Conformational isomerism

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

Comparison of Quantitative Conformer Analyses by Nuclear Magnetic Resonance and Raman Optical Activity Spectra for Model Dipeptides

Abstract: Interpretation of the Raman optical activity (ROA) of peptides is difficult because of molecular flexibility and interaction with the solvent. Typically, simulations and experiments are compared in terms of a qualitative agreement between the spectra. However, on a series of the Pro-Gly, Gly-Pro, Pro-Ala, and Ala-Pro dipeptides more precise conformer ratios could be obtained with the aid of the density functional computations and numerical decomposition of the spectral shapes. All observed transitions were assigned, and the computed transition frequencies were scaled accordingly. Then the populations predicted by the optical spectroscopy agreed within a few percent with an analysis of the spin-spin coupling constants based on the Karplus equations, which was confirmed also by a comparison of calculated and experimental NMR couplings. The results are supported by molecular dynamics simulations and related to the previous conformational studies of similar molecules.

π-Stacking as a Control Element in the (2-PhInd)2Zr Elastomeric Polypropylene Catalyst

Abstract: The crystal structure of the Coates-Waymouth catalyst precursor, bis(2-phenylindenyl)zirconium dichloride ((2-PhInd)-ZrCl{sub 2}, 1) contained two distinct conformers in the unit cell. In one conformer the indenyl ligands were syn to one another (a meso stereochemistry), while in the other cnoformer the indenyl ligands were anti to one another (a rac stereochemistry). To understand why the catalyst formed from 1 produces blocks of isotactic and atactic polymer, a UFF force field conformational study on the catalyst precursor 1 and a model polymer system, (2-PhInd)Zr(CH{sub 3})(C{sub 7}H{sub 15}), was undertaken. A computational model is presented, which contains four main features. (1) The rac and meso conformations of the metallocene system observed by Coates and Waymouth are nearly isoenergetic (within 0.6 kcal/mol of each other). (2) The barrier between the rac and meso conformations is comparable to observed insertion barriers. (3) The shape of the rac conformation is consistent with the established stereochemical model of metallocene polymerizations. (4) Both the rac and meso conformations display {pi}-stacking interactions. Because of these features, the catalyst is able to produce blocks of atactic and isotactic polypropylene. {pi}-Stacking clearly influences the confirmations that the catalyst adopts during the polymerization process. By adding substituents to the phenyl groups more » or changing the solvent, one can alter the {pi}-stacking effects to favor one conformation or the other. 18 refs., 2 figs. « less

Solution conformation of conotoxin GI determined by 1H nuclear magnetic resonance spectroscopy and distance geometry calculations

Abstract: Conformational analysis of conotoxin GI, one of the neurotoxic peptides produced by a marine snail, genus Conus, was performed by a combination of nuclear magnetic resonance spectroscopy (NMR) and distance geometry calculations. The resulting conformers on minimization of the target function were classified into two groups. The difference in the structures of the conformers is mainly due to the difference in the orientation of the side chain of the tyrosyl residue. The results show that the solution structure of conotoxin GI satisfies the conformational requirements for the biological activity of an antagonist toward nicotinic cholinergic receptors elucidated in a series of studies on alkaloids. The structure is discussed on the basis of the results of comparison of the atomic arrangements of the active sites of snake venom peptides and molecular models based on the results of secondary structure prediction.

Structure and dynamics of .alpha.-tocopherol in model membranes and in solution: a broad-line and high-resolution NMR study

Abstract: Nuclear magnetic resonance has been applied to study the conformational dynamics of alpha-tocopherol (vitamin E) in solution and in model membranes. In nonviscous solution, /sup 1/H nuclear magnetic resonance (NMR) showed that alpha-tocopherol is in rapid equilibrium between two or more puckered conformers of its heterocyclic ring. The most likely conformers to be so involved are the two half-chair forms. Deuterium NMR spectra of specifically deuteriated alpha-tocopherol in multilamellar dispersions of egg phosphatidylcholine, measured in the liquid-crystalline state, were characteristic of axially symmetric motional averaging. The orientation of the rotational axis within the molecular framework was determined. Studies on oriented multilamellar membranes revealed that this axis is perpendicular to the surface of the membrane. The profile of quadrupolar splittings along the hydrophobic tail does not have a plateau, in contrast to that of the fatty acyl chains of the membrane lipids. Longitudinal relaxation times (T1) were short. The presence of a minimum in their temperature dependence shows that molecular motion with an effective correlation time tau eff approximately equal to 3 X 10(-9)s is responsible for relaxation. However, the temperatures and absolute values of the minima depend on the position of the deuterium in the molecule, demonstrating that tau effmore » represents a complex blend of motions.« less