Question

Calculate the overall net charge of the following polypeptide at pH = 12.0, to TWO decimal places. Only add a sign if the answer is NEGATIVE. Gly-Pro-Asp-His-Arg

ide chain pKa values of amino acids: Asp 4.0 Glu 5.0 His 6.5 Cys 8.5 Tyr 10.0 Lys 10.2 Arg 12.5

Average N-terminal amino group has pKa = 9.5

Average C-terminal carboxylic acid group has pKa = 2.5

Answer 1

Ans. The dominant form of acidic and basic groups on different pH group is shown in figure.

A difference in pH and pI by 2 units or more is accepted to provide 1 unit charge on the group as applicable.

At pH = pI, the group has 0.5 units.

Assign charge on the groups accordingly as shown in figure.

Net charge on peptide at pH 12.0 = -1.50

Note: The exact charge on Arginine side chain at pH 12 is 0.760 (calculated using Henderson-Hasselbalch equation) because pH in not exactly equal to pI. The difference between pH and pI is not greater than 2.0 units.

So, if you use HH equation, the exact charge on peptide would be – 1.24.