All of the 20 common amino acids have at least pka values

Amino acids are weak acids that have at least 2 pka values. They have two titratable protons that composed of alpha carboxyl (-COOH) and alpha amino (-NH3+), but some amino acids have a third titratable protons with ionizable groups in the R side chain.  In addition, the structure of amino acids in peptides and proteins are determined by the charges of the titratable group and by the R group. 

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