Question
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, or imidazole, that can be protonated.
(C) A carboxyl group that donates a proton and becomes negatively charged.
(D) A hydroxyl group that remains uncharged at physiological pH.
Answer
Choose (B): a nitrogen-containing group that can accept a proton. In the basic amino acids, the relevant side-chain groups are not all identical:
- Lysine has a terminal amine.
- Arginine has a guanidinium group.
- Histidine has an imidazole ring.
These nitrogen-containing groups can bind . Lysine and arginine side chains are ordinarily protonated and positively charged near physiological pH. Histidine's imidazole has a pKa near the physiological range, so its protonation state is especially sensitive to the local environment.
Why the other choices are wrong
- (A) A nonpolar hydrocarbon group lacks the proton-accepting nitrogen that defines the basic side chains in this question.
- (C) A carboxyl group tends to lose a proton and become negatively charged; it characterizes acidic side chains.
- (D) A hydroxyl group is usually uncharged near physiological pH and is not the defining group for lysine, arginine, or histidine.
The broad phrase “amino (nitrogen-containing) group” in the option should therefore be read as a category of protonatable nitrogen-containing functionality, not as a claim that guanidinium and imidazole are amines.
Evidence boundary
This page answers the Pearson classification question. Basic refers here to a side chain's ability to accept a proton; lysine, arginine, and histidine do not contain the same functional group or have identical protonation behavior. Histidine's protonation state is especially sensitive to local environment.
Sources
These references support the concepts and methods used in the explanation above.