A student recently wrote us: "I feel like it should be +2 because we have carboxylic group rather than carboxylate."
For this question, it is important to note that even though there are two nitrogens that could potentially be protonated in highly acidic conditions like the stomach, the amine group in the peptide bond is a weak base because of resonance and will be difficult to protonate. The carboxylate groups are acidic, will not act as bases, and will not be protonate any further. Therefore, the only group that will be protonated at low pH will be the free amine group, giving aspartame a net charge of +1.
Thanks for your question. I hope this helps!
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