Conformational behaviour flexibility

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... 

Finally, the stereodynamic representation of a molecule is a time-dependent representation which adds structural properties to the 3D representations, such as flexibility, conformational behaviour, transport properties, etc. - Dynamic QSAR is an example of a multi-conformational approach. 

The rheological behaviour of polymeric solutions is strongly influenced by the conformation of the polymer. In principle one has to deal with three different conformations, namely (1) random coil polymers (2) semi-flexible rod-like macromolecules and (2) rigid rods. It is easily understood that the hydrody-namically effective volume increases in the sequence mentioned, i.e. molecules with an equal degree of polymerisation exhibit drastically larger viscosities in a rod-like conformation than as statistical coil molecules. An experimental parameter, easily determined, for the conformation of a polymer is the exponent a of the Mark-Houwink relationship [25,26]. In the case of coiled polymers a is between 0.5 and 0.9,semi-flexible rods exhibit values between 1 and 1.3, whereas for an ideal rod the intrinsic viscosity is found to be proportional to M2. 

The behaviour of a series of methyl substituted hept-6-en-l-yl radicals, generated from the corresponding iodides with (TMS)3SiH and AIBN in benzene at 80 °C, has been investigated too. The low regioselectivity and poor stereoselectivities observed for these reactions are not unexpected given the conformational flexibility inherent in the hept-6-en-l-yl radical [24]. 

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