Question

The predominant form of histamine present in human blood is ($p{K}_a$ Histidine$=6.0$)

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The correct option is C

The explanation for the correct option:

C:

Histamine:

• Histamine, an organic nitrogenous substance, is a neurotransmitter for the brain, spinal cord, and uterus as well as a regulator of physiological processes in the gut.
• It also plays a role in local immunological responses.

The predominant form of histamine in blood:

• ${\mathrm{pK}}_{\mathrm{a}}$ of $\mathrm{N}$ of imidazole is $6$ (from histidine ${\mathrm{pK}}_{\mathrm{a}}$​) and that of ${\mathrm{NH}}_2$ is $9·7$. The $\mathrm{pH}$ of blood is $7·4$.
• Since the $\mathrm{pH}$ is lower than the ${\mathrm{pK}}_{\mathrm{a}}$ of ${\mathrm{NH}}_2$ then the compound will be protonated at ${\mathrm{NH}}_2$​ and neutral at $\mathrm{N}$ of imidazole.

Final Answer: Therefore, Option C is the correct option.