glutamic acid

[Martoman]

I don't get how this would be in a neutral pH as there are two carboxyl groups. The only thing I can think of is that one of the carbxyl groups will have its proton and the other not, so the net charge is netural. So if this is the case what is the net charge of an alkaline solution containing this acid? In this situation it acts as an acid... so which protons does it donate?

[kenhung123]

I think both H comes off

[Toothpaste]

I don't get how this would be in a neutral pH as there are two carboxyl groups. The only thing I can think of is that one of the carbxyl groups will have its proton and the other not, so the net charge is netural.

With glutamic acid, three parts of it can be ionised depending on the pH of the environment it's in. So it can have:

• NH3+ (+1)
• NH3+/COO- (0, zwitterion)
• NH3+/COO-/COO- (-1, side chain now ionised)
• NH2/COO-/COO- (-2)

(in order of increasing pH, acidic to basic environment)

So if this is the case what is the net charge of an alkaline solution containing this acid? In this situation it acts as an acid... so which protons does it donate?

I'll try not to over explain because yes there is more to it, so thumbs up for noticing that glutamic acid is more complex than the humble glycine. Well it depends "how alkaline" the solution is since the side chains of amino acids all have specific "acid dissociation constants" A.K.A. a qualitative measure of the tendency to give up a proton. The side chains of each amino acids affect "when" parts of the amino acid is ionised. But yeah, usually the carboxyl group (not the R side chain one) would be ionised "first" if you start from low pH (acidic) and move to a higher pH (basic). The side chain (R) is ionised at around ~pH 4.25 if you're curious, i.e. when you'd start to see the "-1" form. At a lower ~pH 2.2 is when the +1 form exists in equilibrium with 0. So I guess you would say it's in the zwitterion form between pH 2.2 to pH 4.25*.

I don't remember actually having to know when it actually exists in a certain form since the course lacks a lot of detail. Isn't VCE just... basic environment = ta da amino acid acts as a buffer and is now more negative by giving up protons, acidic environment = amino acid is now more positive by accepting protons from the acidic environment.


*at 3.22 which is its isoelectric point to be specific. (calculations are )

CONCLUSION: The VCE course likes to bullshit and dumb things down. Exams would probably say that glutamic acid gives a zwitterion around pH 6-7 because they are lying and are just removing all these amino acid exceptions.

EDIT: see http://vcenotes.com/forum/index.php/topic,26424.msg267995.html#msg267995