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24 August, 04:31

The p K a of lysine's carboxyl group, amino group, and side chain are 2.2, 9.0, and 10.5, respectively. If lysine is in a pH 13 solution, what is the net charge on each lysine molecule?

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  1. 24 August, 04:55
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    The net charge on each lysine molecule would be - 1.

    Explanation:

    When the pH is above 2.2 the deprotonated form of the carboxylic acid is more present, while the amino group and side chain (which is also amino) remain protonated (with a positive charge):

    R-COOH ↔ R-COO⁻

    R-NH₃⁺

    R'-NH₃⁺

    Net charge = + 1

    When pH is above 9.0, the carboxyl group remains deprotonated, while the amino group is deprotonated and the side chain is protonated:

    R-COOH ↔ R-COO⁻

    R-NH₂

    R'-NH₃⁺

    Net charge = 0

    When pH is above 10.5, the carboxyl group remains deprotonated, while both the amino group and the side chain are deprotonated:

    R-COOH ↔ R-COO⁻

    R-NH₂

    R'-NH₂

    Net charge = - 1

    So at pH=13 (which is above 10.5) the net charge is - 1.
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