Study of Enantiomeric Discrimination and Enzyme Kinetics using NMR Spectroscopy

Abstract

Obtaining enantio pure drug molecules is a long standing challenge in asymmetric synthesis implying that the identification of enantiomers and the determination of enantiomeric purity from a racemic mixture are of profound importance. In achieving this target NMR spectroscopy has proven to be an excellent analytical tool. It is well known that normal achiral NMR solvents do not distinguish the spectra of enantiomers. On the other hand, the conversion of substrates to diastereomers using one of the enantiopure chiral auxiliaries, such as, chiral solvating agent, chiral derivatizing agent and chiral lanthanide shift reagent, circumvents this problem. The imposition of diasteomeric interactions circumvents this problem. There is a pool of chiral auxiliaries available in the literature, each of which is specific to molecules of certain functionalities and has its own advantages and limitations. These classical methods have two limitations as they demand the presence of a targeted functional group in the chiral molecule and utilize only chemical shifts to visualize enantiomers. On the other hand in chiral anisotropic medium, due to differential ordering effect, the order-sensitive NMR observables, viz. chemical shift anisotropies (∆σi), dipolar couplings (Dij) and for nuclei with spin >1/2 the quadrupolar couplings (Qi) have enormous power of exhibiting different spectrum for each enantiomer permitting their discrimination. Numerous weakly ordered chiral aligning media have been reported in the literature. Nevertheless there is a scarcity of water compatible medium. Research work presented in this thesis is focused on various aspects, such as, the discovery of new chiral aligning medium for the enantiodiscrimination of water soluble chiral molecules, potential utility of DNA liquid crystal for discrimination of amino acids, on-the-fly monitoring of enzyme kinetics and the preparation of novel composite liquid crystals, hydrogels and thin films. The derived results are discussed in different chapters. Chapter 1 provides a brief introduction to NMR spectroscopy with special emphasis on the conceptual understanding of the tensorial interaction parameters, such as chemical shifts, scalar and dipolar couplings, quadrupolar couplings, effect of r.f pulses, basic introduction to 2D NMR experiments. Subsequently, a broad overview of the enantiomers, specification of their configurations, chirality without stereogenic carbon, chirality in molecules containing different atoms, are discussed. Following this a brief introduction to liquid crystals and their properties, their classification, their orientation in the magnetic field, order parameter are also discussed. The description on the chiral liquid crystals, the differential ordering effect, employment of the orientation dependent NMR interactions, utility of 2H NMR experiments for the visualization of enantiomers and the measurement of enantiomeric composition has been set out in brief. Chapter 2: As far as the organo soluble chiral molecules is concerned (in solvents such as, chloroform, dioxane, tetrahydrofuran and dimethylformamide), it has been well established that an ideal choice of chiral liquid crystal for enantiodiscrimination is poly-�-benzyl-L-glutamate (PBLG). Nevertheless, there is a scarcity of weak aligning medium for water soluble chiral molecules. This chapter introduces the chiral liquid crystal derived from the polysaccharide xanthan, which has numerous applications. The detailed discussion on the preparation of polysaccharide xanthan mesophase is given. The appearance of the mesophse is established by detecting the quadrupole split doublet of dissolved water. Subsequently enantiodiscrimination power of this new medium has been investigated on deuterated D/L-Alanine and (R/S)-β-butyrolactone. For such a purpose the selective 2D-SERF (SElective ReFocussing) experiment has been employed. It has been convincingly demonstrated that the medium has wide applicability for the discrimination of enantiomers, enantiotopic directions in prochiral molecules, measurement of enantiomeric excess and the RDCs in medium sized molecules. The new medium is sustainable over a wide range of temperature and concentration of ingredients, the mesophase is reversible, reproducible, easy to prepare besides being cost effective. It is possible to have the controlled tuning of the degree of order for specific application. Chapter 3: In this chapter the real discriminatory potential of DNA liquid crystalline phase has been explored. It is unambiguously established that; i) the fragmented DNA liquid crystal is able to differentiate between enantiomers of structurally different chiral amino acids; ii) the T1 (2H) values for L/D (alanine) is nearly equal indicating the similar dynamics for both the enantiomers, thus permitting the measurement of ee from the integral areas of the peaks of the contours of 2D spectrum; iii) the enantiotopic discrimination in prochiral compounds has also been successfully explored. Furthermore the analyses of NMR results yielded fruitful information on the analytical potential of DNA chiral liquid crystal, such as, (a) the chiral discrimination is effective on a large range of amino acids with spectral differences ΔΔʋQ‘s and ΔʋQ‘s varying from 80 to 338 Hz, and 50 to 900 Hz respectively; (b) the discrimination phenomenon remain active irrespective of the structure and the electronic nature (polarity) of the fourth substituent around the stereogenic center; (c) compared to an alkyl moiety, the presence of a terminal –OH or –SH group seems to slightly increase both the degree of alignment of the solute and the enantiodiscrimination efficiency compared to alanine; (d) The enantiodiscrimination can be detected easily not only on CD3 and CD groups, but also on CD2 sites exhibiting inequivalent diastereotopic directions; (e) discriminations with rather large differential ordering effect were obtained even for the sites that are situated far away from the asymmetric center; (f) The relative position of quadrupolar doublets from one 2H site to another can be reversed with regard to the absolute configuration (L/D). Chapter 4: Racemases recognize a chiral substrate such as (L-Alanine) and convert it into its enantiomer, i.e., (D-Alanine) and vice versa. Alanine racemase plays a vital role for certain bacteria, providing D-Alanine for peptidoglycan cell-wall biosynthesis. Elucidating the mechanism of enzymatic racemization is crucial for designing new inhibitors that may be useful as a novel class of antibiotics. This requires techniques to discriminate L-and D-Alanine and follow their concentrations as a function of time, so that one can determine the kinetic parameters and study the effect of inhibitors. In this chapter the utility of DNA liquid crystal media for in situ and real-time monitoring of the interconversion of L-and D-alanine-d3 by alanine racemase from Bacillus stearothermophilus has been demonstrated. The enantiomeric excess has been measured at different time intervals to monitor the enzymatic racemization at different time intervals in pseudo 2D NMR. The study unambiguously ascertains the reliability and robustness of utility of NMR in chiral anisotropic phase for monitoring the enzymatic racemization. The method thus provides new mechanistic insight and a better understanding of enzymatic reactions, in particular for alanine racemase. Chapter 5: In continuation with the development of weakly ordered liquid crystals, this chapter reports the spontaneous formation of composite graphene oxide (GO)/double stranded DNA (dsDNA) liquid crystals at higher concentrations of ingredients, and hydrogels at lower concentrations of ingredients, the process of which involves simple mixing in an aqueous phase has been demonstrated. The liquid crystalline phases and hydrogels have been characterized using optical polarized microscopy (OPM), scanning electron microscopy (SEM), Raman spectroscopy and 2H NMR spectroscopy. The observation of strong birefringence in the optical polarized microscope gives evidence for the formation of GO/dsDNA liquid crystals. The strong interaction between the dsDNA and GO was confirmed using Raman spectroscopic analysis. Furthermore, GO/dsDNA thin films have also been prepared and characterized using SEM and OPM. The GO/dsDNA thin film was prepared and its liquid crystal nature was established using OPM and 2H NMR. Importantly, the GO/dsDNA hydrogels were formed without any heat treatment to unwind dsDNA molecules and the porosity of hydrogels can be controlled by changing concentration of the dsDNA. This novel multifunctional composite liquid crystals and hydrogels of GO/dsDNA thus opens up new avenues for many applications like security papers, optical devices such as circular polarizers, reflective displays and drug delivery as well as tissue engineering using GO composite hydrogels.