Talk:Zwitterion

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I don't know about the definition of zwitterion. Other sources claim the definition is an ion with both a negative and positive formal charge somewhere in the structure. I've classified p-Azoxyanisole as a zwitterion based on this definition.

--Edsanville 21:07, 2 Jun 2004 (UTC)

[edit] Zwitterion definition

I am of the understanding that zwitterion refers only to the form of a molecule with one acidic and and one basic group where positive and negative formal charges are present at the same time, such that the net charge is zero. Therefore, a molecule could only be a zwitterion at its isoelectric point. At other pHs it would be anionic or cationic. The way the article is written now makes it seem like any amphoteric molecule is a zwitterion, when it would only be so at a particular pH, and then only if it had one acidic and one basic moiety. I want to make sure this is an accepted definition before I dive in and rewrite the page; any opinions? Porkchopmcmoose 02:05, 30 March 2006 (UTC)

A buffer always contains a weak acid and conjugate base in equilibrium. A zwitterion on its own could not function as a buffer. e.g CH2(COO-)NH3+, on its own has no buffering ability, but mixed with CH2(COOH)NH3+ or CH2(COO-)NH2, can act as a buffer.

Two concerns:

Just because a molecule that has acidic and basic components doesn't mean they both ionize to create a zwitterion. Creating charge or adding/removing proton from some position might destabilize the molecule in some way--for example, destroying aromaticity in 2-pyrrole carboxylic acid. At least in solid state, o-anthranilic acid is a 1:1 mixture of zwitterionic and non-ionic forms.
There are zwitterions that are not due to (effectively) intramolecular acid/base chemistry and equilibria, so juggling pH or other titration/solution games has no effect. Nitromethane and acetonitrile N-oxide are some simple cases.

DMacks 14:14, 14 March 2007 (UTC)

I always understood a zwitterion was any species with positive and negative charged groups at a given pH, I didn't appreciate that it had to be only 1 of each. Are you sure this is technically correct? What about something with 2 positive charges and 2 negatives, is this a zwitterion? Aaadddaaammm 23:19, 31 March 2007 (UTC)

OK, after consultation of a textbook, I'll agree that it has to have no net charge, but can it have many charged groups all cancelling each other out? Ie. is a protein at its pI a zwitterion? Aaadddaaammm 23:22, 31 March 2007 (UTC)