Glucans conformation

Figure 2 Effect of branch frequency on glucan conformation. Conformational characterization of glucans was carried out as described in the experimental section. Curdlan is a linear p(l-3)linked glucan Yeast glucan has a 30% P(l-6) branch frequency and PGG-R glucan has a 50% p(l-6) branch frequency. The Congo Red-single/triple helix complex absorption maxima are indicated.

Previous VUCD studies have been reported for amylose(14) and, by us, for pustulan (9), and dextran (10). Prior to those works, optical studies of glucan conformation in solution included optical rotation and optical rotatory dispersion measurements to approximately 200 nm (15-20), CD studies of chemically modified glucans (21,22.23.24.25). and perturbation methods applied to glucan-ligand complexes (26-33). 

Benzyl derivatives of (1 6)-a-D-glucan, (l->6)-a-D-mannan, and (l-> 6)-a-D-galactan have been studied in 1,4-dioxane. These derivatives have complex and interesting c.d. spectra due to the ww transition of the chromophore with resolved vibrational structure. However, a conformational interpretation of these interesting spectra is not possible at this time. 

Figure 2 compares the conformational transition curves of wild-type yeast glucan (branch frequency = 0.20) and PGG (branch frequency = 0.50). Wild-type yeast glucan required approximately 0.1M NaOH to disrupt the triple helical conformation, whereas this transition is observed at approximately 0.04 M NaOH with PGG. This trend is consistent with the observation that curdlan, an entirely linear p-D(l-3)-linked glucan, requires approximately 0.25M NaOH to disrupt the ordered conformation (76). Hence, it is concluded that the highly branched PGG molecules only form weak inter-chain associations resulting in the formation of predominantly single-helical zones. 

Young, S. H., Dong, W. J. and Jacobs, R. R. (2000). Observation of a partially opened triple-helix conformation in l->3-beta-glucan by fluorescence resonance energy transfer spectroscopy. J. Biol. Chem. 275, 11874-11879. 

It is noteworthy that most of the chemical shift values for all three polymers may be closely approximated ( ) by calculations based on data for monomeric reference compounds. These findings illustrate, therefore, the general validity of studies on low molecular weight model compounds for einalysis of spectra of carbohydrate polymers. Many examples of equally satisfactory comparisons of this kind are to be found in studies on other polysaccharides (11,23). These polymers include glucans (l6), mannans (2k, 2 ), limit dextrins (26), lichenin (2j), agarose (28) and various polysaccharides of fungal and microbial orgins (e.g., 7,8,29-31). Observed departures from expectation have been attributed to specific conformational influences ( 8). 

Vacuum-ultraviolet, circular dichroism (v.u.c.d.) measurements have been made on films and solutions of linear D-glucan in a study of the conformation of dextran and its oligomers.123 Film formation of the linear dextran was accompanied by crystallization, and the v.u.c.d. band was observed at 165 nm, in contrast to nonlinear dextran films displaying a band at 177 nm. The difference was ascribed to hydrogen bonding of the ring-oxygen atom in the crystalline state. 

N 141 "Conformational Analysis of Polysaccharides. Part IV. Long-range Contacts in Some p-Glucans by Model Building in the Computer and the Influence of... 

Evaluation of the fibre patterns showed that the glucan chains have a stretched form with twofold screw symmetry and a fibre repeat period of 0.835 nm, as predicted by model building and conformational analysis. The differences between the polymers exist therefore in their chain packing. A monoclinic unit cell with a=0.581 nm b=1.00 nm Y=96° was derived for polymorph I and orthorhombic cells for II and III respectively with the base plane axes a=0.502 nm b=0.963 nm f°r II and a=0.457 nm b=0.865 nm for III. These cells contain 4 glucose residues, and on account of spatial considerations the ribbon-like chains in projection on the base plane were supposed to be oriented with the longer axis parallel to the b-axis (i. e. with the pyranose rings near parallel to the bc-plane) in both polymorph II and III, whereas in polymorph I a diagonal position appeared more favourable. 

Both in theory and in practice there exist eight gluco-pyranose homopolymers, and some of the molecular conformations of three of these, i.e. cellulose and amylose (l.,2., 3,4), and (1+3)-8-D-glucan (5.,6.,.7) have been established by x-ray analysis. Although (1+3)-a-D-glucan is among the five homopolymers previously unsolved by x-ray diffraction, possible chain conformations were predicted with computers to be an extended ribbon (8.,9.) a single helix (9.), or a double or triple helix (10). 

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