Final answer:
Calculating the isoelectric point (pI) of a peptide involves averaging the pKa values of the ionizable groups present in the peptide. The pI is the pH at which a peptide has no net charge. Accurate calculation requires knowledge of the pKa values of the amino acid side chains and terminal groups.
Step-by-step explanation:
The question involves calculating the isoelectric point (pI) for three peptides. Each peptide contains amino acids that may have side chains with their own characteristic pKa values influencing the overall pI. The isoelectric point is the pH at which a peptide carries no net electric charge. A peptide's pI is calculated by averaging the pKa values of the ionizable groups that are closest to the pH at which the net charge of the peptide is zero.
For peptide a) Ala-Arg-Val-Ser, you need to consider the pKa of the amino (N-terminal) and carboxyl (C-terminal) groups, as well as the pKa of the side chain of any ionizable amino acids within the peptide. Here, Arginine (Arg) has a charged side chain. You calculate the pI as the average of the pKa of the carboxyl group of Ser and the pKa of the guanidinium group of Arg. For peptide b) Phe-Trp-Glu-Ile, Glutamic acid (Glu) has an ionizable side chain with its own pKa to be considered. For peptide c) Met-Leu-Gly-Thr, there are no ionizable side chains other than the terminal amino and carboxyl groups.
To perform the calculations precisely, one would need the pKa values for each relevant group. Since these values are not provided, the exact calculation cannot be done in this response. In a real situation, after obtaining the pKa values from a reliable source, you would use the formula:
pI = (pKa1 + pKa2) / 2
where pKa1 and pKa2 are the pKa values that bracket the pH at which the peptide has a net zero charge.