Comment on the Titration Curves of Amino Acids.

Acid-base titration involves the gradual addition or removal of protons by the addition of a base. Amino acids have characteristic titration curves. Titration curve of glycine has two distinct stages, corresponding to deprotonation from two different groups on glycine. At very low pH, the predominant ionic species of glycine is the fully protonated form (+H3NCH2COOH). On addition of a base, -COOH group starts losing a proton, such that, at the midpoint in the first stage of the titration, equimolar concentrations of the proton-donor (+H3NCH2COOH) and proton-acceptor (+H3NCH2COO) species are present at pH =2.34 = pKa 

(pK1) of the carboxylic acid group: The pKa is a measure of the tendency of a group to give up a proton, with that tendency decreasing tenfold as the pKa increases by one unit. On addition of more base, steep inflexion occurs at pH 5.97, indicating complete removal of the first proton and glycine is present largely as the dipolar ion (+H3NCH2COO). The second stage of the titration corresponds to the removal of a proton from (+H3NCH2COO). The pH at the midpoint of this stage is 9.60 = pKa (pK2) for the (+H3N-) group. The titration is essentially complete at a pH of about 12, at which point the predominant form of glycine is H2NCH2COO

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