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    In f324 jun 12 there is a question asking to draw zwitterio of a section of a protein and in the R group of an amino acid is also a NH2 but the markscheme says only one NH2 protonated, how do I know whether to protonate any other COOH or NH2 if there is one in R group.
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    If there are two NH2 groups, then only one of them will be protonated. OCR don't mind which one. The acid group will be deprotonated.

    Think about the definition of a Zwitterion... no overall charge. If both NH2 groups were protonated, there would be an overall positive charge.
 
 
 
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