Amino Acid Basicity/Acidity

[MedPR]

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If you had an amino acid with side chain NH2, would it be considered basic or acidic?

EK says basic because the amine acts as a base. Makes sense from that point of view.

However, if you stuck this amino acid into physiologic pH (7.4) the amino side chain and amino group on the backbone would both be protonated, while the carboxylic acid would be deprotonated, thus giving this amino acid a net charge of +1. Since it's +1, it has a proton to donate. Doesn't that make it an acid?

 

[chiddler]

It's like asking if you add a bunch of NaOMe, is the product basic or acidic?

OMe- initially grabs a proton from solution to make it MeOH. Now it has a proton to donate. Do you see the analogy?

Can you say that MeOH is acidic? No because it doesn't give up protons very easily.

Similarly, at physiological pH, the amine is protonated. This means it absorbed protons from solution just as NaOMe did and reduced pH just as NaOMe did. It does not give up its proton at physiological pH.

 

[MedPR]

It's like asking if you add a bunch of NaOMe, is the product basic or acidic?

OMe- initially grabs a proton from solution to make it MeOH. Now it has a proton to donate. Do you see the analogy?

Can you say that MeOH is acidic? No because it doesn't give up protons very easily.

Similarly, at physiological pH, the amine is protonated. This means it absorbed protons from solution just as NaOMe did and reduced pH just as NaOMe did. It does not give up its proton at physiological pH.

Oh I see.

When it went into physio pH, it gained 2 protons and donated 1 proton. Net effect of 1 gained proton = base.

Thanks!