Answer :
There are two amino acids that contain two nitrogen atoms and carry an equal amount of positive and negative charge at neutral pH: histidine and cysteine.
Histidine has a side chain that contains an imidazole group, which has a pKa of around 6.0, making it protonated and positively charged at neutral pH. The alpha-amino group and the alpha-carboxyl group are both deprotonated and negatively charged at neutral pH.
Cysteine has a thiol group in its side chain, which has a pKa of around 8.3, making it deprotonated and negatively charged at neutral pH. The alpha-amino group is positively charged and the alpha-carboxyl group is negatively charged, resulting in an overall neutral charge.
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