What is the significance of the isoelectric point in amino acid charge balance?

[leopard]

The isoelectric point is define as the pH t which there is no net charge.

If the carbonyl group has pKa = 2 and the protonated amino group pKa = 9 the amino acid will exist as a zwitteron in solutions with pH between 2 and 9, right? A zwitteron has no net charge, right? So why is it called the isoelectric point, and not the isoelectric region?

 

[Borek]

Zwitterion has no net charge, that's OK, but in the region you mentioned not all aminoacid is in the form of zwitterion - there is an equilibrium mixture of all possible forms - positively charged, negatively charged, neutral. There is only one exact pH value at which concentrations of positively charged and negatively charged forms is identical. At this pH value even if you add electric field there will be no net migration of aminoacid - neutral molecules will stay at their place, while migrations of positively charged and negatively charged will cancel out. At other pH values there will be net shift of the molecule positions.

 

[leopard]

Arginin has pKa values 2.17, 9.04 and 12.48. Am I then right to say that the net charge of arginin at pH 9.04 and 12.48 is +0.5 and 0 respectively?

 

[epenguin]

Arginin has pKa values 2.17, 9.04 and 12.48. Am I then right to say that the net charge of arginin at pH 9.04 and 12.48 is +0.5 and 0 respectively?

I can see the reasoning that got you to that first conclusion, for the first answer that is the right kind of reasoning, but
1) think you have made a mistake about the charges, there is one that can be - and two that can be +
2) the second answer is not so much along the right lines.
When the pH = the pK of a group it is half protonated and half unprotonated (check back you understand why).