Question

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?

Answer 1

The equilibrium equations for the three ionizations in histidine with pK_{a} values are in attached document.

Hisidine has three ionizable functional group namely, the carboxylic group, amino group and nitrogen in the imidazole ring.

The structure of histidine in each ionization state is also attached.

Here, as from the attached diagrams the net charge in first, second and third  ionization state is  +1, 0 and -1 respectively.

Answer 2

Answer:

a. Structures: on the attached picture.

b. 1st ionization: +2, 2nd ionization:+1, 3rd ionization:0 and 4th ionization: -1.

Explanation:

Hello,

It is widely known that each ionizable group of an amino acid could be charged or neutral. The electric charge on the functional group is defined by the Henderson-Hasselbalch formula which relates the pKa and the pH of the solution.

Thus, on the attached picture you will find the three ionizations of histidine with their proper pKa.

However, each structure has its own net charge due to the ionization, thus the first one's is +2, the second one's is +1, third one's 0 and a fourth one's -1, it is known by adding the positive charges and subtracting the negative ones.

Best regards.