Conformational isomerism

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

Submolecular Observation of Photosensitive Macrocycles and Their Isomerization Effects on Host−Guest Network

Abstract: The macrocyclic compounds consisting of photosensitive units as parts of the frame have been extensively studied to mimic photoregulated functions in nature. In this paper, controlled assembly of well-ordered arrays of photosensitive macrocyclic rectangles is demonstrated by using a host-guest molecular template. 4NN-Macrocycle molecules are observed to photoisomerize from trans-trans-trans-trans (t,t,t,t) to a range of isomers including trans-trans-trans-cis (t,t,t,c) and trans-cis-trans-cis (t,c,t,c) isomers after irradiation of UV light. The photoisomers are also observed to affect the guest-host network characteristic appreciably. In the STM observations we can distinguish three (t,t,t,t) conformational isomers, three (t,t,t,c) conformational isomers, and one (t,c,t,c) isomer, which self-assemble into different adlayers with TCDB on a HOPG surface. This study provides a facile approach to study the photoisomerization processes of the azobenzene groups and the conformational photoisomers.

Raman Spectroscopic Studies and Ab Initio Calculations on Conformational Isomerism of 1-Butyl-3-methylimidazolium Bis-(trifluoromethanesulfonyl)amide Solvated to a Lithium Ion in Ionic Liquids: Effects of the Second Solvation Sphere of the Lithium Ion

Abstract: Raman spectra of the ionic liquid, 1-butyl-3-methylimidazolium bis-(trifluoromethanesulfonyl)amide [C4mIm][TFSA] containing a LiTFSA salt were measured for the lithium salt mole fractions xLi = 0.000, 0.053, 0.106, and 0.171 in the temperature range of 273−350 K. The lithium ion solvation number of 2 at ambient temperature is kept constant in higher temperatures examined in this study. Thermodynamic quantities, such as Gibbs free energy, ΔisoG0; enthalpy, ΔisoH0; and entropy, ΔisoS0, for conformational isomerism of TFSA− from trans to cis isomers in the neat ionic liquid and also in the first solvation sphere of the lithium ion were successfully evaluated for the first time. In the neat ionic liquid, the thermodynamics quantities indicates that the trans isomer is slightly stabilized by enthalpy, though the enthalpic advantage is reduced by entropy to yield nearly equal Gibbs free energy. For the TFSA− in the first solvation sphere of the lithium ion, the ΔisoG0, ΔisoH0, and TΔisoS0 were obtained at 298 K...

The “Azido Gauche Effect”Implications for the Conformation of Azidoprolines

Abstract: The “azido gauche effect” was examined both experimentally and theoretically and was found to determine the conformation of, for example, (4R)- and (4S)-azidoproline (Azp) derivatives. For (4R)Azp derivatives, the azido gauche effect induces a preferred C(4)-exo conformation of the pyrrolidine ring, which leads to stabilization of the s-trans amide conformer of, e.g., Ac-(4R)Azp-OCH3 (5R) via an n→π* interaction between the nonbonding electrons of the oxygen of the acetyl group and the carbonyl group of the ester. For (4S)Azp derivatives, the azido gauche effect results in a C(4)-endo conformation of the pyrrolidine ring that does not allow for this stabilizing n→π* interaction of the s-trans conformer. Consequently, a significantly higher s-trans:s-cis amide conformer ratio is observed for (4R)Azp compared to (4S)Azp derivatives (e.g., 6.1:1 versus 2.6:1 in D2O for Ac-(4R)Azp-OCH3 (5R) compared to Ac-(4S)Azp-OCH3 (5S)). These conformational preferences are reflected in the higher tendency of (4S)Azp-cont...

Conformers of the peptides glycine-tryptophan, tryptophan-glycine and tryptophan-glycine-glycine as revealed by double resonance laser spectroscopy

Abstract: The peptides Trp-Gly, Gly-Trp and Trp-Gly-Gly were investigated by UV–UV and IR–UV hole burning spectroscopy. Solid samples of the three peptides were vaporised into an argon jet by laser desorption. The IR–UV spectra of different conformers of the peptides were assigned by comparison with the IR–UV spectra of tryptophan [Snoek et al., Phys. Chem. Chem. Phys., 2001, 3, 1819], the free peptide bond in N-acetyl tryptophan methyl amide [Dian et al., J. Chem. Phys., 2002, 117, 10688] and ab initio calculations performed at the DFT B3LYP 6-31G(d,p) level. Apart from an NH⋯NH2 interaction, the peptide backbone of one conformer of each dipeptide is unfolded. The second conformer of Gly-Trp shows a COOH⋯OC hydrogen bond and the second conformer of Trp-Gly-Gly a hydrogen bond between the peptide backbone and the NH group of the indole ring.

Parameterization of OPLS–AA force field for the conformational analysis of macrocyclic polyketides

Abstract: The parameters for the OPLS-AA potential energy function have been extended to include some functional groups that are present in macrocyclic polyketides. Existing OPLS-AA torsional parameters for alkanes, alcohols, ethers, hemiacetals, esters, and ketoamides were improved based on MP2/aug-cc-pVTZ and MP2/aug-cc-pVDZ calculations. Nonbonded parameters for the sp(3) carbon and oxygen atoms were refined using Monte Carlo simulations of bulk liquids. The resulting force field predicts conformer energies and torsional barriers of alkanes, alcohols, ethers, and hemiacetals with an overall RMS deviation of 0.40 kcal/mol as compared to reference data. Densities of 19 bulk liquids are predicted with an average error of 1.1%, and heats of vaporization are reproduced within 2.4% of experimental values. The force field was used to perform conformational analysis of smaller analogs of the macrocyclic polyketide drug FK506. Structures that adopted low-energy conformations similar to that of bound FK506 were identified. The results show that a linker of four ketide units constitutes the shortest effector domain that allows binding of the ketide drugs to FKBP proteins. It is proposed that the exact chemical makeup of the effector domain has little influence on the conformational preference of tetraketides.