The separation and analysis methods of chiral substances include chiral source synthesis method, crystal split method, chemical split method, enzyme split method, membrane split method, extraction split method and chromatographic split method.
The separation and analysis methods of chiral substances include chiral source synthesis method, crystal split method, chemical split method, enzyme split method, membrane split method, extraction split method and chromatographic split method.
1. Chiral source synthesis method:
Chiral source synthesis is the synthesis of a single enantiomer of a further chiral compound using a chiral compound of a single enantiomer as a starting material, a method commonly used by chemists.
Since the stereostructure of the starting material determines the stereo configuration of the product, it is critical to obtain a chiral starting material for the desired enantiomer. However, the variety and quantity of natural chiral substances are limited and cannot meet actual needs. The use of natural chiral substances to derive the desired chiral compounds has many reaction steps and is difficult.
2. Crystallization split method:
Crystallization resolution is based on the formation of diastereomeric or covalent derivatives of enantiomers and chiral substances, followed by separation of the diastereomers by qualitative differences (eg fractional crystallization), and reduction of the derivatives to the enantiomers body.
The crystal splitting method is divided into a crystal mechanical splitting method and an inoculation crystal splitting method.
The crystal mechanical resolution method is to crystallize the racemic mixture from the solution, and manually separate from the crystal according to the difference in crystal morphology of the two enantiomers. This method is time consuming and laborious and can only be applied to laboratory research.
The inoculation crystallisation method is a stepwise crystal separation method in which a seed crystal of one of the pure enantiomers is added to precipitate the same enantiomer attached to the seed crystal to be seeded.
Although the crystallization splitting method is simple in operation, it takes a lot of time, labor and material resources, and its experiment is greatly deviated from the expectation, so the application is not extensive.
3. Chemical resolution method:
Chemical resolution is the conversion of two enantiomers in a racemate to a diastereomer by a chemical reaction, using a chiral reagent, and then utilizing the physicochemical properties between the diastereomers. The difference is to separate the two. The key to the success of the separation is to choose the appropriate resolving agent. The suitable resolving agent should be able to form diastereomers with enantiomers, and the solubility difference is large. After disintegration, it is easy to regenerate into the original pair. A reflection.
Although this method has been used as an important method of splitting, its limitations are also obvious:
(1) The choice of resolving agent and solvent is more blind.
(2) The yield of the split and the enantiomeric purity of the product are not high.
(3) There are not many types of enantiomers suitable for chiral separation.
4, enzyme split method:
The enzyme selectively decomposes the optically active isomer, so that one optical isomer in the racemate is enzymatically fastened, while the other is slower or does not undergo enzymatic hydrolysis. The conditions are retained to achieve separation.
The main pathways for enzymatic resolution are host selective hydrolysis, enzymatic and transenzyme.
Enzyme-catalyzed side reactions are few, the yield is high, the reaction conditions are mild, the enzyme is non-toxic, easily degradable, and does not cause environmental pollution. However, the method can only be applied to the enzymatic reaction system, the variety of the enzyme preparation is limited, the enzyme is easily destroyed and unstable, and the price of the preparation is high, which hinders the application development.
5, membrane split method:
The biotransfer of amino acids is usually accomplished by a carrier protein embedded in a biofilm. The enantioselectivity of this transfer is very high, and the membrane-cleaving enantiomer is the simulation of this biological process.
In the liquid membrane, a carrier having chiral selectivity is dissolved in a liquid solvent, and is specifically transferred to an isomer to transfer it from the upper phase to the lower phase, thereby realizing separation of enantiomers. . However, due to the poor stability of the liquid film, its industrial application has been greatly limited.
In order to overcome the instability of the liquid film, the solid film has been greatly developed. In the solid film, different enantiomers complete the transmembrane process by selective diffusion or adsorption. The selective diffusion solid film generally does not have a special chiral resolving agent, and the reason for the selective diffusion is that one isomer is more easily diffused in the solid film than the other isomer. Selective adsorption of solid membranes is mainly carried out by chiral resolution using a special intermolecular interaction between a chiral resolving agent embedded in a polymer matrix and an enantiomer. Usually, one isomer is more selective. Adsorbed on the chiral resolving agent, while the other isomer is more free in the polymer matrix.
Due to the inverse relationship between selectivity and permeate flux, the application of selective diffusion solids is limited and can only be compensated by expanding the membrane area or increasing the number of equilibrium stages, which is uneconomical in practical applications. The selective adsorption of the solid film can be simultaneously improved in both selectivity and permeate flux, making it possible for large-scale applications in the chiral separation industry.
6, extraction and separation method:
The extraction and resolution method is a method in which the difference between the affinity of the two enantiomers in the extractant and the chemical action is used to separate the extractant. There are currently three methods of extraction and resolution: affinity extraction and resolution, coordination extraction and separation, and formation of diastereoisomer extraction and resolution.
The extraction and separation method has strong applicability, high efficiency, low cost and continuous operation, and can realize the integration of the extraction and separation process and the racemization reaction.
In the process of resolution, the enantiomers with no application value can be continuously converted into the desired enantiomers, and the desired enantiomers produced by racemization are extracted into the extract phase, and the enriched phase in the raffinate phase has no application value. The enantiomer undergoes racemization to overcome the drawbacks of the meso racemization process.
The extractant chosen during the splitting process is chiral and the choice of extractant is the key to splitting.
7, chromatographic resolution method:
Chromatography is one of the most reliable and commonly used methods for the determination of low levels of enantiomeric impurities, and is capable of determining the enantiomeric purity of complex matrices while facilitating large scale preparation of enantiomers. Chromatography has become the main tool for chiral separation, and chiral separation has become an important research object in chromatographic science.
(1) Thin layer chromatography:
Thin-layer chromatography is one of the easiest chromatographic techniques. It has the characteristics of simple operation, simple equipment, fast analysis, intuitive results and quick change of flow. It has been widely used in various fields such as chemical, chemical, biochemical, pharmaceutical and health. .
Despite the chiral stationary phase price, UV background, color developer, etc., the chiral carrier currently available for thin layer chromatography and chiral compounds that can be separated by thin layer chromatography are few, but it will become a hand. One of the important means of sexual separation plays an important role in the separation, analysis and optical purity determination of optical isomers.
(2) Gas chromatography:
Gas chromatography is an important means of enantiomeric resolution. At present, there are mainly three types of stationary phases for separating enantiomers in gas chromatography: chiral amino acid derivatives, optically active metal complexes, and cyclodextrin chiral stationary phases.
Advantages of gas chromatography: simple mobile phase, high resolution and high efficiency, suitable for resolution of some enantiomeric compounds without aromatic rings (such enantiomers are usually difficult to separate and detect under liquid chromatography conditions).
Disadvantages of gas chromatography: generally carried out at higher temperatures, easily lead to the racemization of chiral selectors, reducing their chiral recognition ability. The separable enantiomers are limited, and generally only enantiomer compounds which are easily vaporized and have high thermal stability can be isolated.
(3) Capillary electrophoresis:
Capillary electrophoresis has the characteristics of simplicity, high efficiency, low sample consumption and almost no waste liquid. As a simple, rapid, economical and convenient modern technology, it has been widely used in drug analysis and clinical medical research.
When the enantiomers are separated by capillary electrophoresis, a chiral selector is generally added to the buffer, and the enantiomer molecules can form a complex having different stability with the chiral selector, resulting in separation of the migration rate. Commonly used methods are: capillary zone electrophoresis, micellar electrokinetic chromatography and capillary electrochromatography.
(4) Supercritical fluid chromatography:
Supercritical fluid chromatography separates compounds with a supercritical fluid (such as liquid CO2) as a flow. The supercritical fluid has a high solubility in the sample compared to the gas mobile phase. The supercritical fluid has a low viscosity and a large diffusion coefficient compared to a conventional liquid mobile phase.
Advantages of supercritical fluid chromatography:
1) It has the advantages of fast analysis speed and high column efficiency.
2) Suitable for analysis of poor thermal stability and low volatility.
3) Because of its good environmental friendliness, it plays a certain role in many fields such as drugs and metabolites, natural products, oils, food, agriculture and environmental samples.
Disadvantages of supercritical fluid chromatography: high requirements on instruments and poor popularity have limited their application to some extent.
(5) High performance liquid chromatography:
High performance liquid chromatography with chiral stationary phase separation for the determination of enantiomers, high column efficiency, wide application range (can be used for the analysis of poor thermal stability and polar pesticides), strong separation ability, is the first choice for chiral drug separation One of the technology platforms.
The reasons why high performance liquid chromatography is widely used in gas chromatography and electrophoresis are:
1) Since all drugs enter the organism and are further affected by the body fluid transport of the organism, almost all chiral drugs can find a suitable environment suitable for their own characteristics in high performance liquid chromatography.
2) High-performance liquid chromatography makes it easier to perform online pretreatment of biological samples, enabling a high degree of automation.
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