Concept #1: Applying Henderson-Hasselbalch to Amino Acids

Example #1: Example of Applying Henderson-Hasselbalch to Amino Acids

Practice: At pH 11.8, what is the % of protonated amino group in the R-group of Lysine. (Lysine’s pKR = 10.8)?

Practice: Draw Glu & calculate the % of -COOH in the R-group of Glu at pH 3.2. (Glu pK  R = 4.1).

Practice: Draw Asp & calculate the pH at which two thirds (2/3) of Asp’s R-group is dissociated. (Asp pKR = 3.9).

Practice: Draw Arg & calculate the pH at which 23% of Arg’s R-group is dissociated. (Arg pK  R = 12.5).

Practice: What is the ratio of [conjugate base] to [conjugate acid] for each of Histidine’s three ionizable groups at pH 7?

A) Amino group ratio: ________________

B) Carboxyl group ratio: ________________

C) R-group ratio: ________________

D) Use the ratios above to determine the average net charge of the ionizable groups & the entire His molecule.

1. Net charge of Amino groups: ____________                                         

2. Net charge of Carboxyl groups: ____________

3. Net charge of R-groups: ____________                                                

4. Net charge of His: ____________ (Hint: sum previous 3 charges).