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)Correct answer is between '9.4,9.4'. Can you explain this answer?

Question

For a 0.1 M aqueous solution of lysine, the pH at which it carries no net charge is ___________. (pKa values for: &alpha;-carboxylic group = 3.1, &alpha;-amino group = 8.0, &epsilon;-amino group = 10.8)Correct answer is between '9.4,9.4'. Can you explain this answer?

Answer

The pKa values for lysine are as follows:
- α-COOH: pKa = 2.16
- α-NH3+: pKa = 8.95
- ε-NH3+: pKa = 10.53

When the α-COOH group is protonated, it carries a +1 charge, and when the α-NH3+ and ε-NH3+ groups are protonated, they each carry a +1 charge. Therefore, at pH values lower than the pKa of α-COOH (2.16), lysine will carry a net positive charge.

When the α-COOH group is deprotonated (pH > 2.16), it carries no charge. However, the α-NH3+ and ε-NH3+ groups still carry a +1 charge. Therefore, at pH values higher than the pKa of α-COOH but lower than the pKa of α-NH3+ (8.95), lysine will carry a net positive charge.

When both the α-COOH and α-NH3+ groups are deprotonated (pH > 8.95), lysine carries no net charge. Therefore, the pH at which lysine carries no net charge is approximately 8.95.

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