Cellulose conformation

Figure 13.3 Model of a strand of cellulose (conformation I, ), showing the hydrogen bonds (dashes) within and between cellulose molecules. ...

Glucose exists in chair and bed conformations and Byrne et al [72] believe that the conformation of the elementary cellulose unit controls the route in which the reaction will proceed, where if an initial chair conformation exists, then the breakdown proceeds (Scheme Via) via furfural derivatives, but the bed conformation will form levoglucosan. Unfortunately, the original cellulose conformation cannot be determined. 

Horii E, Hirai A., and Kitamaru R. 1983. Solid-state -NMR study of conformations of oligosaccharides and cellulose conformation of CHpH group about the exo-cyclic C-C bond. Polym Bull 10 357-361. 

The research listed in Table 2 (81-98) deals mainly with separation problems concerning amino acids, amino acid derivatives, and dipeptides, focusing on the influence of the structure of the chiral support and the eluent temperature on the separation behavior of the racemates. Separation of the aromatic amino acids phenylalanine, P-2-thienylalanine, 4-fluorophenylalanine, and tyrosine could not be achieved on microcrystalline or amorphous cellulose tryptophan isomers, however, could be reproducibly resolved on microcrystalline cellulose layers (83). Lowering the eluent temperature from 30 C to O C enhances enantiomeric resolution. However, developing times of 10 1 h (O C), 7.5 0.5 h (10°C), 5 0.5 h (20°C), and 3.5 h (30°C) have to be tolerated hydrophobic eluent combinations further enhance separation, because they improve formation of the helical cellulose conformation (87). Separation of racemic 3,4-dihydroxyphenylalanine, tryptophan, and 5-hydroxy-tryptophan can be achieved in only 2 h, on a cellulose HPTLC plate (89) these experiments will be described in detail in Section FV.C. 

Fig. 1. A segment of a cellulose chain composed of four P-D-glucopyianose residues (ceUotetiaose), showing (a) the chemical bonds and election clouds around the atoms. The molecule has the twofold hehcal conformation typical of many models of crystalline cellulose, (b) The (Haworth) stmctural formula...

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. 

The proposal of multiple crystalline forms in native celluloses implies that all native celluloses are compositions of two distinct forms, which has been earlier indicated for Acetobacter and Vallonia celluloses, 8). From the resolution of the NMR spectra an estimate of about 60-70 % of the la form in Acetobacter cellulose and of 60-70 % of the lb form in cotton was obtained. A further detailed analysis of conformational features in celluloses seemed to need X-ray diffractometric and Raman spectroscopic confirmation 19-56). 

It is evident that the solid-state NMR data combined with vibration spectra call for a reasessment of earlier views of cellulose I and II. Furthermore, earlier interpretations of diffractometric data that ignore differences in molecular conformations will, according to Atalla 191 be insensitive to subtle structural variations which are central to the phenomena of polymorphy and so clearly indicated by the NMR... 

According to a recent report, the unit cell of cellotetraose hemihydrate in single crystals contains two antiparallel chains, which are conformationally distinct—especially in the sugar geometries.74 However, all hydroxymethyl groups adopt similar gt orientations. Whether this oligosaccharide morphology can be implemented for cellulose II in fibers remains to be seen. 

The discussion is organized in the following order First the advantages of HRC scheme, relative to the industrial (i.e., heterogenous) process are briefly commented on second, the relevance of celMose activation and the physical state of its solution to optimization of esterification are discussed. Finally, the use of recently introduced solvent systems and synthetic schemes, designed in order to obtain new, potentially useful cellulose esters with controlled, reproducible properties is reviewed. A comment on the conformity of these methods with the concepts of green chemistry is also included. 

Fig. 2 Schematic representation of cellulose structures in solution Part A shows the fringed micellar structure. Parts B and C show possible chain conformations of celluloses of different DP. For high molecular weight cellulose, C, intra-molecular hydrogen bonding is possible...

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