Algal cellulose orientation

Orientation. The orientation of the cellulose chain axis in a number of different fibers has been studied in detail (21-22). Much less is known about the cellulose orientation in the plane perpendicular to the chain axis. The orientation in this plane is determined by the lateral arrangement of the microfibrils relative to each other. In algal celluloses, the evidence from x-ray and electron diffraction indicates that the microfibrils are arranged nonrandomly in the plane perpendicular to the chain axis (21-29). Preston (22) proposed the model shown in Figure 1 to explain his x-ray data. There are two different orientations of the microfibrils. The 002 planes in one set of microfibrils are approximately perpendicular to the 002 planes in the second set. In both sets of micro-fibrils, the 101 planes are oriented parallel to the cell wall surface (refer to Figure 1). Preston s model has been confirmed in more recent studies (29). In the remainder of this report, the type of orientation shown in Figure 1 will be referred to as alternating orientation. 

Direct measurements of cellulose orientation in fibers have yielded conflicting results. Evidence from x-ray diffraction studies of wood samples suggested that the cellulose crystallites are arranged randomly in the plane perpendicular to the fiber axis (30-33). Raman spectroscopic studies of cotton fibers dried under tenTTon, however, demonstrated that the methine C-H bonds are oriented preferentially perpendicular to the surface of the cell wall (34). Since the C-H bonds are perpendicular to the 002 plane, the orientation suggested by the Raman evidence differs from the alternating orientation in algal celluloses. 

Figure 1. The cellulose orientation in the plane perpendicular to the chain axis found in algal celluloses.

Work with electron microscopes showed that there is preferential enzymatic activity at only one end of the native microfibrils. This indicates that the reducing ends are all at one end of the microfibril and thus the chains are parallel, not antiparallel [240]. Electron microscopy and diffraction work on algal and bacterial cellulose confirmed the parallel-up nature of the chain orientation in the unit cell and the addition of new glucose residues to the cellulose chain at the nonreducing end [241]. Similar attempts with ramie fibers were not successful. 

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