Question

A polypeptide at a physiological pH (≃7.4) shows a net positive charge. The amino acids in this polypeptide are most likely to be

• A. acidic
• B. basic
• C. non-polar aliphatic
• D. non-polar aromatic

Answer 1

The correct option is B basic

Basic amino acids show a net positive charge at physiological pH. The extra amino groups in their side-chain get protonated (Hᐩ addition) in the solution to form positively charged NH${}_3$ᐩ ions.
Similarly, an acidic amino acid with an extra carboxyl (-COOH) group in its side chain loses a proton $\left(H^{+}\right)$ in solution to change into negative charged $CO{O}^{-}$ ion.

Non-polar amino acids are generally hydrophobic and are normally not involved in ionization of polypeptide. Generally, hydrophobic moieties occur in the interior of the protein molecule so that they are ‘saved’ from water.