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Drawing amino acids under different conditions ...

Hi all,

i was just wondering if anyone on here would be able to help me. i am currently studying ocr a2 chemistry and have got stuck on a amino acid question.

I understand that if a amino acid (zwitterion) is under acid conditions it acts as a base and accepts a proton so the o- becomes oh and the nh3+ stays the same. and when under alkaline conditions it acts as acid and donates a proton so that the nh3+ becomes nh2 and the o- stays the same.

However, when in water i believe there is no change. so if i was asked to draw the structure of say 2-amino-propanoic acid in water, would i draw the zwitterion with both charges present or the actual structure of the amino acid in displayed form?

thanks for any help!
Original post by LTID
Hi all,

i was just wondering if anyone on here would be able to help me. i am currently studying ocr a2 chemistry and have got stuck on a amino acid question.

I understand that if a amino acid (zwitterion) is under acid conditions it acts as a base and accepts a proton so the o- becomes oh and the nh3+ stays the same. and when under alkaline conditions it acts as acid and donates a proton so that the nh3+ becomes nh2 and the o- stays the same.

However, when in water i believe there is no change. so if i was asked to draw the structure of say 2-amino-propanoic acid in water, would i draw the zwitterion with both charges present or the actual structure of the amino acid in displayed form?

thanks for any help!


What pH? Pure water?
Reply 2
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LTID
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LTID
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Original post by Random1357
What pH? Pure water?


Yes my teacher said that a amino acid in water would not change? or its zwitterion wouldnt?
Reply 4
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Kemmist
3
The amino acid would most likely be in its zwitterionic form NH3+ & CO2- when dissolved in water.
The amine is a strong enough base to deprotonate the carboxylic acid.
Original post by LTID
Yes my teacher said that a amino acid in water would not change? or its zwitterion wouldnt?


In pure water zwitterion as said above. Also polar interactions with water molecules stabilise this charged species.
Reply 6
Avatar for Ganglion
Ganglion
9
I am not familiar with the requirements of A2 Chemistry, but essentially the problem is not that simple.

The way an amino acid acts in different pH depends on its isoelectric point. 2-amino propanoic acid, or valine, that you mentioned, has its isoelectric point at pH 6.0, so in water (pH 7.0) the molecules are likely to be either in their zwitterion form or negatively charged.

Without specifying the amino acid, its structure cannot be determined. Another amino acid, e.g. aspartic acid, may contain a carboxyl group in its side chain, and hence have an isoelectric point at pH 2.8. In water, it is therefore negatively charged. The situation is reversed for arginine with isoelectric point at pH 10.8.

I hope this helps. :smile:

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