The functional group most likely to gain a proton and become positively charged is the amino group (-NH2). This group can act as a base by accepting a proton (H+), resulting in a positively charged ammonium form (-NH3+). The nitrogen atom in the amino group has a lone pair of electrons that can bond with a proton, thus gaining a positive charge
. In contrast, other common functional groups like carboxyl (-COOH) tend to lose a proton and become negatively charged (carboxylate, -COO−) rather than gaining one
. Phosphate groups are also acidic and typically lose protons rather than gain them
. Therefore, among typical biological functional groups, the amino group is the one most prone to protonation and acquiring a positive charge.
