Question

You are evaluating the pH-dependent properties of the peptide R-E-T-C-H. You do NOT need to draw the peptide on this exam, but it will be to your advantage to sketch out a schematic image of it to account for the effects of pH on charges of individual groups. What is the net charge on the peptide at pH 7.0? Show how you reached your conclusion with calculations, but please circle your answer so that it can be clearly seen – even if you do calculations on a separate page. Please use the pKa values that we used in class for the N- [8.0] and C- [3.0] termini and for the amino acid side chains. When calculating the charge per group, you can consider any group fully (de)protonated when it is >2.0 pH units from its pKa – but anything 2.0 pH units must be calculated specifically.

Answer 1

At pH 7.0, net charge of peptide R-E-T-C-H will be zero (0).

When pH in solution is greater than the pKa value ( for -COOH), it will be deprotonated. When pH is less than pKb (-NH3+) value of an aminoacid, it will be protonated.

The amino group and carboxy group, which are involved in peptide linkege, they don't participate in this process (protonation or deprotonation). Only side chains, free amino and carboxyl end are involved in this process.

Picture is attached: