How to calculate the pI of a peptide?

[DDS]

How to calculate the pI of a peptide??

Hello everyone, I am hoping someone can shed some light on this question. I know the general idea behind it but i can't seem to put the nail in the coffin.

The sequence is ATLDAK and it asks to : Calculate its approximate pI

This is what i have so far:

...nT-A...T...L... D... A... K-... cT
pK...9.87......3.9...10.54...2.16

I know that there are 2 positive charges so this results in requiring to be deprotinated with two equivalents of base. I also know that the pI is given by the two amino acids that give the zwiterionic charge, or simply zero charge. However, how to identify those amino acids...?

Help please

 

[TeethWhitener]

Use the pKa and what you know about titration. In this case, you have correctly noted that the peptide will have a +2 charge at low pH. Adding 2 equivalents of base will therefore give a neutral species. Examining the pKa’s, we see that the first species to deprotonate will be the C-terminal carboxyl group, and the second species to deprotonate will be the side chain carboxyl on the aspartic acid residue. This will give us a neutral species between roughly the pKa of the aspartate residue and the N-terminal amine group. We can then estimate the pI as the average of these two pKa’s:
$$pI\approx\frac{3.9+9.87}{2}=6.885$$
Keep in mind that this is just a handy estimate for pI. Lots of different factors will affect it, but this quick estimate will usually get you in the ballpark.