The predominant form of histamine present in human blood is ($\mathrm{pK}_{\mathrm{a}}$, Histidine=6.0)

JEE Main · 2018 · 08 Apr 2018 · Offline · Biomolecules

The predominant form of histamine present in human blood is (

pK_{a}

, Histidine=6.0)

(A)(see structure)
✓ Correct
(B)(see structure)
(C)(see structure)
(D)(see structure)

Answer: (A)

Solution

Blood is held near pH

7.4

. Comparing each basic centre of histamine with its

p K_{a}

through the Henderson-Hasselbalch idea, a centre is mostly protonated when the pH is below its

p K_{a}

and mostly free when the pH is above it. The ring nitrogen, with

p K_{a} \approx 6.0

, sits below

7.4

and so stays unprotonated, while the aliphatic side-chain nitrogen, with its higher

p K_{a}

, is protonated as

- NH X_{3} X^{+}

. The form present in blood is therefore the mono-cation that carries its charge on the side-chain nitrogen while the ring is left neutral.

JEE Main · 2018 · 08 Apr 2018 · Offline · Single correct option

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