JEE Main · 2025 · 08 Apr 2025 · Shift 2 · Biomolecules

Valine is a neutral amino acid whose isopropyl side chain carries no ionisable group, so its net charge is set by the carboxyl and amino groups alone. At pH 2, well below the isoelectric point, both groups are protonated and the molecule exists as the cation ${}^{+}{\mathrm{H}}_3\mathrm{N}-\mathrm{CH}(\mathrm{iPr})-\mathrm{COOH}$, which is structure A. At pH 10, above the isoelectric point, the carboxyl group is ionised while the amino group is neutral, giving the anion ${\mathrm{H}}_2\mathrm{N}-\mathrm{CH}(\mathrm{iPr})-\mathrm{CO}{\mathrm{O}}^{-}$, which is structure B. Thus A is the fully protonated cationic form and B is the deprotonated anionic form of valine.