Regular left-handed fragment of amylose: crystal and molecular structure of methyl-a-maltotrioside, 4H20 W. Pangborn, D. Langs Medical Foundation of Buffalo, 73 High Street, Buffalo, New York 14203, USA and S. P~rez Centre de Recherches sur les Macromoldcules V~gdtales*, BP 68, 38402 Saint-Martin D'Hdres, France (Received 23 April 1985; revised 24 July 1985) The crystal structure of meth yl-ct-maltotrioside tetrah ydrate C 19Ha4016, 4H20), has been established by direct methods from 2269 independent reflections and refined to a final R value of 0.054. The crystal belongs to the orthorhombic system, space group P212121 and has a unit cell of dimensions: a = 1.037 (1), b = 2.439 (1) and c = 1.065 (1) nm. The three glucose residues have the 4C 1 pyranose conformation and are ct-(l-4)-linked. The conformation of the glycosidic linkage is characterized by torsion angles (q~,~,) which take the values (82.2, - 148.9) between the non-reducing and the middle residue and (82.8, - 151.8) between the middle residue and the reducing one. The primary hydroxyl groups exist in a gauche-gauche conformation. This structure is also characterized by the lack of intramolecular hydrogen bonding between secondary hydroxyl groups belonging to contiguous residues. The molecules are held together by a complicated network of hydrogen bonds involving all the hydroxylic groups and the water molecules. The three dimensional arrangement corresponds to a regular alternation of antiparallel bilayers strongly linked by water molecules. A survey of the distribution of the glycosidic torsion angles in all known linear ct-(l-4)-linked D-glucose residues, discloses the existence of three stable conformers. This crystal structure provides the first experimental evidence of a regular lefi-handed fragment of the amylosic chain in a highly hydrated neighbourhood. Furthermore, the helical conformation adopted by the trisaccharide gives rise to helical parameters which are close to those found experimentally for native A and B amyloses. The relevance of the present results to the rationalization of the polymorphic transformation of amylose, along with its crystallization habits is also discussed. Keywords: Polysaccharides; amylose;methyl-ct-maltotrioside tetrahydrate;crystal structure Introduction Amylose is the linear fraction of starch, that is a polymer of (1-4)-~t-o-glucose residues, which is deposited in a crystalline fashion within the native granule. Depending on the origin, native starch exhibits two main poly- morphic forms: the A polymorph in cereal starches and the B polymorph in tuber starches. Complexation of native amylose with ionic substances such as alkali or salts gives rise to crystalline materials. By precipitation and further drying of the native amylose, a so-called V structure is obtained, which exists complexed with small organic molecules, water or iodine. Finally chemical derivatives, such as acetylated, methylated or ethylated amyloses can be easily prepared. For most of the poly- morphs of amylose, a determination of the three-dimen- sional structure has been attempted on the basis of X-ray fibre diffraction data. For the A and B forms, double helical chains have been proposed 1,2,3 but has been questioned by several authors 4'5. As for the V type of * Laboratoire Propre du CNRS, associ6fi l'Universit~Scientifique et M&licalede Grenoble, France polymorph, it is currently believed that single helical amylose chains are found in the crystalline regions 6'7. All these crystallographic studies have revealed the charac- teristic ability of the amylosic backbone not only to change molecular shape, but also to yield surprisingly different packing features. In the light of these findings, it is obvious that the amylosic backbone can adopt of variety of conformations. The question arises whether prediction of the most probable conformations based on isolated chains is capable of describing the whole range of observed polymeric conformations. In a continuous dialogue between polysaccharide crystallography and oligosaccharide crystallography, the elucidation of crystal structures of high oligomers is still needed. The purpose of doing so is to accumulate highly reliable information about the crystallographic observed conformational minima, to elucidate the relative contri- bution of hydrogen bonding and crystal packing, and ultimately, to refine the potential energy functions. This point is of importance, as regard to the use of confor- mational analysis calculations coupled with n.m.r, obser- vations in predicting the occurrence of stable confor- mations in solution 8. 0141 - 8130/85/060363- 07503.00 ~P) 1985 Butterworth& Co. (Publishers) Ltd Int. J. Biol. Macromol., 1985, Vol 7, December 363