As is evident in Figure 3-10, glycine has a net negative charge at any pH above its pI and will thus move toward the positive electrode (the anode) when placed in an electric field. At any pH below its pI, glycine has a net positive charge and will move toward the negative electrode (the cathode). The farther the pH of a glycine solution is from its isoelectric point, the greater the net electric charge of the population of glycine molecules. At pH 1.0, for example, glycine exists almost entirely as the form +H3N—CH2—COOH, with a net positive charge of 1.0. At pH 2.34, where there is an equal mixture of +H3N—CH2—COOH and +H3N—CH2—COO", the average or net positive charge is 0.5. The sign and the magnitude of the net charge of any amino acid at any pH can be predicted in the same way.

Amino Acids Differ in Their Acid-Base Properties

The shared properties of many amino acids permit some simplifying generalizations about their acid-base behaviors. First, all amino acids with a single a-amino group, a single a-carboxyl group, and an R group that does not ionize have titration curves resembling that of glycine (Fig. 3-10). These amino acids have very similar, although not identical, p^a values: p^a of the —COOH

group in the range of 1.8 to 2.4, and p^a of the —NHj group in the range of 8.8 to 11.0 (Table 3-1).

Second, amino acids with an ionizable R group have more complex titration curves, with three stages corresponding to the three possible ionization steps; thus they have three p^a values. The additional stage for the titration of the ionizable R group merges to some extent with the other two. The titration curves for two amino acids of this type, glutamate and histidine, are shown in Figure 3-12. The isoelectric points reflect the nature of the ionizing R groups present. For example, glutamate ch2 ch2

cooh pKi coo

cooh coo- coo-

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