH = 9.82. The chart curves up, NH3+ is finally deprotonated to NH2 on the amino side.

 

[popopopop]

I view it more as an equilibrium between an acid and its conjugate base, so when pH is higher than the pKA, this means H+ levels are declining, and the acid will give up its proton (become deprotonated) in an attempt to bring the H+ levels back to its pKA.

Can someone confirm that my Kaplan Orgo book has it wrong? It states: "When pH>pKa1, the carboxyl group is protonated and the amino acid has a positive charge", which I believe should be the opposite.

It sounds wrong because according to the pKa table posted above, the carboxylic acid group is always the first to be deprotonated since they are happy in an environment of 2 pH. The amino side group wouldn't be the one to be deprotonated first since we're comparing it to an acid. Perhaps it's the context of the statement?

 

[DrDreams]

This is correct, because majority of amino acids exist as zwitterions at physiological pH right? That is because cooh has been deprotonated above pKa 1 and it has not yet reach the pKa of the amine group. Hence the COO- and NH3+. Above about a pH of 9.33 etc for the other basic AA the amine will be deprotonated.

 

[kraskadva]

I view it more as an equilibrium between an acid and its conjugate base, so when pH is higher than the pKA, this means H+ levels are declining, and the acid will give up its proton (become deprotonated) in an attempt to bring the H+ levels back to its pKA.

Can someone confirm that my Kaplan Orgo book has it wrong? It states: "When pH>pKa1, the carboxyl group is protonated and the amino acid has a positive charge", which I believe should be the opposite.

Taking just the last part alone "the carboxyl group is protonated and the amino acid has a positive charge" is correct, because the -COOH has no charge and the NH3+ has a positive charge.
Including the first bit "pH>pKa1" however is the opposite.
Double check it you have a ">" or "<" sign though... but if it's as you wrote it, then yes, somebody made a typo.

 

[blackmarble]

Taking just the last part alone "the carboxyl group is protonated and the amino acid has a positive charge" is correct, because the -COOH has no charge and the NH3+ has a positive charge.
Including the first bit "pH>pKa1" however is the opposite.
Double check it you have a ">" or "<" sign though... but if it's as you wrote it, then yes, somebody made a typo.

Thanks, yeah I just checked and indeed somebody made a typo.

 

[hueso]

You can also think of it using Henderson-Hasselbach...

pH=pKa+log[A-]/[HA]

Which is to say, given a pH and the pKa of the group of interest, you can set the equation up and have it look like this:

pH=pKa+log(X)

where X represents the ratio of deprotonated to protonated species. So you can rearrange and get 10^-(pH-pKa)=X

At least that's how I did it.