Electron crystallographic analysis of a polysaccharide structure--direct phase determination and model refinement for mannan I

Three-dimensional electron diffraction intensity data from the polysaccharide mannan I, previously published by H. Chanzy et al. (1987; Macromolecules 20, 2407-2413), were used for an ab initio structure determination. The space group is P2(1)2(1)2(1), with unit cell constants a = 8.92, b = 7.21, c = 10.27 A. After determination of crystallographic phases by evaluation of triplet and quartet structure invariants, 41 values were assigned to 58 unique reflections, 11 linked through a single algebraic unknown a. Two potential maps corresponding to two possible values of a were calculated by a Fourier transformation of the limited 3D data set, combining the phase terms with measured structure factor magnitudes. Either map allowed a monomer model, taken from the crystal structure of methyl alpha-D-mannopyranoside, to be fit very well in the xy plane, since 24 of 34 hk0 data are accessed by the direct phase determination. Although the limited goniometer tilt range is responsible for an incomplete sampling of the 3D reciprocal net, resulting in a series termination effect, a choice could be made between the two possible maps (permuting the unknown phase term a = +/- pi/2), since only one is a good envelope to the monomer structure in the chain direction. After optimizing linkage geometry, a structure that is very close to the initial determination was derived based on a conformational refinement.