NMR based Studies and Applications of Molecular Interactions : From Small Moleculecules to Bio-nanoconjugates
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The work described in this thesis involves the study of weak interactions by NMR spectroscopy and using them to develop novel applications. The two different applications chosen are i) using molecular interactions for chiral discrimination and ii) understanding the nature of the interaction between peptide and nanoparticles to develop potent antibacterial agents. The thesis, which is divided into five chapters starts with a general introduction of NMR spectroscopy for the study of molecular interactions in conjunction with other techniques. The remaining four chapters focus on four different areas/projects that I have worked on. Chapter 1: Introduction This chapter reviews different kinds of molecular interactions along with the introduction to NMR spectroscopy and other techniques used for all the studies. Starting with the application of chiral discrimination the chapter proceeds to the general introduction of antimicrobial peptides, silver nanoparticles and the strategy for peptide resonance assignment. Chapter 2: Chiral discrimination for versatile functionalities There are many chiral agents available for discriminating enantiomers which mainly target specific functional groups. In this study, we have explored a strategy involving ternary complexation to investigate chiral discrimination of different kind of functional groups by NMR spectroscopy. The proposed protocol was employed for the enantiodiscrimination of molecules containing functional groups, such as amino alcohols, secondary alcohols, cyanohydrins, oxazolidones, diols, thiones and epoxides, using a phosphorous based three component mixture. The simple mixing and shaking of enantiopure 1,1’-binaphthyl-2,2’-diyl hydrogenphosphate (BNPA), 4-(dimethylamino)pyridine (DMAP) and a chiral analyte in the solvent CDCl3 served as a chiral solvating agent and resulted in well-dispersed peaks for each enantiomer in the 1H NMR spectrum. Discrimination was achieved not only for the proton at the chiral center but also for multiple proton sites. The J-resolved technique was used for alleviating the spectral complexity pattern to accurately measure the chemical shift difference. The devised approach also permitted the precise measurement of the enantiomeric excess (ee). Chapter 3: Simultaneous discrimination of secondary alcohols and carboxylic acids In this chapter, I describe two novel ternary ion-pair complexes, which serve as chiral solvating agents (CSA), for enantio discrimination of secondary alcohols and carboxylic acids. The superiority of CSA over other auxiliaries arises due to the formation of diastereomeric complexes through non-covalent interactions with the analyte. By exploiting the acid-base interaction strategy and employing DMAP, which further enhanced the hydrogen bonding efficiency the discrimination for both carboxylic acids and secondary alcohols were achieved. The protocol for discrimination of secondary alcohols is designed by using one equivalent mixture each of enantiopure mandelic acid, 4-dimethylaminopyridine (DMAP) and a chiral alcohol. For discrimination of carboxylic acids, the ternary complex is obtained by one equivalent mixture each of enantiopure chiral alcohol, DMAP, and a carboxylic acid. Furthermore, the formation of the complex was supported by calculating the energy-minimized structure of the proposed complex by density functional theory (DFT). The designed protocols also permit accurate measurement of the enantiomeric composition. Chapter 4: Enhanced potency of nanoparticle-antimicrobial peptide conjugates Antibiotic resistance is emerging as the new global health problem. Due to the blatant misuse and overuse of these drugs has resulted in the bacteria becoming resistant to a wide range of antibiotics. Researchers have found an alternative of current antibiotics which are a group of peptides known as antimicrobial peptides (AMP). But using these molecules as drug is rather costly due to high synthesis cost. Further the antibacterial activity of silver nanoparticle is well established. However, due to its toxic nature after, it cannot be used in high concentration. The conjugation of nanoparticles with antimicrobial peptides is emerging as a promising route to achieve superior anti-microbial activity. However, the nature of peptide-nanoparticle interactions in these systems remains unclear. This study describes the interactions of antimicrobial peptide with silver nanoparticles by NMR spectroscopy in conjunction with other biophysical techniques to completely understand the underlying mechanism of interaction between nanoparticles and peptide. It reveals that the conjugation process involves dynamic interaction between the nanoparticle and the peptide. This study also confirms the enhanced antibacterial efficiency of the nano-conjugate towards bacterial killing compared to the nanoparticle or the peptide alone. Chapter 5: Mechanistic insights into the action of nano-conjugates It is well established that antimicrobial peptides act as pore-formers to rupture the bacterial cells. This chapter is focused on studying the mechanism of action of the nano-conjugate with bacterial membrane mimic models. This study for the first time reveals the details of nanoconjugate membrane interaction at an atomic level. The pore formation mechanism and the enhanced efficiency of the nanoconjugate were explored using fluorescence spectroscopy, CD spectroscopy, and NMR spectroscopy. Structural changes of the peptide and the nanoparticle bound peptide have been captured which infers the propensity of the peptide to form a helical structure upon interacting with the membrane. The calculated structure of the peptide and nanoparticle bound peptide remains almost identical in presence of the membrane mimic environment. In the case of the nanoconjugate, the increase in local positive charge concentration makes the system to penetrate the bacterial membrane faster which further allows the nanoparticle to access the intercellular organelles easily. This dual mode of mechanism thus makes this nano-conjugate a promising antibacterial agent towards multi drug resistant bacteria. In summary, the thesis has focused on the studies of weak intermolecular interactions in different chemical and biological systems using NMR spectroscopy. It is demonstrated that in certain chemical systems, such interactions can be exploited to discriminate enantiomers and determine the enantiopurity of compounds by NMR. In the case of biomolecules, such weak interactions exist when protein or peptides interact with nanoparticles. Using silver nanoparticles, it is shown that such interactions result in a stable conjugate system. NMR spectroscopy provides valuable insights into the structure and dynamics of the system. Further, by using anti-microbial peptides conjugated with silver nanoparticles, new superior antibacterial agents can be developed.