Disaccharides conformations

The conformational analysis of monosaccharides, disaccharides, and ohgosaccharides is reviewed. Conformational terms are introduced through examination of the conformations of cyclohexane and cyclopentane then applied to the pyranose, furanose, and septanose rings. Concepts such as the anomeric effect are discussed. Topics of current interest, such as hydroxymethyl group and hydroxyl group rotation and disaccharide conformations are summarized. Physical methods for studying conformation are outlined. 

In a further molecular mechanics study (MM3), the energy surfaces of a,a-, a,p-, and P,P-trehalose were compared with those of the corresponding 2-(6-methyltetrahydropyran-2-yloxy)-6-methyltetrahydropyrans and 5a-carba trehaloses it was concluded that the exo-anomeric effect plays an important role and that linkage-type (a or p) is more important than the presence or absence of exocyclic substituents in detomining disaccharide conformations. The preferred solution conformations of eight (l- )-C-linked disaccharides, such as compounds 13, have been determined on the basis of vicinal H- H coupling constants, and conformational similarities between... 

MM2 has been applied frequently in mono- and disaccharide conformational analyses, but has not been extensively employed in oligosaccharide conformational determinations. The goal of the MM2 energy minimizations is often to allow relaxation of the geometrical parameters, and to provide more accurate estimates of the relative energies, for conformations obtained from rigid-residue conformational searches. ... 

Disaccharide conformations are determined by their free energies, rather that just their mechanical potential energies, and as in the cases of trehalose and neocarrabiose, the solvent can affect the free energy surface governing conformational equilibrium. A more useful description of disaccharide energetics would be a Ramachandran conformational potential of mean force (pmf) which included not only entropic effects but also the influence of the solvent. 

The side chains in the latter are flexible disaccharides on account of poor-quality diffraction patterns, their tentative molecular structures are known only from computer modeling.1" On the other hand, well-defined crystal structures are available for gellan and welan, and they can be correlated with the physical properties of the polysaccharides the details are presented here. Their conformation angles are listed in Table VI. 

The conformational analysis was developped recently by different authors first on the disaccharide units to investigate the role of the charge on C-6 position on the conformation [29-32]. From this study, the 2i or right-handed 3i helices were described as most problable conformations [30,31]. These conformations were also demonstrated from x-ray diffraction [33] or on the basis of circular dichroTsm [34], From these data, the repeat unit in the axial-axial conformation has a 4.3 A° length which will be used to characterize the electrostatic properties. 

From disaccharide analysis, few authors predict the conformation of the poly a-D galacturonan as well as the role of the charge density [35,36] they determine the persistence length Ip that will allow us to explain the behaviour in solution. 

Figure 2. Potential energy maps of a) GalA(l->2)Rha as a function of the glycosidic dihedrals, b) the same disaccharide with the galacturonic acid acetylated at 02 and 03. The conformation used to build integer helices is indicated.

Figure 5. Acetylated RG-I helices generated from the low-energy conformations of the disaccharides (see Figures 2 and 3). Left the AA type with n = -3 and h = 6.75 A right the AB type with n = 2 and h = 7.48 A.

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