[themonk]

Well I got a homework back and it stated that I should have said it never occurs.

My rationale was partially described within the book (Essential Biochemistry, Pratt 2e pg 40). The following was in the book:
pH<2  - COOH and NH3⁺
4<pH<9 - COO^- and NH3⁺
pH>10 - COO^- and NH2

I said the best chance of finding both are within the range of 4 and 9, since "below 2 they are mostly protonated and above 10 they are mostly deprotonated" (quotes summarizing from book).

How exactly is my answer incorrect? Other than the fact that there are only very small amounts of COOH and NH2 in the 4 to 9 range.

[darko]

in aqueous solution, the right answer is never, there is always balance beetwean Zwitter ion, (NH3+, COOH) and (NH2, COO-)

[Doc Oc]

in aqueous solution, the right answer is never, there is always balance beetwean Zwitter ion, (NH3+, COOH) and (NH2, COO-)

Your answer is correct, but those aren't zwitterions, both the amine and acid should be charged (NH₃⁺, COO^-).

The reason is that when you get to an intermediate pKa you are sitting in a range where the amine will be basic enough to grab a proton and the acid is acidic enough to donate one.  That's why it exists as a zwitterion.

[darko]

in aqueous solution, the right answer is never, there is always balance beetwean Zwitter ion, (NH3+, COOH) and (NH2, COO-)

Your answer is correct, but those aren't zwitterions, both the amine and acid should be charged (NH₃⁺, COO^-).

The reason is that when you get to an intermediate pKa you are sitting in a range where the amine will be basic enough to grab a proton and the acid is acidic enough to donate one.  That's why it exists as a zwitterion.

correct, but Zwitter ion is one of three mentioned ionic structures, Zwitter ion (NH3+, COO-) is one, (NH3+, COOH) is second and (NH2, COO-) is third, I did not predict to explain what Zwitter ion is, so those are three different ionic structures, please read more carefully

[themonk]

Alright, thank you very much. I guess I will have to read into it more clearly.