1. Calculate the pI of Histidine and illustrate how you arrived at your answer

Answer 1

Look up the pKa for the amino and carboxyl and pyrole nitrogen group of histidine. The values I found are

pKa1 = 1.82 for the carboxyl

pKa2 = 9.17 for the amino N

pKa3 = 6.00 for the pyrole N

Thus, histidine can exist in 4 forms depending on pH. Going from acidic pH to basic pH, you will have pyrole N with + charge, amino N with a + charge and carboxyl will have no charge

Increasing pH, you have pyrole N with + charge, amino N with a + charge, and carboxyl with - charge.

Increasing pH further, you have pyrole N with 0 charge, and amino N with + and carboxyl with -

Finally, at higher pH, you have pyrole N with 0 charge, amino N with 0 charge and carboxyl with -

The pI is the pH at which the amino acid has no net charge. In this case it will end up being the average of the pKa2 and pK3 or 1/2 (pK2 + pK3) = 1/2 (9.17 + 6.00) = 7.59 = pI