Chiral Separation: The Mechanism of Action

Chiral Separation: The Mechanism of Action

To understand the mechanism of chiral separation, we need to understand the interaction between the chiral compounds being separated and the chiral packing materials or the chiral selectors being used to separate the optical isomers. Since after the incident of the Thalidomide issue, FDA and other regulatory authorities expressed their great concern about the characterization/ identification of drug substances' correct optical isomers status in terms of their efficacy, quality, and safety. The thalidomide issue was a single incident that helped regulators open their insight about the safety concern of drugs relating to the usage of the right optical isomer. Thalidomide (a racemic mixture of R& S-isomer) was used to prescribe a sedative for alleviating morning sickness to the pregnant women, however, later it was identified that one of the optical isomers (R-isomer) was acting like the drug and another isomer (S-isomer) was acting as a teratogenic impurity and caused severe bath defects to hundreds of thousands newly born babies. After the incident the FDA and other regulatory authorities established the rule for optically active isomers to be claimed as drug substances must be isolated and identified as the single form of the isomer, must not be the form of the racemate (mixture of R & S isomer). ?Since then, isolation, separation, and quantitation of the intended optical isomer dealt with a great effort across the world, and scientists spent their time finding suitable techniques to separate the isomers from each other. ?Despite having many alternative approaches to separate optical isomers from each other, the chromatographic technique is considered as the unique one and various types of chiral packing materials and chiral selectors have already been invented. ?Like conventional chromatography, chiral separation also involves some basic interactions between the chemical compound (analyte) and the chiral packing materials. Though several types of chiral packing materials have already been developed, however, cellulose and amylose-based chiral packings are suitably enough to separate most of the chiral compounds. The interactions generally involved during chiral separation are:

i)????Pi-Pi interaction

ii)???Dipole-dipole interaction

iii)??Ion-dipole interaction

iv)??Hydrogen bonding

v)??Van Der Waals forces (London dispersion which is the subtype of Van Der Waal’s forces)

We need to keep in mind, not all the listed interactions are necessarily required to involve when separation is in question. It solely depends on what type of compound it is and what types of functional groups are there to be linked with the chiral carbon. A chiral carbon is required to attach four different types of groups/atom (s) to be claimed as ‘chiral’ and for just an example, let’s imagine a carbon atom in the drug substance having a connection with one amino, one carboxyl, one alkyl and one phenyl group on it and obviously it is a chiral carbon and there should be two optical isomers for this specific chiral center. Since optical isomers are similar in terms of their physical and chemical properties except for their spatial arrangement in the space, thus, separation of optical isomers using chiral packing materials is a little bit different than conventional chromatography. In conventional chromatography, the mechanism of separation is almost the same including the involvement of all the listed five interactions above, however, for the chiral separation, the intended chiral packing material must be capable to exert all the possible interactions so that it can respond to the approaching racemic mixture (the mixture of both the R & S isomers) supposed to be separated.

If we look at the listed above five types of interactions, we will see their independent style to have interacted with the chiral packing/ chiral selectors. This article is outside the scope of describing the different types of interactions in detail. In Conventional chromatography, packing materials are developed keeping in mind the majority portion of the behavior of the compound being analyzed and whether the compound is nonpolar, mild to moderately polar, or highly polar, and based on those behaviors, a broad range of packing materials e.g., nonpolar (C18 >C8> C4> CH3) to mild to moderately polar (amide>amino>Ph>CN>PFP) to highly polar(silica) have already been invented and been used popularly. Depending on the compound’s inherent nature, the right packing material is usually selected to optimize the listed interactions followed by the expected separation.

Though all the listed interactions are involved for both the conventional and the chiral chromatography, the separation in conventional chromatography is pretty straightforward and most of the time the separation is ‘obvious’ since compounds intended to be separated is neither structurally similar nor their physical and chemical properties are same which allow them getting an extra room to be discriminated inside the HPLC column (Chromatographic environment) and this discrimination helps separation taking place. On the other hand, compounds being separated under the chiral packing, the separation is not as straightforward as the conventional chromatography since chiral compounds are structurally similar in terms of having a dozen physical and chemical properties are the same except the spatial arrangement in the space.

For the case of optical isomers, since their physical and chemical properties are the same, so the idea of separating them from each other using the specially designed packing material named ‘chiral packing’ or ‘chiral selector’ is to ensure the interaction of at least three different groups attached to the chiral carbon. These three different groups are supposed to be within the close proximity of the interacting groups attached on the chiral packing/ chiral selectors whereas the fourth group may not be readily available to interact, meaning it is staying far. This approach has been identified as the “Three-point interaction” model in many articles/ textbooks.

Once, three different groups attached to the chiral carbon are found suitably interacting to the relevant three different groups (internationally been attached to the chiral packing/ chiral selectors) using any of the listed five interactions types, ?which means it helps to bind one part of the optical isomer ( for example R-isomer) with the chiral packing material for a while ( little bit longer time) whereas another part of the optical isomer ( for example S-isomer) may be available to interact using only two different groups instead of three different groups and that way discrimination happen and separation take in place. This is one of the well-established concepts of the interaction between the chiral compound and the chiral packing/ chiral selectors. ?

As we already mentioned, like conventional chromatography, five types of interaction may involve during chiral separation, so it is important to make well equipped the chiral packing/ chiral sectors during developing the packing materials so that it is found to be capable to interact/responding to the all the interaction types supposed to be faced by the chiral compound. If we look back to our initial compound having four different types of groups and identified as the chiral compound, the amino, carboxyl, methyl, and the phenyl group may involve different types of five listed interactions e.g., hydrogen bonding, ion-dipole, Van Der Waal’s, dipole-dipole and ‘pi-pi’ interaction respectively. ?Cellulose and amylose based chiral packing material/chiral selectors have been equipped keeping in mind that these selectors should have all the ‘tools’ attached on it so that they can interact all the four different groups to be available on our imaginary chiral carbon occupied four different groups through deploying all the five listed interactions.

In the structure of the cellulose and amylose based chiral selectors, there is a substituted phenyl group attached with the methyl group(s) at different positions including different types of electron withdrawal group ( e.g. Chlorides or Fluorides) attached with the phenyl ring and finally, this intentionally substituted phenyl group has been connected with the Amide group on carbamic acid and the whole dimethyl phenyl or chloromethyl phenyl carbamate has finally been attached through the replacing of?three hydroxyl groups located on the different position of the chair conformation of either cellulose or the amylose structure which can be considered as a “ gigantic actor” having all the tools on it capable to interact all the possible different groups attached with the chiral carbon and capable to exert all the five listed interactions spontaneously as required and discriminate the optical isomers from each other despite having a dozen of physical and chemical properties are the same.

There are a lot of varieties of ???Cellulose and amylose based chiral packing /selectors whose basic structure is phenyl carbamate replaced with different alkyl and halide groups in terms of making the phenyl group either electron enrich, or electron deficit followed by leading/energizing the type of interaction and this single tool (for example -R group) has been attached on the cellulose or amylose structure intentionally. They have been named as a different brand based on the attached group replaced on phenyl group, however, their mechanism of action is the same.

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Note: This article has been written based on the information found in the Scientific articles, Professional discussion forum, and considering different textbooks and internet information.

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