An orthorhombic crystal form of cyclohexaicosaose, CA26 center dot 32.59 H2O: comparison with the triclinic form

Cycloamylose containing 26 glucose residues (cyclohexaicosaose, CA26) cryst allized from water and 30% (v/v) polyethyleneglycol 400 in the orthorhombic space group P2(1)2(1)2(1) in the highly hydrated form CA26 . 32.59 H2O. X- ray analysis of the crystals at 0.85 Angstrom resolution shows that the mac rocycle of CA26 is folded into two short left-handed V-amylose helices in a ntiparallel arrangement and related by a twofold rotational pseudosymmetry as reported recently for the (CA26)(2). 76.75 H2O triclinic crystal form [G essler, K. et al. Proc. Natl. Acad. Sci. USA 1999, 96, 4246-4251]. In the o rthorhombic crystal form, CA26 molecules are packed in motifs reminiscent o f V-amylose in hydrated and anhydrous forms. The intramolecular interface b etween the V-helices in CA26 is dictated by formation of an extended networ k of interhelical C-H . . .O hydrogen bonds; a comparable molecular arrange ment is also evident for the intermolecular packing, suggesting that it is a characteristic feature of V-amylose interaction. The hydrophobic channels of CA26 are filled with disordered water molecules arranged in chains and held in position by multiple C-H . . .O hydrogen bonds. In the orthorhombic and triclinic crystal forms, the structures of CA26 molecules are equivale nt but the positions of the individual water molecules are different, sugge sting that the patterns of water chains are perturbed even by small structu ral changes associated with differences in packing arrangements in the two crystal lattices rather than with differences in the CA26 geometry. (C) 200 1 Elsevier Science Ltd. All rights reserved.