Amino Acid Structure and Classification (1A) Practice Test

A mutation replaces a residue involved in a salt bridge with a partner carboxylate (–COO$^-$) at pH 7.4. Which new residue is most likely to preserve the electrostatic interaction by remaining positively charged at pH 7.4?

General amino acid: $\mathrm{H_3N^+-CH(R)-COO^-}$

A folded protein is exposed to increasing pH from 7.0 to 11.0, and a surface patch becomes more negatively charged, increasing electrostatic repulsion. Which residue type on the surface is most consistent with gaining negative charge over this pH range due to side-chain deprotonation?

General amino acid: $\mathrm{H_3N^+-CH(R)-COO^-}$