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" .


  • 1
    $\begingroup$ Protonated arginine sidechain makes a weak acid. $\endgroup$
    – Buck Thorn
    May 25 '21 at 5:21

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.)

  • 3
    $\begingroup$ I would use "uncharged" instead of "neutral" to avoid confusion but otherwise +1 $\endgroup$
    – orthocresol
    Aug 7 '16 at 1:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.