Sugar ring, conformation

Figure 19. Packing arrangement of sucrose delineated by the observed crystal faces, as viewed along the c axis. The emerging galactose moiety of a raffinose inhibitor molecule has been inserted at the (110) face in the exo conformation. Note that the corresponding hydroxyl of sucrose adopts an endo conformation. The sugar ring inserted at the (lTO) face indicates the position that would be taken by the galactose moiety of a raffinose inhibitor molecule in the endo conformation.

Monosaccharides have many structural variations that correspond to local minima that must be considered. Acyclic carbohydrates can rotate at each carbon, and each of the three staggered conformers is likely to correspond to a local minimum. The shapes of sugar rings also often vary. Furanose rings usually have two major local minima and a path of interconversion. Experimental evidence shows a clear preference for only one chair form for some pyranose rings, but others could exist in several conformers. For exanqple, the and conformers must all be considered as possible structures for L-iduronate, as discussed by Ragazzi et al. in this book. 

We conclude that a) all degrees of freedom should be unconstrained (including those related to the shape of the sugar rings)/ b) the packing forces can significantly influence the conformation even of a polymer main chain and c) a suitable potential set (with all atoms), used in total energy minimization, can account for these effects. With these considerations in mind, we attempted to model the crystal structures of DeS and an isolated chain of Hep. 

C. Altona and M. Sundaralingam, Conformational analysis of die sugar ring in nucleosides and nucleotides. New description using the concept of pseudorotation, J. Am. Chem. Soc., 94 (1972) 8205-8212. 

It has been shown that methyl thiocellotrioside (44d), tetraoside (44i) and pentaoside (44j) are potent inhibitors of endoglucanase I (EGI, family 7) and cellobiohydrolase II (CBHII, family 6) from Humicola insolens [41]. Furthermore, a crystal of EGI of Fusarium oxysporum was soaked in a solution of compound (44j), and a clear density for three sugar rings was seen in subsites -2, -1 and -hi. It has been found that the pyranose ring in subsite -1 adopts a boat conformation leading to a pseudo-axial orientation for the scissile bond (Fig. 2) [66]. 

---