Which one of the nitrogen atoms of histidine | vedclass

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Question

(B) In the imidazole ring of histidine,there are two nitrogen atoms: the pyrrole-like nitrogen $(\alpha-N)$ and the pyridine-like nitrogen $(\beta-N)$

Vedclass · Accepted Answer

$ \beta $

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(B) In the imidazole ring of histidine,there are two nitrogen atoms: the pyrrole-like nitrogen $(\alpha-N)$ and the pyridine-like nitrogen $(\beta-N)$.The lone pair on the $\alpha-N$ is involved in the aromatic sextet of the imidazole ring.The lone pair on the $\beta-N$ is in an $sp^2$ orbital perpendicular to the aromatic ring and is available for protonation.Protonation at the $\beta-N$ leads to the formation of an imidazolium ion,which is stabilized by two equivalent resonance structures,making it the more basic site.

Which one of the nitrogen atoms of histidine is protonated first?

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