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

The correct answer is: A. P at pH 1; Q at pH 12; R at pH 7

The three molecular ionic states, P-R are possible for the amino acid histidine. The pH value at which each state is observed depends on the pKa values of the amino acid’s functional groups. The pKa of the histidine imidazole group is 6.0, and the pKa of the histidine carboxyl group is 1.8.

At pH 1, the histidine imidazole group is protonated, and the histidine carboxyl group is deprotonated. This results in the formation of the ionic state P, which has a net positive charge.

At pH 12, the histidine imidazole group is deprotonated, and the histidine carboxyl group is protonated. This results in the formation of the ionic state Q, which has a net negative charge.

At pH 7, the histidine imidazole group and the histidine carboxyl group are both partially protonated. This results in the formation of the ionic state R, which has a net neutral charge.

Therefore, the correct choice of pH values, respectively for the observation of the ionic states P-R is:

• P at pH 1
• Q at pH 12
• R at pH 7