DL-Amino Acids

Introduction

DL-amino is the mixture of D-amino and L-amino. DL-amino acids are racemic amino acids, that is, L-type amino acids and D-type amino acids in half, with zero rotation. The production process of this amino acid is simpler than that of a single optically active amino acid. So far, nearly two hundred amino acids have been found in living organisms. There are only 20 natural amino acids. Among the traditional natural amino acids, 19 amino acids have an L configuration, and only one amino acid, glycine, has no chiral center. Chemists refer to L-amino acids as "natural" amino acids, and D-amino acids as "unnatural" amino acids.

Application

D-amino acids are mainly used as sweeteners in the food industry. Phenylalanine, aspartic acid and alanine are important raw materials for the synthesis of sweeteners. D- valine can be used to synthesize pyrethroid and chlorofluoroprethrin. D- amino acids are more widely used in medicine. Among them, D-phenylalanine, D-valine, D-proline and D-alanine are the most widely used.

Phenylalanine is the most widely used L-amino acid. The main use of L-phenylalanine is in the production of aspartame (APM). In the field of medicine, L-phenylalanine is not only used directly for infusion of medical compound amino acids and synthetic medicine, but also used to prepare medical culture media.

Chiral Resolution of DL-Amino Acids

L-amino acids can be absorbed directly by the body, and D-amino acids must be converted to L-type in the body before being absorbed. Most of the synthetic amino acids are racemic amino acids. How to efficiently obtain the specular pure enantiomer of amino acids is of great significance for the production of drugs and food. Here are several ways to split DL-amino acids:

Chemical resolution: Chemical resolution is one of the earliest methods for optical isomers resolution, and its mechanism and process are the most mature. Chemical separation principle of amino acid as follows: the racemic amino acid and split agent generate two diastereomer amino acid derivatives (such as amino ester, amide, or salt), using the difference of the chemical and physical properties of enantiomers to separate (usually the solubility difference).

Chemical dispersantFig. 1. Chemical dispersant

Membrane separation: The mechanism of membrane resolution is similar to that of chiral ligand exchange chromatography for the separation of racemic amino acids, depending on the difference in the stability of the ternary complexes formed by the enantiomer of amino acids, the metal cation in solution and the stationary phase containing the chiral selectors.

Chromatographic resolution: Separation principle of chromatographic method: when each component dissolved in mobile phase goes through the fixed phase, the size and strength of the reaction with the fixed phase (adsorption, distribution, ion attraction, exclusion, affinity) are different, and the retention time in the fixed phase is different.

Enzyme separation: Two enantiomers in a racemate, one of which can be converted to other substances under the action of an enzyme, the other one does not change, so as to achieve enantiomer separation.

Extraction resolution: The extraction resolution is based on the difference of solubility of solute in two dissolute phases, which has the advantages of high extraction and separation efficiency, good separation effect and simple equipment.

Induced crystallization: The induced crystallization uses the property that a certain optical isomer of amino acid has a lower solubility than the racemate at a certain temperature and is easy to precipitate out. A certain optical isomer is added into the solution of the outer racemate as a crystal species to induce the isomer with the same crystal species to preferentially precipitate out, so as to achieve the purpose of separation.

Reference:

  1. Xu h m, Xia s w, He c l. synthesis of non-natural L-amino acids by deracemization of DL-amino acids catalyzed by immobilized D-amino acid oxidase [J]. Molecular catalysis, 2012,26 (02) : 192-196.
  2. Hu J q, Huang z p, Li j. Progress in chiral resolution of amino acids [J]. Food and medicine, 2012,14 (01) : 60-64.
  3. Gao j. Study on the B-enzymatic resolution of DL-amino acids by aminopeptidase [D]. Nanjing university, 2013.
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