Amino acid

The first amino acid was isolated in 1805 in the Paris laboratory of Louis-Nicolas Vauquelin and his student Pierre Jean Robiquet from the juice of asparagus (Asparagus officinalis) and was subsequently named asparagine. The last of the common protein-building amino acids, threonine was discovered in fibrin in 1931 and its structure clarified by William Rose in 1935. Rose had found out by experiments with different feeds that the 19 amino acids discovered so far were not sufficient as an additive. He also established the essentiality of other amino acids and determined the minimum daily dose required for optimal growth.

In the period between 1805 and 1935, many of the chemists and pharmacists known at the time were involved in isolating amino acids for the first time and clarifying their structure. Emil Fischer, for example, to whom the Fischer projection also goes back, succeeded in finally elucidating the structure of serine (1901), lysine (1902), valine (1906) and cysteine (1908). Albrecht Kossel (1896 histidine from sturgeon sperm), Richard Willstätter (1900 proline via synthesis) and Frederick Hopkins (1901 tryptophan from casein) also later became Nobel Prize winners. The German chemist Ernst Schulze isolated three amino acids for the first time - glutamine from beets in 1877, phenylalanine in 1881 and arginine from lupins in 1886 - and was involved in the structural elucidation of other amino acids. Heinrich Ritthausen had previously obtained glutamic acid from cereal protein, gluten, in crystalline form in 1866. In 1872, Wilhelm Dittmar clarified the structure of glutamine and glutamic acid, whose salts are glutamates.

As early as 1810, William Hyde Wollaston discovered the sulfur-containing cystine as "cystic oxide" in bladder stones, but it was not until 1884 that Eugen Baumann discovered the monomeric cysteine. In 1819 Henri Braconnot separated glycine from glue and Joseph Louis Proust separated leucine from cereals. Eugen von Gorup-Besánez isolated valine from pancreatic juice in 1856. As early as 1846, Justus von Liebig was able to separate tyrosine from casein for the first time, the structure of which was clarified by Ludwig von Barth in 1869. In the hydrolysate of casein, Edmund Drechsel also discovered lysine in 1889 and later John Howard Mueller in 1922 discovered the sulphur-containing methionine as the 19th amino acid, whose structural formula was given by George Barger and Philip Coine in 1928. In molasses, Felix Ehrlich had already found isoleucine, a structural isomer of leucine, as the 18th amino acid in 1903.

Friedrich Wöhler, whose syntheses in the 1820s opened up the field of biochemistry, did not discover any amino acid, but three of his students were involved, in addition to the aforementioned Gorup-Besánez and Schulze, also Georg Städeler (1863 serine from raw silk). 18 of the 20 amino acids discovered were isolated from plant or animal material, only the two amino acids alanine (1850 Adolph Strecker) and proline (Willstätter) were obtained by organic synthesis. While the analysis of the material composition up to the sum formula could be well accomplished with the methods of the time, the structural formula of many amino acids could often only be finally elucidated by partial steps of the synthesis, which was sometimes only achieved years later. The structure of asparagine and that of aspartic acid was not clarified by Hermann Kolbe until 1862, 57 years after the first description.

Amino acids owe their generic names to two functional groups, their individual names sometimes to a bright appearance (e.g. arginine, leucine), a sweet taste (e.g. glycine) or the material in which they were found (e.g. asparagine, cysteine, serine, tyrosine), features of the chemical structure (e.g. proline, valine, isoleucine) or both (e.g. glucine). e.g. asparagine, cysteine, serine, tyrosine), features of the chemical structure (e.g. proline, valine, isoleucine) or both (e.g. glutamine, glutamic acid) and sometimes also the reactants of their synthesis (e.g. alanine).

The fact that proteins are built up as chains of amino acids linked by peptide bonds was first proposed simultaneously and independently of each other by both Emil Fischer and Franz Hofmeister at the Assembly of German Natural Scientists and Physicians in Karlsbad in 1902 (Hofmeister-Fischer theory).

Amino acids consist of at least two carbon atoms. The unstable carbamic acid has only one carbon atom and is therefore not an amino acid, but a carbonic acid amide. Amino acids can be divided into classes according to the carbon atom at which the amino group is located relative to the carboxy group. If more than one amino group is present in the molecule, the carbon atom whose amino group is closest to the carboxy carbon determines which class of amino acid it is.

• α-Amino acids: The amino group of α-amino acids is located at the second carbon atom, including the carboxy carbon atom. The counting always starts with the carboxy-carbon. Therefore, the IUPAC designation is 2-aminocarboxylic acids. The simplest representative of the α-amino acids is the proteinogenic amino acid glycine. All proteinogenic amino acids are α-amino acids.

The term amino acids often refers to a specific group of α-amino acids consisting mainly of L-α-amino acids: the proteinogenic amino acids. These are the building blocks of all proteins of all life on earth and, along with nucleic acids, the basic building blocks of life.

• β-Amino acids: The amino group of β-amino acids is located on the third carbon atom (counting the carboxy carbon atom). The IUPAC designation is 3-aminocarboxylic acids. The simplest representative is β-alanine.
• γ-Amino acids: The amino group of γ-amino acids is located at the fourth carbon atom (counting the carboxy carbon atom). The IUPAC designation is 4-aminocarboxylic acids. The simplest representative is γ-aminobutyric acid (GABA).

The designation of other classes of amino acids follows the same scheme.

The amino acids of a class are distinguished by their side chain R. If the side chain R is different from the other substituents located on the carbon with the amino group, then a stereocenter is located here and two enantiomers exist of the corresponding amino acid. If the side chain R itself contains further stereocentres, diastereomers also result and the number of possible stereoisomers increases in proportion to the number of further stereocentres. There are four stereoisomers of amino acids with two differently substituted stereocentres.