Question

What is the isoelectric point (pI) of the following peptide? To facilitate answering this question, assume that the N-terminal aminium group has a pKa of 8, the C-terminal carboxyl group has a pKa of 3.5, and that the ionizable side chains have pKas as follows: Asp = 4.0, Glu = 4.0, His = 6, Cys = 8.4, Tyr = 10, Lys = 10, and Arg = 12.5. Also, assume that if the pH is at least two units above or below the pKa of a particular group, then that group is completely unprotonated or protonated, as the case may be.