Question

For a 0.1 M aqueous solution of lysine, the pH at which it carries no net charge is .......... 

(pKa values for: α-carboxylic group = 3.1, α-amino group = 8.0, ε-amino group = 10.8)
 

Hint:

For amino acids, the isoelectric point is the average of the pKa values for the carboxyl and amino groups.

Answer 1

Correct Answer: 9.4

The pH at which lysine carries no net charge is known as its isoelectric point, or pI. To find the pI of lysine, we need to consider the pKa values of its ionizable groups: the α-carboxylic group (pKa=3.1), the α-amino group (pKa=8.0), and the ε-amino group (pKa=10.8).

Lysine, an amino acid, can exist in different ionic forms depending on the pH. The point at which the net charge is zero is where the positive and negative charges balance. At the isoelectric point, lysine is primarily in the zwitterionic form, with its α-amino and ε-amino groups protonated and its α-carboxylic group dissociated.

To calculate the pI, we average the pKa values of the acidic and basic groups that are responsible for the species in the zwitterionic state. For lysine, this involves the pKa of the α-amino group (8.0) and the ε-amino group (10.8):

pI = (pKa_α-amino + pKa_ε-amino) / 2

Plugging in the values:

pI = (8.0 + 10.8) / 2 = 9.4

The calculated isoelectric point (pI) is 9.4