摘要：

An attempt to solve the crystal structure of naturally occurring crystalline cellulose I of the alga Valonia ventricosa through combined stereochemical packing analysis and X-ray intensity analysis is described. From an excellent correlation of results obtained separately by the two analysis methods it was found highly probable that the chains of Valonia cellulose packed with parallel polarity maintained a twofold screw axis for the rigid components of the glucose ring but showed some flexibility of the hydroxymethyl group rotations. The most probable positions of the latter were near the tg position which allowed extensive hydrogen bonding in at least two directions within the unit cell. Based on the results of both packing and X-ray analysis, a new triclinic, two-chain unit cell was proposed for Valonia. The applicability of this unit cell to other native celluloses is discussed. The stereochemical packing analysis was also extended to predict the crystal structure of cellulose II. A highly hydrogen-bonded, antiparallel structure with hydroxymethyl groups again near the tg position was shown to be the most probable structure for this polymorph. The interconversion of celluloses I and II is discussed in the light of stabilities of both structures as predicted by packing analysis.

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