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Chiral Recognition by NMR Spectroscopy- A Theoretical approach.
Illustrating modeling strategies with organic molecules
with overtones for Modeling of Bio-molecules
Department of Chemistry
North Eastern Hill University
SHILLONG 793022 Meghalaya
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Chirality and the Enantiomeric excesses have been playing important role in determining the consequence of events in the biological processes. Hence, the topic of chiral discrimination has acquired importance and NMR spectroscopic methods are evolving to resolve several of the issues associated with chiral discrimination. The NMR chemical shift parameter is one of the important variant indicative of the chirality in the systems and NMR spectroscopic techniques based Multidimensional Pulsed FT NMR experiments are currently increasing in number. Chemical complexing and the study of the resulting complex by NMR is one of the chemical spectroscopic approaches. The number of theoretical calculations on such complexes and the pros and cons of the various theoretical methods known for calculating chemical shifts have not been well known and such studies can indicate the actual systems to choose for complexing, even if the calculated chemical shift does not reproduce actually measured spectra in the few known cases. Thus, a situation arises to set out a premise for molecular modeling and the considerations seem to be revealing trends, also on the outlook for molecular modeling, in general and the inferences do not confine only to small molecules for its validity.
To be more specific, Derivatives of crown ethers have been used as host molecules for complexing the guest chiral molecules and the typical crown ether molecule prevalent is not a small molecule, but can have as many as 40 to 60 atoms with H, O and C as constituents. It is possible to find systems to handle this size for chemical shift calculations and for the complexes. However more details are to be documented on the mechanisms inherent in these complex formations for the different enantiomers, and the chemical binding that is important different from any other weak interaction which sensitively alters the NMR spectrum. It is necessary 1) to choose appropriate combination of Host and Guest systems simple enough to make the crucial factors evident, and 2) to choose the theoretical methods (Quantum Mechanical versus Molecular mechanics) to minimize the computational effort and obtain a vivid picture of the details. This approach enables critically choose a system for an experiment to be effective. These are the strategies typically for Modeling biological systems and it has been possible to some extent obtain the required indications.
Starting with the large molecules of the type as is known from the publications till now (1 & 2) logically it has been possible to evolve smaller systems of guest and host which can result in calculable NMR differentiations. These calculations can typically be of two minutes to five minutes of calculation time for gradual cumulative inferences on the chiral discrimination. These modeling strategies reveal significant information to support the till now known approaches of complexing and suggest required alterations when the target molecules in question vary only slightly from one another.
1,. Chiral NMR Discrimination of Secondary Amines Using (18-Crown-6)-2, 3,11,12-tetracarboxylic Acid,A.E.Lovely and Thomas J. Wenzel, Organic Letters, 2006, Vol.8, No.13, pages 2823-2826.