Interconversion of the Ialpha and Ibeta crystalline forms of cellulose by bending.

Bending cellulose in a plane normal to the hydrogen-bonded sheets of chains causes a longitudinal displacement of the sheets with respect to one another. The magnitude of this displacement is shown to be sufficient to interconvert the Ialpha and Ibeta forms of cellulose within a bending angle of 39 degrees when the curvature of the sheets of chains comprising the microfibril is modelled as a series of concentric circular arcs. Bending through an angle of 90 degrees is more than sufficient to convert the Ialpha form into Ibeta and back again. Cellulose microfibrils emerging from the cellulose synthase complex in the plasma membrane must bend sharply before they can lie parallel with the inner face of the cell wall. The scale of the changes induced by bending is sufficient to ensure that whatever crystal form would be expected from the geometry of the biosynthetic complex, it is likely be radically altered before the cellulose is incorporated into the cell wall.