Dissociation Constant

In aqueous solution, amino acids are in a state of dissociation equilibrium that is dependent on the pH. In acidic conditions, they exist as cationic forms (A+); in basic conditions, as anionic forms (A"); in between, as mainly zwit-

terions (A^, where the cationic and anionic forms are equal.

When an amino acid solution is in a neutral state and acidic or alkaline agents are added slowly, an abrupt change in pH at the inflection is observed that plateaus at constant value. The more added, the move abrupt the change in pH at the infliction. Amino acids show a double buffer action in both acidic and basic regions, which are named pKl and pK2, corresponding to the inflection point in the titration curve.

The pKi, pK2, and pi values are calculated from the titration curves with the Henderson-Hasselbalch equation.

The equilibrium constant in acidic solution is expressed as Kj = [A±][H+]/[A+]. Conversely, in basic solution, the equilibrium constant is K2 = [A"][H+]/[A±]. When 50% of amino acid is dissociated, [A±] = [A+]; [A ~ ] = [A±], ie, Kx = [H+] and K2 = [H+], then pK, and pK2 each show the pH in solution.

When a direct current is applied to an amino acid solution and the pH in solution is lower than pK'ly the amino acids transfer toward the cathode; when pH is higher than pK2, they transfer toward the anode. This phenomenon is called electrophoresis and is used as a effective tool for the separation of amino acids. In between pK1 and pK2, no migration of amino acids is observed because the apparent charge of amino acid in solution is zero, which assumes that the concentration of cations in solution is equal to the concentration of anions:

It follows that [H+] at the isoelectric point = ^JK1K2, and in negative logarithmic terms, where pi is pH at the isoelectric point, pi = pKx + pK2/2. For neutral amino acids, such as glycine, an amphoteric ion theoretically exists only at the point of pi, but in the broad range of pH between 4.3 and 7.7, no amino acid transfer in electrophoresis observed. This range is said to be the isoelectric region, in which more than 99% of amino acids are dissociated as amphoteric compounds.

For acidic amino acids such as L-aspartic acid, the amino group and carboxyl group on the a-position and an additional carboxylic group on the R-site are dissociated. This compound has three pK values: pKt (a-carboxylic group), pK2 (/i-car boxy lie group), and pKz (a-amino group), named in the order from the acidic region. The isoelectric point is defined as pi = pKx + pK2/2.

On the other hand, a basic amino acid such as lysine has an additional amino group on the R-site, corresponding to pK' values that exhibit pKt for the a-carboxyl group, pK2 for the a-amino group, and pKz for the co-amino group, respectively. At neutral pH 7, lysine exists in cation form, when moved toward an alkaline condition, the ratio of cation form gradually decreases and reaches an isoelectric point of pi = pK2 + pi£3/2.

The pK1 values show that the amino acids are considerably stronger acids than acetic acid. Because of intramolecular protonation of the amino moiety by the carboxyl group, the amino acids solutions are weakly acidic.

The differences in acidity and basicits are used in the separation of amino acid mixtures by ion exchange chromatography and electrodialysis. The dissociation constant pK and the isoelectric point pi for various amino acids are summarized in Table 7 (32).

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