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The present work focuses on the structural properties, spectroscopic signatures, intermolecular hydrogen bonding interactions, chemical and biological activity of nicotinamide (NIC) based on its monomeric and trimeric models using density functional theory and vibrational spectroscopy. FT-IR and FT-Raman spectra were obtained using the double-side forward-backward acquisition mode under vacuum. UV-Vis absorption spectra were recorded in methanol and compared with the calculated values. Geometry optimization and vibrational wavenumbers were obtained with the aid of Gaussian 09 program packages. The structural analysis of NIC revealed that the trimeric model was in better agreement with the experimental values than the monomer due to the incorporation of nearest hydrogen bond interactions. Spectroscopic results showed that NH2 and C = O groups of NIC were involved in intermolecular interactions in the trimeric model. The natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses determined the presence, strength as well as nature of the hydrogen bonds were partially covalent. The lesser value of the HOMO-LUMO energy gap for the trimeric model indicated higher reactivity than monomer. Moreover, chemical reactivity was calculated using molecular electrostatic potential surface (MESP) and reactivity descriptors. The docking studies for NIC with several targets explored its biological activity. [ABSTRACT FROM AUTHOR]

Ezetimibe (EZT) being an anticholesterol drug is frequently used for the reduction of elevated blood cholesterol levels. With the purpose of improving the physicochemical properties of EZT, in the present study, cocrystals of ezetimibe with L-proline have been studied. Theoretical geometry optimization of EZT-L-proline cocrystal, energies, and structure-activity relationship was carried out at the DFT level of theory using B3LYP functional complemented by 6-311++G(d,p) basis set. To better understand the role of hydrogen bonding, two different models (EZT + L-proline and EZT + 2L-proline) of EZT-L-proline cocrystal were studied. Spectral techniques (FTIR and FT-Raman) combined with quantum chemical methodologies were successfully implemented for the detailed vibrational assignment of fundamental modes. It is a zwitterionic cocrystal hydrogen bonded with the OH group of EZT and the COO - group of L-proline. The existence and strength of hydrogen bonds were examined by a natural bond orbital analysis (NBO) supported by the quantum theory of atoms in molecule (QTAIM). Chemical reactivity was reflected by the HOMO-LUMO analysis. A smaller energy gap in the cocrystal in comparison to API shows that a cocrystal is softer and chemically more reactive. MEPS and Fukui functions revealed the reactive sites of cocrystals. The calculated binding energy of the cocrystal from counterpoise method was -11.44 kcal/mol (EZT + L-proline) and -26.19 kcal/mol (EZT + 2L-proline). The comparative study between EZT-L-proline and EZT suggest that cocrystals can be better used as an alternative to comprehend the effect of hydrogen bonding in biomolecules and enhance the pharmacological properties of active pharmaceutical ingredients (APIs)., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Prajapati, Pandey, Tandon, Sinha and Shimpi.)