An electron diffraction study of the mannan I crystal and molecular structure

Abstract

The crystal and molecular structure of Mannan I has been determined by a constrained linked-atom least-squares refinement utilizing intensities derived from electron diffraction and stereochemical restraints. In this polymorph, chains crystallize on an orthorhombic lattice with a = 0.892 nm, b = 0.721 nm, and c = 1.027 nm. Systematic absences are consistent with the space group P2,2121. A density of 1.57 g~m-~ requires four mannose residues per unit cell, indicating that the chemical repeating unit is also the crystallographic asymmetric unit. Intensities were measured from diffraction patterns produced by specimens which were arranged with c* either parallel to the electron beam or rotated about a* or b*. Several zones including (OOl), (Oil), and (TO11 were collected, digitized, and reduced to integrated intensities. The best model obtained using the base plane data coupled with a stereochemical refinement model yields = 0.245 and R = 0.221. It corresponds to a system of highly extended 2-fold helices stabilized by intramolecular 03-05 hydrogen bonds and packing into (110) sheets of alternately 'up and 'down chains along the apparent growth plane. The association of adjacent chains is stabilized by intermolecular 02-05 hydrogen bonds. The (a,*) angle pair defining the main chain conformation is (-81.1°, -160.Cl0), and 06 is arranged in an approximately gt fashion. Subsequently attempts were made to incorporate the intensities from higher layer lines into the refinement process.