Helical ribbon-like structure

Examination of (1 4)-linked xylans have indicated a three-fold, left-handed helical structure [65]. hi the case of the AX from rice endosperm flour [66], this structure was confirmed by both X-ray diffraction and conformational analysis using the PS79 computer program. Although it does not seems to be a desirable conformation for xylans to make a complex firmly associated with the two-fold ribbon-like structure of cellulose, the existence of such interactions was documented [67]. 

Figure 17.16 Ribbon diagram representations of the structures of domain B1 from protein G (blue) and the dimer of Rop (red). The fold of B1 has been converted to an a-helical protein like Rop by changing 50% of its amino acids sequence. (Adapted from S. Dalai et al.,...

Although the existence of the 30nm fiber is widely accepted, several models have been proposed for its structure (Felsenfeld and McGhee, 1986). Thoma and colleagues proposed a solenoid model, in which the nucleosomes are ordered in a spiral manner (Thoma et al, 1979). Woodcock and colleagues postulated a helical ribbon model, in which the nucleosomes are arranged in a zig-zag manner and the sheet of the zig-zag nucleosomes winds up helically to form a ribbon-like stmcture (Woodcock ef a/., 1984). 

Figure 45 (a) ORTEP view of the molecular adduct 39 35 (H-bonds are represented by thin lines), (b) ORTEP view of the inclusion complex between benzene and adduct 39 35. (c) Side view of the H-bonding network of adduct 39 35. (d) Simplified representation of the view in (c) showing the right-handed helical motif of the ribbon like H-bonded core of the assembly, (e) Single strand for H-bonded units extracted from the triple-stranded heli-cate structure in 39 35 showing left-handed helicity. (f) Stereoview of the inclusion complex between benzene and adduct 39 35 [60],... 

Figure 19. Typical helical and tubular structures in bile, (a and b) High pitch helical ribbon and helically grown tubule, respectively, (c and d) Similar structures of low pitch, (e) Fracture of a low pitch tubule, (f) Subsequent growth of the low pitch tubule in (e) into a plate-like cholesterol monohydrate crystals after 12 h. Bar, 20 urn. Reproduced from ref. 202 (Chung et al., Proc. Natl. Acad. Sci. USA 1993, 90,11341) with permission from the Academy of Sciences of the USA.

It seems that the kind of linkage is extremely important in determining the conformation of the chain which in turn will be responsible for the secondary and tertiary structure adopted by the polysaccharide. Obviously, the ribbon-like units of chitin can pack readily into dense, rectangular cross-sectioned microfibrils ideal as the skeleton material of cell walls. Therefore, helical conformation, secondary and tertiary structure are dictated by well-defined variations in the main chain chemistry. 

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