Conformational isomerism

Dl A. Supercoiling. Supercoiling is a topological property of closed-circular DNA molecules. Circular DNA molecules can exist in various conformations differing in the number of times one strand of the helix crosses the other. These different isomeric conformations are called topoisomers and maybe characterized in terms of the linking number, Ek. A linear DNA molecule having Nbase pairs and h base pairs per turn of the helix, if joined end to end, has the following ... 

The probability distribution of isomeric conformations in PDMS is investigated by both conformational energy considerations and by molecular dynamics simulations. A comparatively smooth distribution of isomeric states is obtained from both approaches. A new RIS treatment, compatible with the molecular mechanics and dynamics considerations, is introduced for describing the conformational statistics of PDMS. 

Know the meaning of chair conformation of cyclohexane, equatorial, axial, geometric or cis-trans isomerism, conformational and configurational isomerism. 

When R, = R2, such as for iPr or cyclohexyl groups, the substituent assumes one of the bisected isomeric conformations, A or B (Scheme 52), which are sometimes better represented as enantiomeric pairs of slightly twisted conformations separated by a low barrier. The position of the equilibrium between these conformations depends on the relative effective size and shape of the flanking substituents X and Y. More precisely, it is a quantitative indication of the differences in through-space interactions between [2(R/X) + (H/Y) in A and [2(R/Y) + (H/X) in B for a given geometry. 

If in the study of the polymers primary structure we did not especially take into account particular characteristic parameters of the compounds because "substitution effects" are so clearly marked, it is clear that in studying the secondary structure of the polymers, we must be very careful and critical, about the preparation and characterization of the compounds as far as the isomeric conformation, tacticity, and/or crystallinity... are concerned. It is indeed expected that the effects that must be experimentally evidenced will be (very) fine, and small in amplitude. It is the reason why the compounds, whose XPS spectra are discussed below, were synthesized and characterized by specialized laboratories in the world (practical informations will be given elsewhere (40, 41). 

Electron diffraction studies have shown (30) that, in its vapor phase, the bicyclopropyle molecule possesses two isomeric conformations a nonrigid s-trans form (A) and a nonrigid left form (B) (Equation 19) able to oscillate respectively from 80° to 18° around an equilibrium position. 

As mentioned earlier, n-undecane exhibits a premelting transition at about 10 degrees below the melting temperature. The entropy change due to this solid-state (rotor phase) transition is reported to be 27-29 Jmol" K" [1]. The rotational isomeric (conformational) character of individual chains remains unperturbed by these low-temperature transitions. The physical meaning of this entropy should cause controversial arguments for and against... 

The actual origin of the backbone colour changes is still unclear. Chain end effects were considered as well as bond isomerization , conformational transitions and aggregation phenomena without reaching a definite conclu-... 

Nuclear magnetic resonance Determination of steric configuration in homopolymers composition of copolymers, including proteins chemical fvmctionality, including oxidation products determination of structural and geometric and substitutional isomerism, conformation, and copolymer microstructure. (a-c)... 

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