Hernández Ramírez Juan
Amino acids
Activity 2-Biochemistry
1. Ionization State of Histidine (4 points)
Each ionizable group of an amino acid can exist in one of two states, charged or neutral. The
electric charge on the functional group is determined by the relationship between its pKa and the
pH of the solution. This relationship is described by the Henderson-Hasselbalch equation.
a) Histidine has three ionizable functional groups. Write the equilibrium equations for its three
ionizations and assign the proper pKa for each ionization. Draw the structure of histidine in each
ionization state. What is the net charge on the histidine molecule in each ionization state?
Equilibrium Equations
𝑝𝐻 = 𝑝𝑘𝑎 + 𝑙𝑜𝑔 [𝐴−]
[𝐻𝐴]
Equation For Carboxyl Group.
𝑝𝐻 = 1.82 + 𝑙𝑜𝑔 [𝐶𝑂𝑂−]
[𝐶𝑂𝑂𝐻]
Equation for R group.
𝑝𝐻 = 6 + 𝑙𝑜𝑔 [𝑅]
[𝑅+]
Equation for amino group.
𝑝𝐻 = 9.17 + 𝑙𝑜𝑔 [𝑁𝐻2]
[𝑁𝐻2+]
First ionization state: +2
Second ionization state: +1
Third ionization state: 0
Fourth ionization state: -1
