Conformational isomerism

About: Conformational isomerism is a research topic. Over the lifetime, 11563 publications have been published within this topic receiving 199312 citations.

Structure of isolated tryptophyl-glycine dipeptide and tryptophyl-glycyl-glycine tripeptide: ab initio SCC-DFTB-D molecular dynamics simulations and high-level correlated ab initio quantum chemical calculations.

Abstract: The tryptophyl-glycine (Trp-Gly) and tryptophyl-glycyl-glycine (Trp-Gly-Gly) peptides have been studied by means of molecular dynamic simulations combined with high-level correlated ab initio quantum chemical and statistical thermodynamic calculations. The lowest energy conformers were localized in the free energy surface. The structures of the different Trp-Gly and Trp-Gly-Gly conformers coexisting in the gas phase have been for the first time reported and their scaled theoretical IR spectra unambiguously assigned and compared with previous gas-phase experimental results. Common geometrical features have been systematically observed for the sequence Trp, Trp-Gly, and Trp-Gly-Gly. In addition, the peptide backbone of Trp-Gly-Gly has been compared with that of the previously studied Phe-Gly-Gly (Reha, D. et. al. Chem. Eur. J. 2005, 11, 6803). From the observed systematic structural behavior between these peptide analogues, it is expected that the gas-phase conformers of other similar aromatic small peptides would present equivalent geometries. The DFT methodology failed to describe the potential energy surface of the studied peptides since the London dispersion energy (not covered in DFT) plays a significant role in the stabilization of most stable conformers.

A Microwave and Quantum Chemical Study of the Conformational Properties and Intramolecular Hydrogen Bonding of 1-Fluorocyclopropanecarboxylic Acid

Abstract: The structural and conformational properties of 1-fluorocyclopropanecarboxylic acid have been explored by microwave spectroscopy and a series of ab initio (MP2/6-311++G(d,p) level), density functional theory (B3LYP/aug-cc-pVTZ level), and G3 quantum chemical calculations. Four “stable” conformers, denoted conformers I−IV, were found in the quantum chemical calculations, three of which (conformers I −III) were predicted to be low-energy forms. Conformer I was in all the quantum chemical calculations predicted to have the lowest energy, conformer III to have the second lowest energy, and conformer II to have the third lowest energy. Conformers II and III were calculated to have relatively large dipole moments, while conformer I was predicted to have a small dipole moment. The microwave spectrum was investigated in the 18−62 GHz spectral range. The microwave spectra of conformers II and III were assigned. Conformer I was not assigned presumably because its dipole moment is comparatively small. Conformer II i...

Conformational preferences of 1,4,7-trithiacyclononane: a molecular mechanics and density functional theory study

Abstract: Conformational preferences of 1,4,7-trithiacyclononane were studied using a highly efficient sampling technique based on local nonstochastic deformations and the MM2(91) force field. The results show that conformers that the molecule adopts in the crystal state were found to be low-energy conformers (LECs) within 5 kcal mol−1 of the global minimum. A conformation with C1 symmetry was the global minimum and the C3 and C2 conformations were calculated to be 0.03 and 1.78 kcal mol−1 higher in energy, respectively. The structures were further minimized using Density Functional Theory (DFT) calculations with two different functionals. The C2 and the C1 conformations were found to be LECs with the C3 conformation more than 4.0 kcal mol-1 above the global minimum. The relative energies and structural ordering obtained using the BP86 functional are in agreement with the previously reported relative energies calculated using second-order Moller–Plesset (MP2) ab initio calculations. With the energy ordering being dependent on the molecular mechanics force field used, the approach of MM→DFT (searching exhaustively the available conformational space at the MM level followed by generating the energy ordering through DFT calculations) appears to be appropriate for thiacrown ethers.