Zwitterionnnnnn

[destain]

So i'm wondering when an amino acid is dissolved in water, does the Z group follow the regular protein chain, as in the regular protein chain will become NH3+ and COO-, and in acidic and basic solution the Z groups do that too if there is a COOH or NH2, but how about in water where it is neutral, what happens?

[LessIsMore]

So i'm wondering when an amino acid is dissolved in water, does the Z group follow the regular protein chain, as in the regular protein chain will become NH3+ and COO-, and in acidic and basic solution the Z groups do that too if there is a COOH or NH2, but how about in water where it is neutral, what happens?

1. According to the answers in the Heinemann text book it doesn't really matter whether the NH2 or COOH in the Z group gets protonated/deprotonated. But I'm a little sceptical, lets say you have two deprotonated COO- vs one protonated NH3+ wouldn't you get an unbalanced balance of charges?
2. In water, an amino acid would be consider a zwitteron or dipolar ion when it takes the form of H3N+-CHZ-COO-

If my explanation isn't coherent enough, please refer to the attached picture, you should understand what's happening.

[Somye]

I'm pretty sure it would lead to a basic/acidic envirnment (depending on if it's an amine or carboxyl)

[destain]

so whether or not an amino acid is in basic/acidic/neutral solution, the Z group always follows the regular protein chain in becoming NH3+ or COO- if it has a COOH or NH2?

[AllAboutTheLGs]

When amino acids are in zwitterion state
do their z groups become protonated/deprotonated too
or is that only for acidic/basic mediums?

[destain]

yeah thats what im asking thanks LOL

[Mao]

Yes, the Z groups do ionise. But that requires knowing what the isoelectric points and pKa of the groups, which aren't in the data booklet.

Know your acidic/basic forms and don't worry too much about whether or not you get ionisation in neutral water.

[polkadot]

So if the question was 'Would you expect a solution of asparagine to be acidic, basic or neutral when dissolved in water?' (Insight 2011) Neutral would be an acceptable answer?

[destain]

So when an amino acid is in water the Z groups ionise as well...So Z groups ALWAYS follow the normal amino chain to protonate or deprotonate? (in vce anyway)

[Mao]

So if the question was 'Would you expect a solution of asparagine to be acidic, basic or neutral when dissolved in water?' (Insight 2011) Neutral would be an acceptable answer?

The Z-group in Asparagine does not act as acid nor base.

[destain]

but since it has a NH2, would it become basic in general when placed in water?

[nooshnoosh95]

http://chemed.chem.purdue.edu/genchem/topicreview/bp/1biochem/amino2.html

i found that the COOH on a Z group always becomes deprotonated
but the NH2 on a Z group becomes protonated in arginine and  lysine
aswell as the NH in proline which becomes H2N+
the rest stay neutral

[destain]

wow thats confusing i have no idea what to do now....what to follow haha..

[Mao]

The Z-group in Asparagine does not act as acid nor base.

The functional group in the Z-group is an amide, it does not accept nor donate H+.

[Bismuth]

I'll try to clarify the matter:

On the 2009 VCAA paper, you could have drawn aspartic acid as a zwitterion (Q.6aii). As the assessor's report reiterates as key knowledge, "the net charge on a zwitterion is zero".

Therefore on aspartic acid and other amino acids with Z groups that look like they can have lose/gain a proton, it is only the carboxyl group attached attached to the second carbon  that will have its proton donated and only the amino group attached to this carbon that can accept. Thus, an amino acid's Z group remains as depicted in the data book so don't change these for zwitterions.

Hence, as mentioned on the assessor's report, students who deprotonated the Z group on aspartic acid would have no gained the mark.

When in high/low pH conditions, Z groups are affected as well. Looking at the VCAA 2007 Exam 2, there is a question (Q.5bii) where they ask students which side chain would be positively charged at pH 2. The answer is of course -CH2CH2CH2CH2NH2. Likewise, when they give very basic conditions (pH = 12 - 14) the carboxyl group on Z groups will lose a proton and the amino group won't be positive. Therefore, all amino groups and carboxyl groups are affected.

This outlook is based on what VCAA wants. It's what I think is the best approach unless they specify otherwise in a question.