Question

At pH _____ histidine is found mainly in form II
a) 1
b) 5
c) 7
d) 9

Answer 1

Histidine is mainly found in form II at pH 7, where the side chain's amine group is mostly deprotonated due to the pH being one unit above its pKa value of approximately 6.0.

Step-by-step explanation:

At pH 7, histidine is found mainly in form II. Histidine, a basic amino acid with a side chain that contains an amine group, has a pKa value of approximately 6.0. At a pH lower than the pKa, the side chain's amine group is protonated to -NH3+, and at a pH higher than pKa, it is deprotonated to -NH2. Considering that the pH of the medium is one unit above the pKa, the amine group will be largely in its deprotonated form, hence form II is the predominant species of histidine at pH 7.