Question

The predominant form of histamine present in human blood is:
$(pKa=6.0)$

• A.
• B.
• C.
• D.

Answer 1

The correct option is A

The N-atoms present in the ring will have same the $pKa$ values each (6.0), while the $N$ atom outside the ring will have a different $pKa$ value ($pKa>7.4$)
Therefore, the two $N$-atoms inside the ring will remain unprotonated in human blood because their $pKa$ is lower than the $pH$ of the blood ,while the $N$-atom outside the ring will remain protonated because its $pKa$ is higher than the $pH$ of blood.