Dehydration of nigeran crystals: Crystal structure and morphological aspects

Abstract

The crystal structure of anhydrous nigeran, poly[(1 → 3)-α- d-maltose], obtained from Penicillium crustosum has been determined by a combined electron diffraction. X-ray diffraction and packing analysis. The electron diffraction pattern from solution-grown single crystals shows some (h k I) reflections in the baseplane pattern. The polymer crystallizes with two chains passing through the cell of dimensions: a = 17.76, b = 6.0 and c = 14.62 A ̊ (fiber repeat). The space group is P2 12 12 1. 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 R e = 0.25 and R x = 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 hydrogen bond between contiguous α-(1 → 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 hydrogen 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 two extremes correspond to the “dry” and “wet” crystal forms. The transition between the two conformations takes place reversibly in the crystalline state. This is observed on solution-grown crystals, as well as 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.