At biological pH levels, the protonated form is in equal concentration with the

Question

At biological pH levels, the protonated form is in equal concentration with the unprotonated form of basic side chain of the amino acid below.

Which labeled N is the one protonated in the protonated form? a or b? I know that protonating nitrogen labled a will not destroy the aromaticity of the ring; however the question asks for the Nitrogen that is already in the protonated form, and Nitrogen a doesn't seem to be already protonated. Protonating nitrogen b will destroy the aromaticity, but since it is attached to the H already, wouldn't it be considered already protonated? I feel like the answer would be b. But I am not sure, so please help. Thank you

Explanation / Answer

"N" a will be protonated as the lone pair of electrons are not involved in aromaticity, where as if we are protonating Nb then the system will lose its aromaticity. Even thogh it is already having a proton attached to it its to satisfy its valency.

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