Skew conformation, definition

The conformation shown in 8 represents one of a number of skewed conformations in which the rotation around the C-C axis, from the position shown in 2 (or 3), has occurred such that the hydrogens of the rear methyl group have moved in a clockwise direction by an amount 0 < 0 < 60°. This rotation around the carbon-carbon bond in ethane is rapid at room temperature, and is sometimes described as free rotation . This is not strictly true as a small, though definite, energy barrier is encountered. 

The torsional rigidity of an epoxide (or episulfide or ethyleneimine) group and of a carbon-carbon double bond must have a definite effect on the shape of any ring to which the former are attached or in which the latter is incorporated. When these groupings are associated with a pyranoid ring, the latter must adopt a half-chair, a boat, or a skew conformation, and it has been calculated that, for cyclohexene, the half-chair is energetically more favored than the boat by 2.7 Kcal./mole. The conformation of... 

The conformational behaviour in solution of a dermatan-derived tetra-saccharide has been explored by means of NMR spectroscopy, especially by NOE-based conformational analysis. RDCs were also measured for the tetrasaccharide in a phage solution and interpreted in combination with restrained MD simulations. The RDC-derived data substantially confirmed the validity of the conformer distribution resulting from the NOE-derived simulations, but allowed an improved definition of the conformational behaviour of the oligosaccharides in solution, which show a moderate flexibility at the central glycosidic linkage. Differences in the shapes of the different species with the IdoA in skew and in chair conformations and in the distribution of the sulphate groups were also highlighted.28... 

FIGURE 5.6 Definition of the skew-line convention used to define the absolute configuration of tris- or bis(chelate) complexes and conformation of diamine chelates. 

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