Titration of Aspartic Acid

Aspartic acid is an amino acid, or one of the building blocks of protein. It's one of only two amino acids with an acidic sidechain, which means that it has a net negative charge in the body, whereas most amino acids are neutral. The titration of aspartic acid reflects this.

Amino Acids

Amino acids are the building blocks of protein, and there are 20 of them that occur "normally" in nature, though there are also modified amino acids that are less common. All amino acids have a central carbon to which four groups are attached. Three are common to all amino acids: a carboxylic acid group -- which has acidic properties -- a basic amine group and a hydrogen. The fourth group, called the sidechain, is different in each amino acid.

Aspartic Acid

Aspartic acid has a sidechain with the chemical formula CH2COOH. The "COOH" component of the sidechain is called a carboxylic acid and has acidic properties. Coupled with the carboxylic acid that appears on all amino acids, this means aspartic acid has two acidic groups attached. In the body, both acidic groups react and lose their hydrogen atoms, resulting in the formation of a negatively charged version of aspartic acid called aspartate.

Titration

A titration is the neutralization of an acid with base or of a base with acid, generally for the purpose of gaining information about the acid or base being titrated. In the case of amino acids, titrating with base can produce a graph called a titration curve, which has either two or three inflection points known as "equivalence points." These are used to find points on the titration curve called "half-equivalence points," which in turn can help to identify the amino acid.

Aspartic Acid Titration

Aspartic acid has three pKa values, where a pKa value indicates how acidic each group on the amino acid is. These are 2.10, 3.86 and 9.82, and they correspond to the main carboxylic acid, the sidechain carboxylic acid and the amine group, respectively. On the titration curve for aspartic acid, as with all titrations, the half-equivalence points occur at the pKas for the molecule being titrated, which is useful for biochemists.

References

• "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
• "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
• "Chemistry: The Molecular Nature of Matter and Change"; Martin Silberberg, Ph.D.; 2008