Why is arginine's positive side chain classified as basic and not acidic?


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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Updated on August 01, 2022


  • K-Feldspar
    K-Feldspar 1 day

    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" .


    • Buck Thorn
      Buck Thorn about 1 year
      Protonated arginine sidechain makes a weak acid.
  • orthocresol
    orthocresol almost 6 years
    I would use "uncharged" instead of "neutral" to avoid confusion but otherwise +1