Question

. The pI is called ________________. The rule of calculating pI of an amino acid is that first, write the dissociation equation from fully protonated form to fully deprotonated form, label the charge of each form; second, identify the zwitterionic form (zero charge) and find the closest pKs (left and right side in the dissociation equation); third, average these two pKs. Write the dissociation equations for amino acids, glutamate, histidine, and calculate their pIs.

Answer 1

The isoelectric point is that the pH at which the compound is in an electronically neutral form.

For diss equations, please find them in the enclosed file.

The pIs of 2 amino acids:

• Glutamate: pI = 3,2
• Histidine: pI = 7,6

Explanation:

Formula for the pI calculation: pI = (pKa1 + pKa2)/2

Given 3 pKa :

• Acid glutamic with an acid sidechain:

Use the lower 2 pKas (corresponding with 2 -COOH groups)

pKa1 = 2,19; pKa2 = 4,25; so pI = 3,2

• Histidine with 2 amino groups:

Use the higher 2 pKas ( -COOH group and -NH= group)

pKa1 = 6; pKa2 = 9,17; so pI = 7,6