Amino Acids May Have Positive Negative or Zero Net Charge

Charged and uncharged forms of the ionizable —COOH and —NH3+ weak acid groups exist in solution in protonic equilibrium:

While both R—COOH and R—NH3+ are weak acids, R—COOH is a far stronger acid than R—NH3+. At physiologic pH (pH 7.4), carboxyl groups exist almost entirely as R—COO- and amino groups predominantly as R—NH3+. Figure 3-1 illustrates the effect of pH on the charged state of aspartic acid.

Molecules that contain an equal number of ioniz-able groups of opposite charge and that therefore bear no net charge are termed zwitterions. Amino acids in blood and most tissues thus should be represented as in A, below.

NH3+

Structure B cannot exist in aqueous solution because at any pH low enough to protonate the carboxyl group the amino group would also be protonated. Similarly, at any pH sufficiently high for an uncharged amino

NH3+

Around pH 3; net charge = 0

Around pH 3; net charge = 0

In strong alkali (above pH 11); net charge = -2

Figure 3-1. Protonic equilibria of aspartic acid.

group to predominate, a carboxyl group will be present as R—COO-. The uncharged representation B (above) is, however, often used for reactions that do not involve protonic equilibria.

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