Isoelectric Point

[MedPR]

I don't really understand what this is. I mean, its just the pH at which the zwitterionic form of the amino acid exists in highest concentration, right?

So do the nonpolar amino acids all have the same isoelectric point since they all only have 2 acidic sites?

Also, if in an electrophoresis experiment you used a pH 10 media, would the most acidic amino acid migrate the most? or the least?

Also can someone give me the general guidelines for how to know if the acidic sites are protonated or deprotonated based on pKa and pH of solution?

I think I understand all of these concepts individually, but I can't put them all together.

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[theseeker4]

I don't really understand what this is. I mean, its just the pH at which the zwitterionic form of the amino acid exists in highest concentration, right?

It is the pH at which the net charge of the amino acid is neutral on average. This would be the zwitterionic form for amino acids with neutral R groups, but would vary for the others.

So do the nonpolar amino acids all have the same isoelectric point since they all only have 2 acidic sites?

No, see http://www.askiitians.com/iit_jee-Carbohydrates_Amino_Acids_Peptides/Isoelectric-Point

Also, if in an electrophoresis experiment you used a pH 10 media, would the most acidic amino acid migrate the most? or the least?

For gel electrophoresis, typically proteins are separated by size. I don't know how it would be used for individual amino acids on a constant pH gel. You could isoelectrically separate (separate by pKa's) the amino acids on a gel that has a gradient of pH's, but I don't know if there would be any real difference between amino acid movement on a continuous pH medium. Someone who knows more about this than I do can probably answer this one better.

Also can someone give me the general guidelines for how to know if the acidic sites are protonated or deprotonated based on pKa and pH of solution?

Basically, if the pKa > pH, it will be deprotonated, while it will be protonated if the pKa < pH.

 

[mcloaf]

For gel electrophoresis, typically proteins are separated by size. I don't know how it would be used for individual amino acids on a constant pH gel. You could isoelectrically separate (separate by pKa's) the amino acids on a gel that has a gradient of pH's, but I don't know if there would be any real difference between amino acid movement on a continuous pH medium. Someone who knows more about this than I do can probably answer this one better.

Biochemists do this pretty regularly--the technique is called isoelectric focusing.

 

[theseeker4]

Biochemists do this pretty regularly--the technique is called isoelectric focusing.

Right, and that is based on a gradient of pH's, not a single uniform pH across the gel, unless I am misunderstanding something.....

 

[TheShaker]

I would think the most acidic amino acid will migrate the most at 10 pH or any basic pH. The acidic proton would be taken away from the amino acid (lets use aspartate as an example) and leave it with the negative charge. This causes it migrate to the far end (positive charge) faster due to its negative charge. Compare that with a neutral or basic amino acid which will not lose a proton and stay at neutral charge. Those should have less migration since they are not attracted to the positive charge of the cathode.

I have a feeling it would be the same if it was in acidic solution (ex. pH 2). In this situation, the basic amino acids will accept a proton and become positive which causes them to move to the cathode more slowly since positive repels positive. The acidic and neutral amino acids will not lose or gain protons so they will remain neutral and migrate normally.