What Is Chiral Purity And How To Perform: Learn In 3 Minutes

Chiral Purity quantifies the proportion of one enantiomer in a mixture, with higher chiral purity indicating a greater concentration of the desired isomer. In pharmacology, for example, one enantiomer of a drug may be therapeutic, while the other could be harmful or inactive. Therefore, determining the chiral purity is critical to ensure the correct enantiomer is present in the desired amount.

What is Chiral purity?

Chiral purity is the percentage of a specific desired enantiomer present in a chiral substance, relative to its mirror image or other enantiomer. It measures how much of the desired enantiomer is present relative to the other in a sample mixture.

Chiral purity is calculated by the following using GC or HPLC:

It quantifies the proportion of one enantiomer in a mixture, with higher chiral purity indicating a greater concentration of the desired isomer. This concept is crucial in fields like pharmaceuticals, where the efficacy and safety of a particular enantiomer may differ significantly from its counterpart.

Expert Tip: Chiral Purity

Chiral purity 100% means the sample is composed entirely of one enantiomer, and 0% indicates a racemic mixture (equal amounts of both enantiomers).

Why is Chiral Purity Important?

1. Pharmaceuticals: Many drugs are chiral, and their enantiomers can have vastly different effects. One enantiomer might be active and beneficial, while the other could be toxic or have no effect. For instance, in drugs like thalidomide, one enantiomer was helpful for treating nausea, while the other caused birth defects.
2. Food and Chemical Industries: Chiral purity ensures that the desired flavour, aroma, or chemical reaction is achieved.
3. Environmental Concerns: Some enantiomers might be more persistent or toxic in the environment, so controlling chiral purity is important.

How to Perform Chiral Purity Analysis?

The following analytical techniques are used to perform Chiral purity:

1. Chiral Chromatography (HPLC or GC)
2. Polarimetry
3. NMR Spectroscopy
4. X-ray Crystallography
5. Chiral Reagent Derivatisation

1. Chiral Chromatography (HPLC or GC):
   • High-Performance Liquid Chromatography (HPLC) or Gas Chromatography (GC) with a chiral stationary phase are used to separate enantiomers.A chiral column will allow the two enantiomers to be separated based on their different interactions with the stationary phase.The separation can then be quantified by integrating the peak areas corresponding to each enantiomer.
   Steps:
   • Prepare a sample for analysis.
   • Choose a suitable chiral column (based on the nature of the compounds).
   • Inject the sample into the chromatograph.
   • Analyse the chromatogram to calculate the relative proportion of each enantiomer.
2. Polarimetry:
   • This technique measures the optical rotation of chiral substances. Each enantiomer rotates polarized light in a different direction—one to the left (levorotatory) and one to the right (dextrorotatory).
   • If you know the specific rotation of pure enantiomers and the concentration of your sample, you can use polarimetry to determine chiral purity.
   Steps:
   • Prepare a solution of your sample.
   • Measure the optical rotation using a polarimeter.
   • Compare the observed rotation with that of a known pure enantiomer to determine the purity.
3. NMR Spectroscopy:
   • Nuclear Magnetic Resonance (NMR) can be used to analyse chiral compounds, especially using chiral solvents or derivatisation techniques. In some cases, chiral NMR shifts can give insight into the purity of the compound.
4. X-ray Crystallography:
   • If the compound crystallises, X-ray diffraction can be used to determine the structure of each enantiomer and quantify their relative amounts.
5. Chiral Reagent Derivatisation:
   • This method involves reacting the sample with a chiral reagent that forms a distinguishable product with each enantiomer. The resulting products are then analysed (often via HPLC or GC) to determine the enantiomeric excess.

Steps for Measuring Chiral Purity using HPLC:

1. Prepare the Sample:
   • Dissolve your compound in an appropriate solvent (like methanol or acetonitrile) to make a sample solution.
2. Select a Chiral Column:
   • Choose a column with a chiral stationary phase. There are various types of chiral columns, such as cyclodextrin-based, protein-based, or a simple chiral selector on silica.
3. Chromatographic Conditions:
   • Set up the mobile phase (solvent mixture) and flow rate based on the compound you’re analyzing.
   • Optimise the temperature and detection wavelength (typically UV) for maximum sensitivity.
4. Inject and Analyse:
   • Inject a known amount of your sample into the HPLC system.
   • Use the chromatogram to identify the elution times of the enantiomers.
5. Calculate Chiral Purity:
   • Calculate the area under the peaks corresponding to each enantiomer.
   • The chiral purity can be calculated as:

Factors That Can Affect Chiral Purity Results:

• Temperature: Affects the retention times and efficiency of separation in chromatography.
• Solvent: Some solvents may have interactions with the chiral molecules, affecting separation.
• Detection Method: Ensure that the detector is sensitive enough to detect both enantiomers accurately.

Conclusion:

To assess the chiral purity of a compound, you can use various methods such as chiral chromatography (HPLC or GC), polarimetry, NMR spectroscopy, or X-ray crystallography. Out of these techniques, chiral chromatography is widely used in the pharmaceutical industry

Related

• Relative Response Factor (RRF) in Pharmaceutical Analysis

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Further Reading

• Chiral purity