The different niobium sites in a Dion-Jacobson triple-layered perovskite, RbSr 2 Nb 3 O 10 , and its acid-exchanged version, HSr 2 Nb 3 O 10 , were investigated by using solid state NMR and DFT methods. 93 Nb electric field gradient (EFG) and chemical shift anisotropy (CSA) tensor values were extracted and site assignments made by using DFS, VOCS, MQMAS, and { 1 H}-93 Nb CP NMR techniques. The exterior niobium site exhibited a quadrupolar coupling (C Q ) of 45 MHz and a CSA span of 820 ppm while the interior site exhibited a reverse trend with a C Q of 93 MHz and a CSA span of 530 ppm. Both EFG and CSA tensors for the sites exhibited near axial symmetry although the CSA tensors had a greater deviation. The symmetry of the tensors is in conflict with the previously proposed structure for RbSr 2 Nb 3 O 10 and implies a lower symmetry space group and a possible tilting of the octahedra. Acid exchange altered the EFG tensors for both sites reducing their C Q values to 39 and 86 MHz, respectively, while not substantially changing the tensor symmetry. The effect on the CSA tensor occurred only for the surface site with the span reduced to 560 ppm. The effect of the tilting of the octahedra and alteration of bond angles and bond lengths on the EFG tensor was investigated through periodic DFT calculations. Interior and exterior sites of the rubidium form exhibited different tilt along the c-axis of 14°and -3°, respectively, based on the 93 Nb EFG values. Calculations on the acid-exchanged composition implied that structural alterations must occur with both sites. The exterior octahedra experience changes in both bond length and bond angles, while only a subtle elongation of the interior site is found.
Pauling and Corey proposed a pleated-sheet configuration, now called α-sheet, as one of the protein secondary structures in addition to α-helix and β-sheet. Recently, it has been suggested that α-sheet is a common feature of amyloidogenic intermediates. We have investigated the stability of anti-parallel β-sheet and two conformations of α-sheet in solution phase using the density functional theoretical method. The peptides are modeled as two-strand Acetyl-(Ala)2-N-methylamine. Using stages of geometry optimization and single point energy calculation at B3LYP/cc-pVTZ//B3LYP/6-31G* level and including zero-point energies, thermal, and entropic contribution, we have found that β-sheet is the most stable conformation, while the α-sheet proposed by Pauling and Corey has 13.6 kcal/mol higher free energy than the β-sheet. The α-sheet that resembles the structure observed in molecular dynamics simulations of amyloidogenic proteins at low pH becomes distorted after stages of geometry optimization in solution. Whether the α-sheets with longer chains would be increasingly favorable in water relative to the increase in internal energy of the chain needs further investigation. Different from the quantum mechanics results, AMBER parm94 force field gives small difference in solution phase energy between α-sheet and β-sheet. The predicted amide I IR spectra of α-sheet shows the main band at higher frequency than β-sheet.
Computational chemistry is a field of current active research in science that can supplement and complement the regular experimental methods and validate or propose new theoretical formulations. It has contributed from basic to advanced levels of investigation in a synergistic manner. The mathematical experiments can be pursued with minimal resources and are sustainable making it a good approach for institutions with limited resources. The freely available computer programs that are suitable for solving various types of problems in chemistry and interdisciplinary areas are briefly mentioned. The general features and specificity of some selected codes that are adaptable for undergraduate and graduate-level courses and research in Nepal are highlighted. A recommendation is made for the incorporation of adequate theoretical courses and parallel laboratory sessions in the existing syllabus for obtaining a trained and skilled workforce qualified for national and global careers in computational chemistry and also for research and development in Nepal.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.