Answer:
The correct answer is 7.68
Step-by-step explanation:
Isoelectric point (pI) is the pH at which the amino acid has net a charge of zero. We have to choose the two values of pKa at which the neutral form of the amino acid is predominant. Histidine (Hys) is a basic amino acid. From pKa's data, we deduce:
pKa1 = 2.3 (corresponds to α- carboxyl group)
pKa2= 6.04 (corresponds to R group)
pKa3= 9.33 (corresponds to α- amino group)
Since it is a basic amino acid, the neutral form will exist between the higher pKa's values. Thus, the pI will be in a pH value between 6.3 and 9.33.
So, we have to calculate the average of pKa2 and pKa3:
pI= (pKa2+pKa3)/2= (6.04+9.33)/2= 7.68
Therefore, the pI of Hys is 7.68.