The Student Room Group

Protonation

I feel like this is a silly question but I’m a bit confused on how amino acids are protonated? I thought since they’re proton donors they would be deprotonated at a low pH?
protonation is gain of proton.

amino acid has amine group which can accept proton since nitrogen act as a base, and a carboxylic acid group which is an acid as it can donate proton.

Carboxylic acids are partially dissociating, so higher proton concentration (lower pH) would mean they are less likely to dissociate.

therefore, at low pH its more likely the surrounding proton will go to the basic amine group rather than the carboxylic acid group deprotonating.
Reply 2
Original post by Terrafied
protonation is gain of proton.

amino acid has amine group which can accept proton since nitrogen act as a base, and a carboxylic acid group which is an acid as it can donate proton.

Carboxylic acids are partially dissociating, so higher proton concentration (lower pH) would mean they are less likely to dissociate.

therefore, at low pH its more likely the surrounding proton will go to the basic amine group rather than the carboxylic acid group deprotonating.


That makes sense thank yiuuuu
Reply 3
amino acids normally consist of zwitterions, they undergo self neutralisation. The basic amino group on one end will ne protonated by the acid group on the other end, thats why amino acids are ionic solids with high melting points!

However the structure of the amino acid zwitterion will vary depending on the pH of the solution it is in.

At high pH values the hydroxide ion present in the alkaline solution will simply deprotonate the ammonium ion:

As a simple example the same thing happens with an ammonium ion in strongly alkaline solutions:

OH- + NH4+ ........................... NH3 + H20

the same thing will happen to the RNH3+ group on the end of the amino acid. The carboxylate ion will obviously be present on the other end of the amino acid.

In strongly acidic solutions the carboxylate ions (conjugate base) will simply be protonated to reform the COOH carboxyl group. The amino group on the other end will be obviously protonated in the acidic solution.

If you google the isoelectric points of common amino acids then you will get the pH at which the amino acids exits in its molecular form

hope this helps a bit
at low pH, the concentration of hydrogen ions is high

so, (a more visual representation)
protonation of amino acid.png
the lone pair of electrons on the N is available for protonation/ can be donated
(R represents the rest of the amino acid)

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