question_answer

The predominant form of histamine present in human blood is \[\text{(p}{{\text{K}}_{a}},\,\,Histidine=6.0)\]                         [JEE Main Online 08-04-2018]

A)   

B)  

C)   

D)  

Correct Answer: B

Solution :

Isoelectronic point \[=\frac{5.74+9.8}{2}=7.77\] Histamine has two basic center which are Aliphatic amino group and imidazole ring. In blood, the aliphatic amino group (pKa = 9.4) will be exist whereas the second nitrogen of imidazole ring (pKa = 5.8) will not be protonated in blood which pH is 7.4. \[\text{pH=pKa+log}\frac{\text{ }\!\![\!\!\text{ Salt }\!\!]\!\!\text{ }}{\text{ }\!\![\!\!\text{ Acid }\!\!]\!\!\text{ }}\] \[\text{7}\text{.4=}\underset{\begin{smallmatrix}  \downarrow  \\  \text{It should be high for balancing} \end{smallmatrix}}{\mathop{\text{7}\text{.77+log}\frac{\text{ }\!\![\!\!\text{ Salt }\!\!]\!\!\text{ }}{\text{ }\!\![\!\!\text{ Acid }\!\!]\!\!\text{ }}}}\,\]