zwitterions

Can someone confirm if I have this the right way round:

When you dissolve an amino acid in water, the zwitterion is the dominant species but there are small proportions of the positive and negative ions due to reaction with water. These ions are not in equal concentration and so H+ or OH- has to be added. This is why the isoelectric point is not always PH7.

For purpose of A level:
Isolectric point is the PH at which amino acids exist as zwitterion. (eventhough it is just the dominant species)

secondly, depending on the Whether the amino acid is in acid or alkaline conditions it will either take a positive or nagtive ion For the purpose of A level.

However, Really once again, the positive or negative amino acid ion will only be the dominant species. -(or maybe if there is enough OH-/H+ it will go to completion I'm not sure)?

Also does the amino acid exist as a zwitterion in the solid form as well?

Thanks

No, on dissolution the zwitterion is not necessarily the dominant species, it depends on the pH.

Thanks,
Right, i know.
ok so is this right:

As a solid amino acids exist as an ionic lattice of zwitterions. When dissolved in water you will probably have to add some H+ or OH- until there is no net charge. at this point (isoelectric point), the zwitterion will be dominant. But for A level purposes (at least in my a level) is says the isoelectric point is where the amino acid exists as a zwitterion but i think this is just a simplification.

So then when you go on to add either lots of H+ or OH- the exams questions usually ask like what is the ion present at this stage and the answer is always either the positive or negative ion. So i guess you are assuming the reaction is going to completion and disregarding other ions which might be in the solution (be it very small concentrations) and only focusing on the one with the greatest concentration.

something like this or thereabouts?

I am not really clear as to what you are actually trying to say.

The isoelectric point is the pH at which the zwitterion exists. This is not a simplification in so far as it is a definition.

Both the acid group and the basic group are weak and as such only partially dissociated. As the pH decreases the basic group equilibrium moves more towards the side of the protonated base and the acidic group moves more towards the side of the molecular carboxylic acid group.

-NH₂ + H+ <==> -NH₃⁺
-COOH <==> -COO^- + H+

Logically there must be some pH value at which the concentration of the positive species, -NH₃⁺, exactly equals the concentration of the negative species -COO^-. This is the isoelectric point.

Its just that on a website (Chemguide) i read that when you dissolve a zwitterion in water it reacts with water molecules acting as both an acid and a base:

(zwitterion) + H20 <--> (negative ion as H20 has accepted H from NH3+ group) + H30+

But NH3 is a weak acid so equilibrium is far to the left.

And the same equation again but this time the zwitterion reacts with water which instead acts as an acid. But as Coo- is only a weak base the equilibrium lies well to the left.

But the positions are actually different depending on the R group in the zwitterion. Apparently for simple ones there will be more negative ions in solution because the first one lies more to the right. What you do is add some H+ and move the first one to the left to reach the isoelectric point.

So i understand that the isoelectric point is the PH where the concentration of both the positive and negative ion (formed from the zwitterion are in equal concentration) but the definition in my book states the isoelectric point is when the amino acid exists as a zwitterion.

COO- is a strong conjugate base because COOH is a weak acid.

I don't see where your problem lies!

Two functional groups that both interact with water giving two equilibria.
Zwitterion exists to some extent at most pH values, BUT the isoelectric point is when there is no overall charge on the amino acid, i.e. the concentration of the conjugate acid = the concentration of the conjugate base. At this point there will be a maximum of zwitterion in the solution.

Thanks,
So when you say conjugate acid and base concentration being equal is that to do with the concentrations of the positive and negative ion concentration being equal (in the equilibrium reaction with water)?

And at this point there is a maximum of zwitterion but is it ok to say the isoelectric point is where the amino acid exists as a zwitterion?

Finally, if you add lots of acid or base,will this will react with the zwitterion and form either a positive or negative ion so that is now the dominant species?

I am talking about the conjugate base formed from the carboxylic acid group, and the conjugate acid formed from the amine group.

The isoelectric point is when there concentrations are equal.

This could also be expressed as there being a maximum of zwitterion present.




yes



yes

In low pH the aminoacid becomes a cation as the conjugate acid of the amine group is formed

H₂N-CHR-COOH + H⁺ --> ⁺H₃N-CHR-COOH

In high pH the aminoacid becomes an anion as the conjugate base of the carboxylic acid is formed.

H₂N-CHR-COOH + OH^- --> H₃N-CHR-COO^- + H₂O

Hi, thanks for your time.
so the equilibrium showing the initial form of the amino acid being dissolved in water (i think):

NH2 + H+ <--> NH3+
COOH <--> COO- + H+

And so i am im right in saying when the concentrations of NH3+ and COO- are equal this is the isoelectric point where the zwitterion is present. But, For these two equilibria the position must lie far to the left which would suggest you are left with the original form of the amino acid in solution.

But would i be right in saying that the attached image on your last post would be more representative of what is actually happening because there will be internal transfer of H+ between the two groups and the addition of H+ or OH- to shift the equilibrium to when there is no net charge in the solution is the isoelectric point. At this point the amino acid will exist as a zwitterion.

Also, when you add lots of H+ or OH- would it be right to consider both the zwitterion form and original form (as you did) being turned into either the cation or anion?