Molecular Biology: Protein Structure and Functions - Stepanov V.M. 2005

Amino Acids
Acid-Base Properties of Amino Acids

The acid-base properties of Amino Acids are determined by the simultaneous presence of amino and carboxyl groups in their Structure, which, being situated in close proximity, exert a profound mutual influence on each other.

In a neutral medium, across a fairly wide pH range, a-amino acids exist as dipolar ions; therefore, the conventional formula for amino acids, such as Glycine — NH₂—СН₂—СООН, is merely a simplification. It would be more accurate to write NH⁺₃—СН₃—СОO for neutral solutions. It is precisely in this form of dipolar ions — zwitterions (formerly referred to as a "betaine-like structure") — that amino acids exist in neutral solutions and crystals. The Influence of the positively charged ammonium group in the a-position makes the a-carboxyl group significantly stronger compared to the carboxyl of aliphatic acids. For instance, while the рК_а of acetic acid (the pH at which dissociation is fifty percent complete) is 4.7, the carboxyl рК_а of glycine is 2.34. Similar рК_а values are characteristic of the carboxyl groups in other a-amino acids (Table 1.1).

Naturally, carboxyl groups that are distant from the a-ammonium group, such as the ß-carboxyl of aspartic acid or the у-carboxyl of glutamic acid, do not experience such a significant positive charge effect and closely resemble conventional Fatty acids in their acidic properties and their ability to donate and accept a proton.

Class="center">Table 1.1 Amino acids incorporated into Proteins during Translation (Proteinogenic Amino Acids)

* The complete structure of Proline is shown.

Similarly, the a-amino group in a-amino acids is influenced by the carboxylate anion attached to the same carbon atom, making it considerably less basic than the amino group of primary aliphatic amines: its рК_а is 9.6–9.7, compared to 10.7 for ethylamine. METABOLISM/18.html">The Influence of the carboxyl group on The properties of the amino group, particularly its basicity, diminishes as the distance between them increases. Consequently, the c-amino group of Lysine is practically indistinguishable in its chemical characteristics from aliphatic amines, with a рК_а of 10.5.

If an acid is added to a neutral solution of an a-amino acid containing dipolar ions, thereby increasing the hydrogen ion concentration, the hydrogen ions will bind to the carboxylate groups, converting them into uncharged carboxyl groups. As a result, The amino acid—whose charges were initially balanced—acquires a net positive charge, meaning it will migrate toward the cathode in an electric field. Conversely, if the hydrogen ion concentration is lowered by adding an alkali, the hydroxyl ions will react with the a-ammonium groups, abstracting a proton and converting them into uncharged amino groups. Consequently, the amino acid acquires a net negative charge and will migrate toward the anode in an electric field:

The pH value at which both the amino and carboxyl groups of an amino acid are fully ionized yet their net charge is zero is called the isoelectric point (p_i). For a-amino acids lacking ionizable side chains, the pI value is equal to the arithmetic mean of the рК_а values of the a-amino and a-carboxyl groups. If an amino acid contains additional ionizable groups, their contribution must be factored into the pI calculation.

For example, the pI of aspartic acid cannot be close to 6, as it is for Monoaminomonocarboxylic Acids, because at that pH its ß-carboxyl group would be almost completely dissociated, leaving the aspartic acid with a net single negative charge. To suppress this dissociation, the pH must be lowered significantly—to 2.77—in order to protonate the ß-carboxyl by approximately 90%, which leaves about 0.1 of a negative charge on this group. At such a low pH, protonation of the a-carboxyl group also begins, retaining about 0.9 of a negative charge. Together, these charges balance the single positive charge of the amino group, thereby establishing the isoelectric state of aspartic acid. It is easy to see that in this case the pI is close to the arithmetic mean of the рК_а values of the a- and ß-carboxyl groups. The same principle applies to calculating the pI of glutamic acid, which is 3.24. Similarly, when calculating the isoelectric point of lysine (pI = 9.82), the midpoint between the рК_а values of the a- and ε-amino groups is used.