Final answer:
The isoelectric point (pI) of the tripeptide Lys-Phe-Arg is approximately 6.25, calculated by averaging the relevant pKa values of the ionizable groups.
Step-by-step explanation:
To calculate the isoelectric point (pI) of the tripeptide Lys-Phe-Arg, you must consider the pKa values of the ionizable groups in the amino acids. Lysine (Lys) has a side chain pKa of around 10.5, phenylalanine (Phe) has no ionizable side chain, and arginine (Arg) has a side chain pKa of around 12.5. The pI is calculated as the average of the pKa values of the ionizable groups that are on either side of the zwitterion form (neutral form) of the amino acid in the tripeptide.
Lysine has a second pKa value that will affect the pI because of the positive charge on its side chain. To find the pI, take the average of the pKa value of the carboxyl group (which will be approximately 2 for the C-terminal Arg) and the pKa value of the amino group of Lys. Therefore, the pI of Lys-Phe-Arg will be the average of 2 and 10.5, which is approximately 6.25.