Dissociation Constant

Amino acids exit aqueous solutions as dipolar ions (zwitterions), and glutamic acid may be represented as follows (16). Following Bronsted, an acid may be defined as a substance capable of combining with a proton. When an amino acid is titrated with HCl and NaOH, a curve exhibiting two points of inflection is obtained. Thus amino acid can react either as an acid or as a base, that is, as an ampholyte.

The dissociation constants for glutamic acid are pK1 2.19 (COOH), pK2 4.25 (COOH), and pK3 9.67 (NH3+). (It is customary to designate the pK values of amino acids in order of decreasing acidity.) The isoelectric point (pI) of glutamic acid is the pH 3.2 calculated from the relation pI = pK1 3 pK2/2, at which there is no net charge on the amino acid molecule. At this pH, no migration will occur in an electric field.

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