The prevailing paradigm regarding the structures of native celluloses during most of the last century held that cellulose is inherently crystalline in the native state and that removal of other cell wall constituents results in exposure of the cellulose in its native state. This is best represented by the models of native celluloses presented by Preston (1) and by Frey-Wyssling (2) in their respective classic treatises. The paradigm has been the basis of all crystallographic models of the structures of native celluloses. This view was based on early observations of the birefringence of cellulose, and after X-ray diffraction was discovered, by observations of X-ray diffraction by cellulosic substances. As new instrumental methods have been devel­oped and as progress has been made in computational modeling ofcelluloses, it has become clear that questions of structure are more complex.

The evolution of crystallographic models of cellulose has been reviewed elsewhere (3). There had been little consensus regarding the crystallographic structures of the native form well into the early 1980s. The reason for the uncertainty was that different investigators had data sets from different tissues and species, and the different investigators used dif­ferent constraints on their solutions of the structures. The constraints are necessary be­cause the data sets are not adequate for achieving a definitive solution of the structural problem.

In 1984, Atalla and VanderHart (4, 5) reported that native celluloses are composites of two forms, Ia and Ip, which coexist in all native forms. Two new instrumental methods for that time, Raman spectroscopy and solid state 13CNMR, contributed to the finding. In the initial reports, the two forms were described as “two distinct crystalline forms.” In retrospect, use of the term “crystalline” was unfortunate. Raman spectroscopy also led to the conclusion that the two forms have the same conformation but different hydrogen bonding patterns (6).

The crystallographic studies have been limited historically by the inadequacy of the num­ber of reflections observed for a definitive solution. Diffraction patterns include at most about 300 reflections from celluloses, whereas a definitive solution requires many more re­flections. The approximately 300 reflections achieved in the most recent studies of cellulose structures (7, 8) are in contrast to 1257 reflections observed in the study of cellobiose by Chu and Jeffrey (9) and 1724 reflections in the study of methyl p-cellobioside by Ham and Williams (10). In the crystallographic studies of cellulose cited above, the reflections are complemented with constraints imposed on the solutions of the structural problem. The key constraints are assumptions regarding the symmetry of the crystal structures that have been controversial since Honjo and Watanabe first reported that such symmetry is not consistent with the electron diffraction patterns (11).

Regarding the organization of celluloses in their native states in higher plants, the central flaw in crystallographic studies is that the constraint represented by the assumption of translational symmetry, implicitly imposed in the mathematical analysis of diffractometric data, is not consistent with the curvature of the microfibrils nor does it accommodate the naturally occurring long-period helical twist. Both the curvature and the twist are distinctive of the morphology of the cell walls and of the tissues and species where they occur. To understand the genomic encoding of species and tissue specificity, it is necessary to have a structural paradigm that can be related to the variability in morphology.

Structures derived from diffractometric measurements require that the forms Ia and Ip belong to different space groups. Ia must have one chain per unit cell with a distinctive conformation, and Ip has two nonequivalent chains per unit cell distinct from the chain in the Ia form. These findings cannot be reconciled with the two forms of native cellulose coexisting in a nanofibril 3-5 nm in diameter. These findings also contradict those by Sugiyama and coworkers in the classic study reporting the lattice images of Valonia ventricosa fibrils (12), which shows that the fibril is a single crystal though it is approximately 65% Ia and 35% Ip. Finally, recent observations of the Raman spectra of celluloses Ia and Ip leave little question that the conformations in both forms are nearly identical. These matters have been discussed in detail elsewhere (13); they will be considered in overview here.

From a more conceptual perspective, the fundamental flaw of the crystallographic models is that they do not provide a basis for rationalizing the species and tissue specificity of the blends of the two forms that occur in native tissues. It is clear that the distinctive patterns of different native celluloses are related to the aggregation of the most elementary fibrils at the levels immediately above the aggregation of the individual nanofibrils emerging from individual cellulose synthase complexes (see Chapter 5 for more discussion).

To clarify terminology used in the following discussions, we define nanofibrils as the aggregates of cellulose chains emerging from individual synthase complexes, whereas a microfibril is used to define the next level of aggregation. A microfibril is thus an aggregate of nanofibrils distinctive of a particular tissue in a particular species, and it is usually the first level observable through high magnification microscopy, whether electron microscopy, or during the last decade or so, atomic force microscopy.

The point of departure for our discussion will be the observations by electron microscopy that microfibrils of native celluloses are first and foremost biological structures spatially orga­nized in a periodic helical form. Furthermore, their period varies with the lateral dimensions of the fibrils and is species — and tissue-specific. Recent molecular modeling studies of ag­gregates of cellulose chains in a hydrated environment have shown that the periodicity is inherent in the nature of cellulose molecules (14).