Racemization
The racemization denotes an approximation of the concentrations of two mirror image molecules ( enantiomers ). When the racemization is complete, a racemate results , the 1: 1 mixture of the two enantiomers.
properties
Racemization is a term from stereochemistry which describes the conversion of non-racemic enantiomeric mixtures or pure enantiomers of chiral chemical compounds into the racemate, with an existing enantiomeric excess being completely degraded. When the racemization is complete, a racemate is formed whose ee = 0 (the ee is the enantiomeric excess ). If the ee value ≠ 0, one speaks of partial (partial) racemization.
Racemization is used to obtain desired enantiomers from a mixture with undesired enantiomers. If the ( R ) -enantiomer is obtained from a racemate by resolution in a process, 50% of the starting material remains as the ( S ) -enantiomer. The ( S ) -enantiomer can be converted back into the racemate or partially racemized and used again as starting material by racemization. If the racemization is carried out in situ at the same time as the racemate resolution, one speaks of dynamic racemate resolution .
The mechanism of racemization depends strongly on the type of substance. Racemization occurs roughly as a result of S N 1 reactions . This creates a trigonal-planar arrangement around the now positively charged carbon atom ( carbenium ion ). Therefore the carbon atom can now be attacked from both sides. The reaction produces both enantiomers, even if a single enantiomer is used as the starting material. For example, ( R ) -3-bromo-3-methylhexane reacts with water to ( R ) -3-methyl-3-hexanol and ( S ) -3-methyl-3-hexanol in a ratio of 50:50, so it is formed ( RS ) -3-methyl-3-hexanol, a racemate.
An enantiomerically pure secondary alcohol with a stereocenter on the carbon atom to which the hydroxyl group is attached can be converted into the racemic secondary alcohol by oxidation to the relevant ketone and subsequent reduction of this ketone with sodium borohydride, i.e. racemized.
Age determination
As a rule, biomolecules are formed enantiomerically pure due to the enzymatic biosynthesis , ie only one of the two possible enantiomers is formed. However, racemization occurs over longer periods of time, beginning at the time of death, for example with the amino acids . The determination of the racemization to determine the age by amino acid dating is suitable for periods of 1,000 to 100,000 years.
Peptide synthesis
In peptide synthesis , the partial racemization of individual amino acid building blocks , which can occasionally be observed, is an undesirable process , since it results in two or more peptides of different stereochemistry ( diastereomers ) which are often difficult to purify due to their great chemical similarity. The diastereomeric peptides formed in this way usually have significantly different physiological - z. B. pharmacological - effects. The racemization in the peptide synthesis can be reduced by selecting suitable coupling reagents . A racemization test is often carried out after a peptide synthesis .
Web links
Individual evidence
- ↑ Heinz GO Becker, Werner Berger, Günter Domschke, Egon Fanghänel , Jürgen Faust, Mechthild Fischer, Fritjof Gentz, Karl Gewald , Reiner Gluch, Roland Mayer, Klaus Müller, Dietrich Pavel, Hermann Schmidt, Karl Schollberg, Klaus Schwetlick, Erika Seiler and Günter Zeppenfeld: Organikum , Johann Ambrosius Barth Verlag, 1993 , 19th edition, pp. 216-217, ISBN 3-335-00343-8 .
- ↑ PM Masters, JL Bada, JS Zigler: Aspartic acid racemization in heavy molecular weight crystallins and water insoluble protein from normal human lenses and cataracts. In: Proceedings of the National Academy of Sciences . Volume 75, Number 3, March 1978, pp. 1204-1208, PMID 274711 , PMC 411438 (free full text).
- ^ Josef Riederer: Works of art viewed chemically. Springer-Verlag, 2013, ISBN 978-3-642-81588-1 , p. 142.