Structural studies on triacetates of mannan and glucomannan

Abstract

Mannan triacetates prepared from material extracted from ivory nut and Tubera salep were studied by means of electron and X-ray diffraction. The former is uniquely constituted of acetylated d-mannopyranosyl units linked by a (1 → 4)-β-linkage whereas the latter contains acetylated (1 → 4)-β- d-glucopyranosyl randomly distributed in the backbone with a ratio of mannose to glucose of about 3:1. However, there seems to be no effect on crystallisation due to the presence of the glucosidic units on the conformation of the chain. Single crystals of ivory nut triacetate were prepared by slowly cooling a dilute solution of nitromethane and butanol. The crystals were long narrow laths which provide electron diffraction data after annealing at 190°C in a vacuum. Two different unit cells were derived from the acetylated Tubera salep X-ray data. A first unit cell with a = 1·18 nm, b = 1·54 nm and c = 1·60 nm contains eight sugar units, whereas the second unit cell with a = 0.369 nm, b = 0·96 nm and c = 1·58 nm would accommodate 16 residues. The latter agrees best with the base-plane parameters derived from electron diffraction of single crystals. The X-ray fibre diagram was interpreted in terms of a two-fold helix and an asymmetric unit composed of two triacetyl mannopyranosyl units. This means that two chemically identical mannose units would not be conformationally equivalent along the backbone. The presence of glucose units in the backbone does not seem to perturb the crystalline conformation. The ‘isomorphous replacement’ hypothesis was invoked to explain this observation. The helical parameters derived herein for Tubera salep mannan triacetate are different from those reported earlier for the same acetylated glucomannan but crystallised using a different technique. This is attributed to the occurrence of polymorphism in this material.