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At isoelectric point amino acids exist as zwitterions. As a molecule it is electrically neutral, but still has a positive and a negative charge. Then why are amino acids still least soluble at isoelectric point?

The positive and the negative charges of the amino acid can interact with the negative and the positive charges of water and can dissolve, right? Where am I going wrong?

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    $\begingroup$ They can and do dissolve. But at other conditions they dissolve even better. $\endgroup$ – Ivan Neretin Aug 19 '20 at 8:59
  • $\begingroup$ It has also the other side, their mutual interaction within the solid lattice is even more stronger. $\endgroup$ – Poutnik Aug 19 '20 at 9:11
  • $\begingroup$ you are probably going wrong because you are not considering the pH of the solution $\endgroup$ – Paras Bisht Aug 19 '20 at 10:01
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In order for molecules to precipitate out of solution, they need to aggregate together. Amino acids that have zero net charge can aggregate together much more easily than those that are charged.

Molecules that have net charge need counterions to aggregate with them to offset the charges or the electrostatic repulsion will be too great.

The concept of isoelectric point is more often applied to polypeptides and proteins than to individual amino acids. In those much larger molecules, the tendency to aggregate when there is no net charge can be quite high. This is especially true if there are any hydrophobic regions that can favorably associate with each other and reduce the interaction with water.

It is important to note, as mentioned in the comments, that this does not mean that all amino acids and proteins are insoluble at their isoelectric point, simply that they tend to be less soluble at that pH than at pH values where they carry net charge.

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  • $\begingroup$ Good answer. Ions don’t form solids (they need to combine with counterions, which typically involves decreasing entropy). $\endgroup$ – Karsten Theis Aug 19 '20 at 13:17

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