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What is the charge of the amino acid lysine at $\mathrm{pH}=2$? $\mathrm{p}K_\mathrm{a}$'s are carboxylic: 2.18; amino: 8.95; side chain: 10.79.

The side chain of lysine is an amine group, so I assume that when fully protonated it has positive charge ($\ce{NH3+}$).

At $\mathrm{pH} = 2$, everything will be protonated because their $\mathrm{p}K_\mathrm{a} > \mathrm{pH}$.

This gives a net 0 (carboxy) + 1 (amino) + 1 (side chain) = +2 charge. Why do sites such as this say that at $\mathrm{pH} = 2$, lysine's charge is only +1, not +2?

There's probably something really simple I don't know. If anyone can help point it out, that would be great.

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At $\mathrm{pH}=2$, the charge will be:

(charge near neutral pH, 2 groups positive, one negative) + (the fraction of the pKa = 2.18 group that is protonated)

=1 + (the fraction of the $\mathrm{p}K_\mathrm{a} = 2.18$ group that is protonated)

$=1 + 1/(1 + 10^{\mathrm{pH}-\mathrm{pK}}) =1 + 1/(1 + 10^{2-2.18})=1.60$

So the charge is +1.60, which is closer to 2 than 1, as you are saying.

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  • $\begingroup$ Well it worth to pointing that's average charge and modal value is +2 $\endgroup$ – Mithoron Mar 16 '18 at 17:21
  • $\begingroup$ I'll mark this answer as correct in a second, but if +2 is the average charge, then what is 1.6? $\endgroup$ – Felix Hu Mar 16 '18 at 17:35
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    $\begingroup$ @LittleDragon 1.60 is the average charge. 1.60 is the charge that you would measure by electrophoresis. $\endgroup$ – DavePhD Mar 16 '18 at 17:39

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