DEHYDRATION OF NIGERAN CRYSTALS - CRYSTAL-STRUCTURE AND MORPHOLOGICAL ASPECTS

The crystal structure of anhydrous nigeran, poly[(1 .fwdarw. 3)-.alpha.-D-maltose], obtained from Penicillium crustosum was determined by a combined electron diffraction, X-ray diffraction and packing analysis. The electron diffraction pattern from solution-grown single crystals shows some reflections in the base-plane pattern. The polymer crystallizes with 2 chains passing through the cell of dimensions: a = 17.76, b = 6.0 and c = 14.62 .ANG. (fiber repeat). The space group is P212121. The glycosidic torsion angles of this poly(disaccharide) were determined using an iteration process. The results are analyzed on the basis of the effect of side-group orientations on backbone conformation and are compared with those obtained from a survey of relevant linear and cyclic oligosaccharides. The crystallographic reliability index was Re = 0.25 and Rx = 0.30 when the model was tested against electron and X-ray diffraction data, respectively. The poly(disaccharide) chain is a 2-fold helix stabilized by an intrachain H- bond between contiguous .alpha.-(1 .fwdarw. 4) residues. The hydroxymethyl side-chains are in the gauche-gauche form. Two such chains pack with anti-parallel polarity and the 2-fold screw axis coincides with the macromolecular axis. A network of H- bonds holds the chains together in the crystal. In the condensed state, the nigeran chains are more or less extended depending on the state of hydration. The 2 extremes correspond to the dry and wet crystal forms. The transition between the 2 conformations takes place reversibly in the crystalline state. This is observed on solution-grown crystals, and in vivo where nigeran is found to occur in crystalline domains. Such a transition is analyzed in terms of the conformational flexibility of the backbone and in terms of the affine deformation concept. Morphological and textural transformations occurring as a result of drying are suggested to have important consequences on the subsequent microbiology such as enzymic digestion.