The isoelectric point of glycine is 6.0. When glycine is in a buffer with a pH of 6.0, which form predominates?

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Correct Answer: A. H3N+-CH2-COO–
Recall that you can use isoelectric point (PI) to predict an amino acid’s charge at any given pH. Specifically, if PI < pH, then this means that pH is high -- which means that [H+] is low, and charge is negative. Likewise, if PI > pH, then this means that pH is lower — which means [H+] is high and charge is positive. In this case, PI = pH, which means that charge is zero (the amino acid is neutral). Thus, the answer is A since this is the amino acid where both charges cancel each other out, leaving a neutral zwitterion glycine.
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