A priori crystal structure prediction of native celluloses

The packing of beta-1,4-glucopyranose chains has been modeled to further el aborate the molecular structures of native cellulose microfibrils. A chain pairing procedure was implemented that evaluates the optimal interchain dis tance and energy for all possible settings of the two chains. Starting with a rigid model of an isolated chain, its interaction with a second chain wa s studied at various helix-axis translations and mutual rotational orientat ions while keeping the chains at van der Waals separation. For each setting , the sum of the van der Waals and hydrogen-bonding energy was calculated. No energy minimization was performed during the initial screening, but the energy and interchain distances were mapped to a three-dimensional grid, wi th evaluation of parallel settings of the cellulose chains. The emergence o f several energy minima suggests that parallel chains of cellulose can be p aired in a variety of stable orientations. A further analysis considered al l possible parallel arrangements occurring between a cellulose chain pair a nd a further cellulose chain. Among all the low-energy three-chain models, only a few of them yield closely packed three-dimensional arrangements. Fro m these, unit-cell dimensions as well as lattice symmetry were derived; int erestingly two of them correspond closely to the observed allomorphs of cry stalline native cellulose. The most favorable structural models were then o ptimized using a minicrystal procedure in conjunction with the MM3 force fi eld. The two best crystal lattice predictions were for a triclinic (P-1) an d a monoclinic (P2(1)) arrangement with unit cell dimensions a = 0.63, b = 0.69, c = 1.036 nm, alpha = 113.0, beta = 121.1, gamma = 76.0 degrees, and a = 0.87, b = 0.75, c = 1.036 nm, gamma = 94.1 degrees, respectively. The c orrespond closely to the respective lattice symmetry and unit-cell dimensio ns that have been reported for cellulose 1 alpha and cellulose 1 beta allom orphs. The suitability of the modeling protocol is endorsed by the agreemen t between the predicted and experimental unit-cell dimensions. The results provide pertinent information toward the construction of macromolecular mod els of microfibrils. (C) 2000 John Wiley & Sons, Inc.