Helical conformations of gellan gum

Abstract

We have obtained X-ray diffraction patterns of high quality from oriented polycrystalline fibres of the extracellular microbial polysaccharide gellan gum. The diffraction from the lithium salt form indexes on a unit cell with dimensions a=b=1.56±0.03 nm, c (fibre axis)= 2.82±0.05 nm, α= β=90°, γ=120°, with strong meridional reflections observed on the 3rd, 6th and 9th layer lines. The chemical structure consists of a tetrasaccharide repeat: å3)-β- d-Glcp-(1å4)-β- d-GlcpA-(1å4)-β- d-Glcp-(1→4)-α- l-Rhap-(1→. We have examined molecular conformations consistent with this chemical structure, with the measured density of 1.47 g ml −1, the unit cell dimensions and the periodicity and symmetry deduced from the X-ray fibre diffraction patterns. The axial projection of 2.82/3=0.94 nm is unusually short for an unbranched tetrasaccharide repeat (less than half the theoretical maximum), so in addition to very contradicted single helices, a number of intertwined double helix structures capable of generating the appropriate symmetry and layer line spacing were investigated. Computer model building using a linked-atom least-squares (LALS) system yielded four stereochemically acceptable models for a single molecule: a right-handed (3 1 contracted single helix, right- and left-handed (6 1 and 6 5) contracted double helices and a left-handed (3 2) extended double helix. Examination of the packing of molecules in the unit cell eliminated both 6-fold double helix models. Even after extensive and detailed calculations, the best of each of the surviving models still yield relatively poor agreement between the measured and calculated structure factors. The reliability factors for 3 t single helices packed parallel and antiparallel, extended 3 2 double helices packed parallel and antiparallel were computed to be 0.86, 0.79, 0.71 and 0.65 respectively. This discrepancy between the measured and calculated structure factors can be related to the general distributio of diffracted intensity seen in the X-ray diffraction pattern. Most of the diffracted intensity is concentrated on or close to the equator, and remains so even when the lattice sampling is suppressed by reducing crystallinity whilst maintaining orientation. These observations suggest models with concentrations of electron density on planes closely parallel to the fibre axis, as would be expected from almost fully extended polysaccharide chains. Of all the models considered the extended 3 2 double helices, packed antiparallel, would be favored on general considerations and best fit with the measured X-ray intensities.