I am not very clear how zwitterions are related/linked with isoelectronic points.
I have the following 4 points that I need clarification. Hope someone can help me out here.
1) Are zwitterions very soluble in water, as a result of ion-dipole interactions with water?
2) Can I say that for at a protein's isoelectronic point, the structure of protein is actually the zwitterionic structure? Or is this only true for glycine?
3) The COO- and NH3+ groups in a zwitterionic structure, refers to the groups that are at opposite ends of the alpha-carbon, right?
4) Why are proteins least soluble in water at isoelectronic point? I thought, it would be soluble still, since the groups are in the ionized state.
>>> 1) Are zwitterions very soluble in water, as a result of ion-dipole interactions with water? <<<
See point 4 below. (after reading points 2 and 3 first.)
>>> 2) Can I say that for at a protein's isoelectronic point, the structure of protein is actually the zwitterionic structure? Or is this only true for glycine? <<<
It depends on the particular amino acid's 'R' group. Assuming the 'R' group is neutral (ie. neither acidic nor basic, eg. COOH, SH, NH2, etc), then yes, at the isoelectronic point (the pH for which the amino acid has a net charge of zero), the negative formal charge at the COO- is counterbalanced by the positive formal charge at the NH3+. In other words, the zwitterionic structure.
For amino acids with acidic or basic 'R' groups, depending on the pH, you can have dianionic, anionic, zwitterionic, cationic, dicationic, etc. Every amino acid has it's own unique pKa values for the alpha-carboxylic group, the alpha-amino group, and the 'R' group.
>>> 3) The COO- and NH3+ groups in a zwitterionic structure, refers to the groups that are at opposite ends of the alpha-carbon, right? <<<
Yes, they are called the alpha-carboyxlic groups and the alpha-amino groups.
>>> 4) Why are proteins least soluble in water at isoelectronic point? I thought, it would be soluble still, since the groups are in the ionized state. <<<
At the isoelectronic point, in zwitterionic form, there is tendency for ionic bonding (ie. electrostatic attraction) between the negatively charged COO- of one amino acid, with the positively charged NH3+ of a neighbouring amino acid. In other words, amino acids would rather ionic bond with each other than (form ion-dipole interactions) with the polar water molecules.
If on the other hand, the pH is lower than the isoelectronic point, the amino acid (assuming the 'R' group is neutral) is in its cationic form NH3+, which means they repel away from each other, and can freely form ion-dipole interactions with the polar water molecules, and hence dissolve.
Similarly for high pH, anionic form COO-. Hence aminio acids are more soluble in conditions more acidic or more basic than its isoelectronic point.
But that is not to say at its isoelectronic point, amino acids are totally insoluble. You can only state that "an amino acid is LEAST SOUBLE at its isoelectronic point".
On a related note, you might wonder, what makes NaCl soluble when CaSO4 is insoluble? It has to do with Enthalpy and Entropy considerations. (Gibbs Free Energy).
Enthalpy - endothermic lattice dissocation enthalpy VERSUS exothermic hydration enthalpy (ie. energy released from ion-dipole interactions). Consider charge density of cations and anions involved to see who wins.
Entropy - increase in entropy due to dissolving of ionic solid into aqueous ions VERSUS decrease in entropy due to ion-dipole interactions causing water molecules to be more arranged more orderly around the aqueous ions. Again, charge density of ions must be considered.
For a more in-depth discussion on this topic, visit :
http://www.chemguide.co.uk/inorganic/group2/problems.html