Why are amino acids least soluble at their isoelectric point?

Question

What other physical and chemical properties do amino acids have at their isoelectric point?

Answer 1

If pI is the isoelectric point of an amino acid then

at pH at pH>pI the molecule has a net negative charge

For either case molecules have the same type of charge and thus repel each other and stay "happily" in solution, solvated by water.

At pH=pI the net charge is zero. This means that molecules of the amino-acid can come together (since they are not repelling each other anymore), aggregate and thus precipitate out of solution. They will still have charged groups but the positively charged group (-NH3+) of one molecule will interact with the negatively charged group (-COO-) of another.

The same principle stands for proteins and colloids.

Answer 2

I was not aware that individual amino acids had isoelectric points. Proteins have isoelectric points - this is the pH at which the protein is neutral.
Amino acids exist in three possible states:
At low pH ( ~ less than 3) everything is protonated: NH3+ and CO2H
At intermediate pH ( ~ 3 - 9), carboxylic acids are deprotonated, but amines are not yet deprotonated: NH3+ and CO2-. This is the zwitterionic form.
At higher pH (~ greater than 9) both the carboxylic acid and the amine are deprotonated: NH2 and CO2-.

Amine salts are less acidic than carboxylic acids, which means you can never have both a neutral amine and a neutral carboxyl group. This is a common mistake in many textbooks; I have heard of professors who will take a marker and X out every structure with an NH2 and CO2H in it.

I would say amino acids are most water soluble in their zwitterionic form. In acidic methanol you can probably dissolve many of the amino acids in their protonated form.

Keep in mind that there are several amino acids that have amines and carboxylic acids on their side chains, which introduces more possible states. As I mentioned before, just remember that you can never deprotonate any amine unless you have deprotonated all of the carboxylic acids.

Answer 3

at the isoelectric point the amino acid carries no net charge. however, i think that the isoelectric point is the zwitterionic form which should be rather soluble. but if you find the one where the amine is deprotonated and the carboxylic acid is protonated, then it would be uncarged completely which would make it less soluble.