Question

The predominant form of histamine present in human blood is:
[pKa′​ Histidine =6.0]
A.

B.

C.

D.

Answer 1

Hint: We will check the structure of each of the following structures and then we can decide which is the predominant form of histamine present in blood. Histamine is a nitrogenous organic compound which is present in blood.

Complete step by step solution: We are given with the information that pKa’ value for histidine is 6.0.
Histamine is involved in local immune response, as well as regulating physiological function in the gut and acting as acting as a neurotransmitter for brain, spinal cord and uterus. Histamine is also present in inflammatory response.
Nitrogen atom present in the ring will have same pka value as that of histidine i.e. the given pka’ value because it remains unprotonated. This is because the given pka value is less than that of blood.
Similarly the nitrogen atom present outside the ring have a different pka’ value. The different pka’ value will be above the pka’ value of blood i.e. 7.4 because it is in protonated form because the given pka’ value will be greater than blood.
Now we will check all the options and check the above conditions.
In the first option nitrogen the ring is protonated, but that of the outside one is not protonated. So this is wrong.
In the second option the nitrogen in the ring nitrogen is not protonated whereas the outside nitrogen is protonated. So this option is correct.
We will also check the other options.
In the third option ring nitrogen as well as the nitrogen present outside are not protonated. So this option is incorrect.
In the fourth option ring nitrogen as well as nitrogen outside the ring is protonated. So this option is also incorrect.

The correct option is B.

Note: For solving this question the conditions mentioned above should be known or else this question cannot be solved. The structure which fulfills the condition is stable so the stable compound will be predominant in blood.