Three molecular ionic states, P - R, are possible for the amino acid histidine. Identify the correct choice… · ABC26OR0492
Subject: Organic Chemistry · Chapter: Natural Products · Topic: Natural Products & – General · Exam: GATE 2000-2022 · Marks: 2 · Difficulty: Medium
Three molecular ionic states, P - R, are possible for the amino acid histidine. Identify the correct choice of pH values, respectively, for the observation of the ionic states P - R. 
(a)P at pH 1 : Q at pH 12 : R at pH 7
(b)P at pH 7; Q at pH 1; R at pH 12
(c)P at pH 12; Q at pH 7; R at pH 1
(d)P at pH 12 ; Q at pH 1 ; R at pH 7
Answer
Answer: A ✓ checked by 4AB · confidence high
Explanation
P, the fully protonated cation, exists at pH 1. Q, with only COO⁻ and neutral amine and imidazole, exists at pH 12. R, the zwitterion with imidazolium, is the near-neutral form (a).
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