What is the Isoelectric Point of Lys-Ser-Asp-Ala Tetrapeptide?

[leopard]

How can I find the pI of a tetrapeptide like Lys-Ser-Asp-Ala?

There are four ionizable groups that I have to take into account:

Alpha amino of Lys: 8.95
Amino side chain of Lys: 10.79
Carboxyl side chain of Asp: 3.86
Alpha carboxyl of Ala: 2.34

The two first are positively charged ionizing to uncharged groups. The two last are uncharged groups ionizing to negatively charged groups.

pI is somewhere between 3.76 and 8.95 of course, but that's not a very accurate answer.

 

[epenguin]

Sure ‘somewhere’ is not very accurate. What reason is there for that ‘somewhere’ to be nearer one of those figures than the other?

Firstly you should make sure you could do that question for a molecule with only 2 ionising groups say a carboxyl and an amino group. E.g. with pKs of 3.86 and 8.95?

Then at the isoelectric point (pH) of that molecule in what state are groups with pKs of 2.34 and 10.79 going to be?