The amino acid shown has three ionisable groups with $p K_{a}$ values 2.18 (COOH), 8.95 ( $α$ - $NH X_{3} X^{+}$ ) and 10.53 (side-chain $NH X_{3} X^{+}$ ). Take the predominant species at pH 12, in which a group is counted as deprotonated when its $p K_{a}$ is below the pH. Give the net charge of the molecule at pH 12 as an integer.

Numerical answer type, tolerance 0.5.

Answer: -1

At pH 12 all three pKa values are below the pH, so the carboxyl group is

COO X^{-}

(-1) and both amino groups are neutral

NH X_{2}

(0). The net charge is -1; a Henderson-Hasselbalch calculation (side chain 97% deprotonated) gives an average of -0.97, consistent with -1. The amino acid is lysine.

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Answer: -1

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