Louis Pasteur, one of the pioneering stereochemists, recognized the omnipresence and the prominence of chirality and went on to state that the universe is chiral. Chirality bears very indispensible rationale in the everyday living of the universe (e.g., plants, animals). Any object lacking the elements of symmetry (i.e. plane, center, and axis of symmetry and helicity) and existing in two forms that are non-superimposable mirror images of each other is referred to as a chiral object.
Chirality in bioactive molecules
Chirality is the rule rather than the exception in biological systems, controlling the utilization of nutrients, influencing the functioning of genes and enzymes, and regulating interactions at receptor sites. For instance, the human body can only utilize (-)-amino acids but not the unnatural (+)-enantiomers. Also, humans can metabolize only the (+)-glucose and not (-)-glucose. (+)-leucine tastes sweet while (-)-leucine is bitter. Aroma characteristics are highly dependent on chirality. Caravone the (+)-form has carway smell while the (-)-form has spearmint smell. R-Limonene smells orange while its mirror image twin smells lemon. Table 1 presents selected examples agrochemicals that exhibit enantioslectivity in their biological effects.
Knowingly or unknowingly we are consuming many natural products as a medicine or as a dietary supplement which are benefiting us with sound health and life style. Natural products and their synthetic derivatives are highly used in the treatment of cancer. In a recent analysis of the antineoplastic agents marketed in western countries and Japan revealed that of 155 compounds in total ever since introduced since the 1940s, 47.1 per cent were either unmodified natural products among that (25 compounds, 16.1 per cent) or semisynthetic derivatives of natural products (48 compounds, 38.1 per cent). Table 2 lists selected chiral drugs that have been derived from natural products which have played a key role in the treatment of cancer.
Thalidomide is considered to be the most noteworthy example of differential physiological activity between isomers is the drug, with the (R)-enantiomer having sedative properties, whereas the (S)-enantiomer is teratogenic. Thalidomide was synthesized as a racemic mixture, but purification to obtain only the (R)-enantiomer was not a viable solution as a liver enzyme converts the R-enantiomer into S-enantiomer. In recent years, there has been increased interest in the administration of chiral drugs as the single enantiomer rather than the racemic mixture.
Enantioseparation- a case study
In this context, it is important to take measures to establish the optical purity of the compound as to establish its compositional purity. In the absence of such proof, the biological activity data will remain compromised. Generally plants produce one enantiomer. However, during maturation, storage, or processing racemization takes place and the amount of enantiomers can be changed. Enantiomers produce various physiological, pharmacological, and toxicological effects. Due to these effects, the determination of absolute configuration seems to be necessary. NMR, MS, CD, optical rotation, and X-ray crystallography have been used to solve this problem. However, chiral HPLC combined with these classical methods may be the method of choice for determination of absolute configuration. Advantages of this method are the short analysis time, wide availability of chiral columns, and effectiveness. The establishing of the absolute configuration of unknown compounds can be done in one step from the comparison of the retention time of the investigated solute with the reference enantiomer whose absolute configuration is well known, but further confirmation of the absolute configuration with the help of Mass Spectra or Circular Dichroism is necessary.
One of the classical example is Citrus aurantium (bitter orange) is a plant belonging to the family Rutaceae. The most important biologically active copstituents of the C. aurantium fruits are phenetylamine alkaloids (i.e., octopamine, synephrine, tyramine, N-methyltyramine and hordenine). Synephrine is present in the peel and the edible part of Citrus fruit. Of the adrenergic amines of natural origin, synephrine has been found to be the main constituent of C. aurantium fruits and extracts; the other alkaloids are either absent or present in only low concentrations. It is known that synephrine and the other amines found in C. aurantium have adverse effects on the cardiovascular system, owing to adrenergic stimulation. Synephrine is cited as ‘the active component’ of plants and dietary supplements used in weight loss.
C. aurantium is listed in its Poisonous Plants Database; however, C. aurantium orange oil extract, peel, flowers, and leaf are listed in its food additive database, an inventory often referred to as "Everything" Added to Food in the United States (EAFUS) (FDA CFSAN, 1996, 2003). It became one of the most popular stimulants present in weight-loss products after the US Food and Drug Administration had interdicted the use of ephedrine-containing dietary supplements. Synephrine is a primary synthesis compound with pharmacological activities such as vasoconstriction, elevation of blood pressure and relaxation of bronchial muscle.
Synephrine exists in three different positional isomers; however, only p- and m-synephrine have been described in weight-loss products and each positional isomer have two enantiomers. Each positional isomer can also be found in two enantiomeric forms, as a result of the presence of a chiral carbon.
Only the R-(-) form of synephrine is present in fruits and this enantiomeric form is described as more active than the S-(+) form. Notwithstanding, it is important to notice that extractive procedures employing high temperatures or prolonged heating periods can promote optical isomerization of synephrine and can give fallacious results. S-(+)-p-synephrine appeared to possess more antidepressant-like activity than R-(-)-p-synephrine. And more investigations are on the pipeline
Pellati et.al (2007) developed a method an enantioselective LC method with photodiode array detection (PDA) for the enantioseparation of (±)-synephrine from C. aurantium L. var. amara by using a protein-based chiral stationary phase with cellobiohydrolase as the chiral selector (Chiral-CBH) with a mobile phase consisting of 2-propanol (5%, w/w) in sodium phosphate buffer (pH 6.0; 10 mM) and disodium EDTA (50 mM) with a flow rate of 0.8 mL/min and detection at 225 nm. The corresponding chromatogram is given below (Fig. 1).
By using the developed method Fellati et.al estimated the amount of Synephrine present in C. aurantium fruits, dry extract and dietary supplement.
Conclusion
Mother nature is not even handed. Chirality is a key characteristic of natural products and stereochemistry is often crucial for biological activity. I call upon academia to provide more space for chirality in all the deliberations on natural products research. Further while exploring bioactive molecules of natural origin it is important to have a chiral perspective.
(Valliappan Kannappan is Professor of Quality Assurance and Sai Sandeeep M is Research Scholar, Department of Pharmacy, Annamalai university)