The question asks about a naturally occurring amino acid that has only one basic functional group in its chemical structure. Let's examine the options provided:
Arginine: This amino acid is known to have a guanidinium group as its side chain, which is basic in nature. Thus, arginine has more than one basic group, including its amino group.
Lysine: This amino acid contains an additional amino group in its side chain, making it possess more than one basic group.
Histidine: Histidine contains an imidazole group in its side chain, which is basic. So, it also has more than one basic functional group.
Isoleucine: Isoleucine's side chain is non-polar and aliphatic. It does not contain any additional basic functional groups apart from the basic α-amino group that is present in all amino acids.
Based on the functional groups present in each of these amino acids, Isoleucine is the only one that contains just the single basic α-amino group in its structure.
