JEE MAIN - Chemistry (2018 (Offline) - No. 3)
The predominant form of histamine present in human blood is (pKa, Histidine = 6.0)
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Explication
Blood is slightly basic in nature as it's pH is around 7.4 .
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In Histamine there is two basic centers
(1) $$\,\,\,$$ Aliphatie amino group
(2) $$\,\,\,$$ Basic nitrogen of imidazole ring.
Where pKa of Aliphatic amino group is 9.4 and pKa of basic nitrogen of imidazole ring is 5.8.
As more pKa value means more basic, So, aliphatic amino group is more basic.
As pH of blood is 7.4 so pKa of blood is < 7.4. So, $$-$$NH2 group will act as more basic compare to blood. So, blood will act as acid and donate H+ to $$-$$NH2 group.
So, Histamine in blood will become this :
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In Histamine there is two basic centers
(1) $$\,\,\,$$ Aliphatie amino group
(2) $$\,\,\,$$ Basic nitrogen of imidazole ring.
Where pKa of Aliphatic amino group is 9.4 and pKa of basic nitrogen of imidazole ring is 5.8.
As more pKa value means more basic, So, aliphatic amino group is more basic.
As pH of blood is 7.4 so pKa of blood is < 7.4. So, $$-$$NH2 group will act as more basic compare to blood. So, blood will act as acid and donate H+ to $$-$$NH2 group.
So, Histamine in blood will become this :