Question

The isoelectric point (pI) of a peptide is the pH at which the peptide does not migrate in an electric field. Since the peptide is zwitterionic, there are the same number of positive charges as negative charges on the peptide population. The pI can be estimated fairly accurately (within 0.1 or 0.2 pH units) from the pK values of all the proton dissociable groups in the peptide. Using pK values from the table at the right, estimate the pI value of the following hexapeptide: Gly-His-Glu-Lys-Phe-Cys

Answer 1

You are not providing the data for the pKa, however, let me guide you and tell you how you will do it with the values of table you have.

Step 1: Determine the total positive charge on the peptide (n) when all acidic and basic groups are fully protonated. Step 2: List the pK values of all acidic and basic groups in order from lowest (pK1) to highest.

Step 3: Calculate the pI as the average of the values for pK(n), the proton dissociation forming a neutral species from a 1 species, and pK(n 1), the proton dissociation forming a +1 species from the neutral species.

Once you realize this, you can actually calculate the pl. Suppose only 4 of them are neutral, the pl would be calculated as:

pl = pK1 + pK2 + pK3 + pK4 / 4

This is how you'll do it. A best way to know the contribution of the charge, is drawing the molecule and put the value of pK where it belongs. Sorry for not give you the whole answer, but I need the table info to have an idea of the pKs given. At least this is my attemp to guide you in how to do it.

Hope this may help you. If it's not, then post again with the tabla data.