In the context of charged amino acids, a basic amino acid has an R group that contains which of the following functional groups that can accept a proton and become positively charged at physiological pH?

• A. A nonpolar hydrocarbon group composed only of carbon and hydrogen
• B. An amino (nitrogen-containing) group, such as an amine/guanidinium/imidazole, that can be protonated
• C. A carboxyl group that donates a proton to become negatively charged
• D. A hydroxyl group that remains uncharged under most physiological conditions

Answer: (B)