clarification

[sam0001]

 what does glutamic acid look like in a low pH? my teacher said you should draw it as a zwiterron. im not sure though. that doesnt seem to make sense. ideas?

[Hellrocks]

Your teacher is unfortunately wrong. At low pH, the side chain, carboxyl and amino group would all be protonated. As low pH indicates an acidic environment, favouring protonation.

[Ravit]

I might be wrong, but i don`t think that if you were to dissolve glutamic acid in a low pH environment, would lead to the production of a zwitter ion. Instead the low pH would cause the primary amine functional group to bond with an hydrogen atom from the surrounding environment giving the nitrogen atom an overall positive charge. The carboxyl functional groups would not be affected by the acidic environment and therefore would remain neutral.
Therefore the resultant molecule would be cation, more specifically a cation that can act as an triprotic acid.
if you were to draw the resultant molecule you would have add another hydrogen to the nitrogen atom and draw a plus sign above nitrogen to show that it is positively charged, the rest would remain the same.
Could some confirm whether my reasoning is right?

[jasoN-]

Yep that's right, you'd get a cation, with the basic group being protonated (pH<pKa).
Carboxylic groups (with pKa ~2 and ~4 for glutamic acid) will remain unionised at low pH (<2)
So no, you won't get a zwitterion. Expect formation of zwitter ions within pH ranges between 3-9 give or take for this particular amino acid.

[sam0001]

Yeah that's what I thought! But what about ph greater than 3 because we don't learn about isoelectric points, do we just draw it protonated?

[jasoN-]

Not really sure what you're asking, but to draw the structure just look at what pH you're given, and the pKa's of the corresponding ionisable groups of the structure.
So again using glutamic acid, and at a physiologic pH (7.4) for example;
We have pKa values of:
- pK1(COOH) ~2.16
- pK2(NH2) ~9.58
- pKR(COOH side chain) ~4.15
The carboxylic and amino groups will both be ionised (carboxylic deprotonated to form carboxylate R-COO-, and amine group protonated to get ammonium R-NH3+) and the side chain carboxylic acid will also be ionised (R-COO-)
Basically the amino acid is in its zwitterionic form, with an overall charge of -1
Tips:
- pH<pKa: protonated
- pH>pKa: deprotonated
- for acids: protonation = non-ionised, deprotonation = ionised
- for bases: protonation = ionised, deprotonation = non-ionised

[sam0001]

Umm...I'm not too sure what pka values are at this point. That's unit 4! But thanks