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FIGURE 3-15 Alanylglutamylglycyllysine. This tetrapeptide has one free «-amino group, one free a-carboxyl group, and two ionizable R groups. The groups ionized at pH 7.0 are in red.

ever, the R groups of some amino acids can ionize (Table 3-1), and in a peptide these contribute to the overall acid-base properties of the molecule (Fig. 3-15). Thus the acid-base behavior of a peptide can be predicted from its free a-amino and a-carboxyl groups as well as the nature and number of its ionizable R groups.

Like free amino acids, peptides have characteristic titration curves and a characteristic isoelectric pH (pi) at which they do not move in an electric field. These properties are exploited in some of the techniques used to separate peptides and proteins, as we shall see later in the chapter. It should be emphasized that the pKa value for an ionizable R group can change somewhat when an amino acid becomes a residue in a peptide. The loss of charge in the a-carboxyl and a-amino groups, the interactions with other peptide R groups, and other environmental factors can affect the pKa. The pKa values for R groups listed in Table 3-1 can be a useful guide to the pH range in which a given group will ionize, but they cannot be strictly applied to peptides.

Biologically Active Peptides and Polypeptides Occur in a Vast Range of Sizes

No generalizations can be made about the molecular weights of biologically active peptides and proteins in relation to their functions. Naturally occurring peptides range in length from two to many thousands of amino acid residues. Even the smallest peptides can have biologically important effects. Consider the commercially synthesized dipeptide L-aspartyl-L-phenylalanine methyl ester, the artificial sweetener better known as aspartame or NutraSweet.

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