Ramie cellulose chain conformation

In order to decide whether cellulose molecules have an extended or a folded-chain conformation in the native cell-wall, Muggli and co-workers (see also. Ref. 10) devised an interesting experiment based on determination of the molecular-weight distribution in ramie microfibrils before and after they had been cut into 2-ju.m sections. As shown in Fig. 14, should the molecules have an extended conformation, many of the molecular chains would be cut in a random fashion (Gaussian distribution), resulting in substantial diminution in the molecular weight. On the other hand, in the folded conformation, a few macromolecules would be cut, but the rest would retain their... 

Other workers (19-20) have interpreted these differences in the NMR spectra and other data in alternative ways. They believe that celluloses I and II have the same skeletal conformation but are packed in different lattices. In this theory, the differences within the cellulose I family are derived from the size of the unit cells. Valonia contains a larger 8 chain unit cell, whereas ramie contains a mixture of the 8 chain unit cell and the smaller Meyer and Misch unit cell. Therefore the interpretation of the NMR spectra remains controversial. 

The cellulose I spectra were also compared with spectra of ceTTulose II recorded from a mercerized ramie fiber. Figures 7 and 8 show the Raman spectra of these three celluloses. Spectra were recorded with the electric vector of the incident light parallel and perpendicular to the chain axis. These spectra can be divided into two regions. The region below 1600 cm (Figure 7) is most sensitive to the conformation of the cellulose backbone (especially below 700 cm" ). 

The use of synchrotron X-ray data collected from ramie fibers after ad hoc treatment in NaOH provided a revised crystal-structure determination of mercerized cellulose II at 1 A resolution." The unit-cell dimensions of the P2i monochnic space group are a = 8.10 A, h = 9.04 k,c= 10.36 k,y= 117.1°. As with the regenerated cellulose, the chains are located on the 2i axes of the cell. This indicates that the different ways of preparing cellulose II result in similar crystal and molecular structures. The crystal structure consists of antiparallel chains having different conformations, but with the... 

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