Amino Acids Have Characteristic Titration Curves

Acid-base titration involves the gradual addition or removal of protons (Chapter 2). Figure 3-10 shows the titration curve of the diprotic form of glycine. The plot has two distinct stages, corresponding to deprotonation of two different groups on glycine. Each of the two stages resembles in shape the titration curve of a monoprotic acid, such as acetic acid (see Fig. 2-17), and can be analyzed in the same way. At very low pH, the predominant ionic species of glycine is the fully pro-tonated form, +H3N—CH2 —COOH. At the midpoint in the first stage of the titration, in which the —COOH group of glycine loses its proton, equimolar concentrations of the proton-donor (+H3N—CH2—COOH) and proton-acceptor (+H3N—CH2—COO-) species are present. At the midpoint of any titration, a point of inflection is reached where the pH is equal to the pKa of the protonated group being titrated (see Fig. 2-18). For glycine, the pH at the midpoint is 2.34, thus its —COOH group has a pKa (labeled pK1 in Fig. 3-10) of 2.34.

(Recall from Chapter 2 that pH and pKa are simply convenient notations for proton concentration and the equilibrium constant for ionization, respectively. 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.) As the titration proceeds, another important point is reached at pH 5.97. Here there is another point of inflection, at which removal of the first proton is essentially complete and removal of the second has just begun. At this pH glycine is present largely as the dipolar ion +H3N—CH2—COO-. We shall return to the significance of this inflection point in the titration curve (labeled pi in Fig. 3-10) shortly.

The second stage of the titration corresponds to the removal of a proton from the —NHj group of glycine. The pH at the midpoint of this stage is 9.60, equal to the pKa (labeled pK2 in Fig. 3-10) for the —NHj group. The titration is essentially complete at a pH of about 12, at which point the predominant form of glycine is H2N—CH2—COO-.

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