Conformation amylose

Single Wavelength. Evidence for Distortion of Cyclohexa-amylose in Aqueous Solution. Optical Rotation and Amylose Conformation"... 

Rees DA (1970) Conformational analysis of polysaccharides. Part V. The characterization of linkage conformations (chain conformations) by optical rotation at a single wavelength. Evidence for a distortion of cyclohexaamylose in aqueous solution. Optical rotation and the amylose conformation. J Chem Soc B 877-884... 

At least two generalizations can made from a study of these accessible conformations of glucosides. Although a large number of amylose conformations is observed, a great many more are possible. 

Convergent energy minimization in force-field calculations, carried out initially on a- and /3-D-glucopyranose, has been used to predict the conformations of /3-maltose and cellobiose. ° The four conformations predicted for the former were close to those found in Z-ray structures and n.m.r. experiments, and there was a good correlation with the irregularities known to be present in amylose conformation. There were six local minima identified for cellobiose, the two lowest of which corresponded to the conformations found in X-ray structures. 

Fig. 4.34. Amylose conformation (for explanation see text) (according to Rees, 1977)...

Specific optical rotation values, [a], for starch pastes range from 180 to 220° (5), but for pure amylose and amylopectin fractions [a] is 200°. The stmcture of amylose has been estabUshed by use of x-ray diffraction and infrared spectroscopy (23). The latter analysis shows that the proposed stmcture (23) is consistent with the proposed ground-state conformation of the monomer D-glucopyranosyl units. Intramolecular bonding in amylose has also been investigated with nuclear magnetic resonance (nmr) spectroscopy (24). 

FIGURE 7.22 Suspensions of amylose in water adopt a helical conformation. Iodine (b) can insert into the middle of the amylose helix to give a bine color that is characteristic and diagnostic for starch. 

Derivatized amylose is the basis for the Chiralpak AD CSP. This CSP has been utilized for the resolution of ibuprofen and flurbiprofen, as well as other members of the family of nonsteroidal inflammatory drugs (NSAIDs) [39, 61]. Ibuprofen was not resolved on the Chiralpak AD CSP in LC. Pressure-related effects on stereoselectivity were observed by Bargmann-Leyder et al. on a Chiralpak AD CSP [58]. No corresponding effect of pressure on selectivity was observed with a Chiralcel OD CSP. The authors speculated that the helical conformation of the amylose-based CSP is more flexible than that of the cellulose-based CSP. 

Lewis and Johnson compared the c.d. spectra of amylose and cyclomaltohexaose, and showed that amylose is helical in aqueous solution. Cyclomaltohexaose is chromophorically equivalent to amylose, and it is known to assume a pseudohelix having zero pitch, and thus, no helical chirality. The conformation of amylose is clearly different from that of cyclomaltohexaose, as their c.d. spectra are very different (see Fig. 9). The difference in conformation was considered to be a matter of helical chirality. To confirm this, these workers measured the c.d. spectrum of an amylose-1-butanol complex presumed to have the V-form of helical conformation with the 1-butanol complexed in the channel of the helix. The c.d. spectrum of the complex is identical to that of amylose in aqueous solution. 

Amylose is an instructive example, showing the relationships between different conformations and chemical properties (28). In the discussion of gelation mechanisms, the evaluation of the fine structure of the polysaccharides is a very important point. 

Chemically, both varieties of starch are polymers of glucose with the a-D-glucose units in the 4Ci conformation. The glucose units are linked -(1 ->4)- in both amylose and amylopectin but in amylopectin roughly one residue in twenty is linked -(1 ->6)-, which forms branch points. The proportion of branch points varies, depending on the source of the amylopectin. 

Amylose complexes (wet precipitates) were prepared with fluoro-benzene, 1,1,2,2-tetrachloroethane, 1,1,2,2-tetrabromoethane, bromo-form, and ferf-butyl alcohol. The conformation and packing of the amylose chains complexed with halogen-substituted hydrocarbons are the same as found in the complex with tert-butyl alcohol, namely,... 

From the positive Cotton effect observed in the amylose case and the negative Cotton effect in the SPG case, it is clear that the oligosilane adopts opposite screw sense helical conformations in the two cases.337 It now appears possible to associate an d/-screw sense helix with a positive sign Cotton effect (and vice versa), although other experimental confirmations of this are desirable. 

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