Conformation, of carbohydrates

An electronegative substituent adjacent to a ring oxygen atom also shows a preference for an axial orientation. This is known as the anomeric effect , and is particularly significant to the conformations of carbohydrates (B-71MI20100, B-83MI20100). 

From the previous discussion, it is clear that relaxation experiments constitute a very powerful tool for investigation of the structure and conformation of carbohydrate molecules in solution. However, the nature of the individual problem may determine which relaxation experiment should be chosen in order to extract interproton distances to the desired accuracy of < 0.2 A. Although the limitations and relative merits of all of the various relaxation methods have not yet been systematically studied, accumulated experience provides some direct knowledge about the range of errors associated with relaxation experiments. 

Several evidences, reported in the literature and briefly reviewed in the present article, demonstrate that the carbohydrate recognition at the surface of organized systems is somewhat different from that observed in isotropic media. These differences lie in (1) the conformation of carbohydrate which is affected by hydrophobization and by the nature of the surrounding lipids, (2) cluster effects from which can result in high energies of binding and which are affected by the fluidity of the lipid system, (3) entropy changes at the surface of a supra-molecular structure. 

New glycolipids have to be synthesized to get further insights into liquid crystal properties (mainly lyotropic liquid crystals), surfactant properties (useful in the extraction of membrane proteins), and factors that govern vesicle formation, stability and tightness. New techniques have to be perfected in order to allow to make precise measurements of thermodynamic and kinetic parameters of binding in 3D-systems and to refine those already avalaible with 2D-arrays. Furthermore, molecular mechanics calculations should also be improved to afford a better modeling of the conformations of carbohydrates at interfaces, in relation with physical measurements such as NMR. 

Recall that in the latter, certain types of substituents adjacent to oxygen in the ring actually prefer axial arrangements. This observation has been codified in what is commonly referred to as the anomeric effect and is responsible in part for the conformations of carbohydrates. Is it possible that conformational preferences seen in substituted tetrahydropyrans will carry over into preferences in transition-state geometries for Claisen rearrangements ... 

A. Carbohydrate radicals in synthesis B. Stereochemistry and conformations of carbohydrate 507... 

It may be concluded, from the analysis of the Raman results, that the information provided by Raman spectroscopy is, in essence, similar to that of infrared spectroscopy. The exploitation of the data, namely, the frequencies and intensities due to the molecular vibrations, is of a certain benefit in giving some insight as to the conformations of carbohydrates, and their interactions with the environment. As laser-Raman spectroscopy is applicable to solids, as well as to aqueous solutions, the linear relationship between Raman intensities and mass concentrations, and the specificity and high quality of the spectra experimentally obtained, make this technique particularly promising in investigations of the chemistry and biochemistry of carbohydrates. 

A general survey of conformation of carbohydrates is presented in A. D. French and J. W. Brady, Computer Modeling of Carbohydrate Molecules, American Chemical Society, 1990. 

In considering the overall conformations of carbohydrate chains it is useful to start by distinguishing between (1) ordered conformations, in which the values of the torsion angles are fixed by cooperative interactions between residues and (2) disordered conformations, in which continuous fluctuation occurs. 

The shapes or conformations of carbohydrates strongly influence both their reactivity and their physical properties. Several reviews of various aspects of the conformational analysis of carbohydrates have been published [1,2,3,4,5,6,7]. 

The interactions between substituents are important for the stabilities of the conformations of carbohydrates. Non-geminally disubstituted cyclohexanes can exist as cis- and trans-isomers. The relative stabilities of cis- and trans-1,4-disubstituted cyclohexanes or the relative stabilities of the diaxial versus the diequatorial conformers of trans-l,4-disubstituted cyclohexanes can be predicted successfully from the A values of the two substituents [8]. Another factor arises when ds-l,3-disubstituted-l,3-cyclohexanes are considered the interaction between syn-1,3-diaxially related substituents, as shown in Fig. 2. Corey and Feiner summarized values for this interaction [28]. For carbohydrates, the most important of those available are the OH/ OH interaction (7.9kJmol [29]), the CH3/OH interaction (7.9-11.3 kJmol [30,31,32]) and the OAc/ OAc interaction (8.4 kJ moT [33]). 

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