Probing the role of intermolecular interactions in the model aqueous biological media by linear and nonlinear vibrational spectroscopy

Abstract

Important aspects of water behavior occur at interfaces as well as in the bulk media. The chemical, physical and biological behaviors of water media are different at the interfacial regions and in the bulk aqueous systems. The study of these properties in the biologically relevant interfaces and in the bulk of aqueous solutions grabbed substantial attention due to the various applications in science, technology, medicines, and industries. The challenge is to understand the behavior and the impact of any guest molecules in the pristine aqueous system in the bulk as well as at the interfacial region. The molecular structure and behavior of substance or molecules present in the bulk and at the aqueous interface are very much influenced by the presence of various inter- and intramolecular interactions present in the respective medium. Therefore, extracting better insights about these interactions would provide important information to improve the current understanding of various model biological aqueous media. The objective of this thesis is to explore various aspects of inter- and intramolecular interactions and probe their role for different model biological aqueous systems. We have used linear and nonlinear vibrational spectroscopic tool to probe the model aqueous systems. The attenuated total reflectance Fourier transform infrared (ATR-FTIR) technique based on linear optical process has been used to probe the bulk properties of the aqueous systems. The sum frequency generation vibrational spectroscopy (SFG-VS) based on second order nonlinear optical process has been employed to probe the interfacial structure of the aqueous system. We have studied the self-assembly process of L-phenylalanine (Phe) amino acid in the aqueous as well as in the solid phase. The motivation behind the present study was to probe the role of various intermolecular interactions which play an imperative role in the formation and control of various morphological nanostructures during the self assembly of L-Phe. Recently, nanostructured morphology originated from the selfassembly process of molecules has attracted substantial attention due to its role in the toxic fibril formation and its immense potential in the design and fabrication of novel biomaterials for biomedical applications. To the best of our knowledge, for the first time, we have reported the impact of the electrostatic induced effect on self-assembly process of a single amino acid by exploiting the existence of various ionization states of the amino acid. By tuning the pH of the aqueous solution, we were able to observe the considerable role of intermolecular electrostatic interaction over the anticipated hydrophobic π-π stacking interaction. The finding of our study demonstrates the significant role of intermolecular interactions for the formation of various morphological nanostructures during the self-assembly process of small molecules. Polymers play an imperative role in the enhancement of aqueous solubility of drugs through maintenance of solution state supersaturation for a considerable time of absorption. It is important to select suitable polymers in drug solubilization rate enhancement thereby improving the dissolution and bioavailability of a drug. The drugpolymer interaction in the aqueous medium plays an important role towards enhancing the rate of solubilization of a drug. In the present study, we have employed sum frequency generation (SFG) vibrational spectroscopy to probe the drug polymer interactions at simulated air/gastric fluid or biorelevant medium (BM) interface. The model drug and polymers employed are atorvastatin calcium (ATC) and Eudragit EPO (E-EPO) and polyvinylpyrrolidone K30 (P-K30), respectively. The SFG spectroscopic findings reveal dramatic changes in the water structure to reveal the role of hydrogen bonding network for the drug-polymer-aqueous interactions at the air/BM interface. For the case of ATC drug and E-EPO polymer combination, we find the clear evidence of enhanced orientational ordering with significant increase in the population of strongly hydrogen-bonded water molecules. In contrast, drug with the P-K30 polymer combination supports enhanced water ordering with dominant signature from the weakly hydrogenbonded water molecules in the OH-stretch region. From the solubility study, we have witnessed an enhancement of solubility rate of ATC,  42 times with the combination of E-EPO and merely  2.6 times with the presence of P-K30. Our finding suggests that the solvation environment of the polymer provides the adequate platform for hydrogen bonding network in the drug-polymer-BM aqueous solution. We strongly believe that the enhanced strongly hydrogen bonding environment may be responsible for increasing the solubility rate of the drug in the BM-polymer aqueous solution. The current observation introduces a new method for probing drug-polymer interactions, which may help in the selection of polymers in drug formulation development. The study of aqueous binary mixtures provides a platform to extract better molecular-level insights about various fundamental molecular and biological processes and their properties like hydrophobic hydration, solubility, protein folding, micelle formation and aggregation, and lipid bilayer formation. In the present study, we have probed the bulk and interfacial structure of N, N-dimethylformamide (DMF) solvent and its impact on the hydrogen bonding environment of the water molecules in the binary mixtures of DMF-water. DMF is a polar and hydrophilic aprotic solvent used for peptide coupling in pharmaceuticals and various model biological systems. The bulk study of aqueous binary mixture of DMF and water is performed by employing ATR-FTIR vibrational spectroscopy. We used SFG-VS for investigating interfacial arrangements, orientation order, and molecular conformations of DMF and water molecules at the air/aqueous interface. We have investigated the interfacial and bulk molecular structure of the binary mixture in the CH- and OH- stretch region by varying the molar concentration of DMF. From the bulk studies, we observed that the strength of the OH-stretch modes decreases with the increase in the presence of DMF molecules in the binary mixture. In addition, there is also a continuous blue shift towards the higher frequency seen in the OH-stretch region with the increase in DMF molar concentration. In contrast, no significant shift in the OH-stretch region is noticed from the SFG spectra collected from the air/binary mixture interface as a function of DMF concentration. However, the impact of DMF is found to be very disruptive in nature towards the hydrogen bonding network which is existing at the interfacial region for the binary mixture. We have quantitatively calculated the molecular tilt angle of the methyl group as a function of DMF concentration. For the case of neat DMF the observed tilt angle is 17.7° and the value decreases to 1.7° for 0.5 mole fraction and then value increase with further decrease in DMF concentration. The tilt angle reaches to 20° for the dilute DMF concentration of 0.05 mole fraction. This indicates that the methyl groups of DMF molecules at the air/binary mixture interface are more ordered and putting the methyl groups of the DMF molecule towards air for the intermediate DMF concentrations.