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Arginine has a positive side chain.

Doesn't this mean it wants to lose a proton H+? Isn't something that wants to lose or donate a proton an acid?

Yet my book states, "arginine has a positively charged side chain, i.e it is basic" .

Arginine

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    $\begingroup$ Protonated arginine sidechain makes a weak acid. $\endgroup$
    – Buck Thorn
    May 25 at 5:21
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Your textbook is imprecise. We consider whether a sidechain is acidic or basic based on the action of the sidechain when it is neutral.

Because arginine's sidechain is basic, it preferentially picks up a proton, and hence becomes positively charged. (In the same vein, carboxylic acid sidechains are acidic because they preferentially lose a proton, and exist as the negatively charged carboxylate anion.)

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    $\begingroup$ I would use "uncharged" instead of "neutral" to avoid confusion but otherwise +1 $\endgroup$
    – orthocresol
    Aug 7 '16 at 1:32

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