Cyclooctaamylose crystallizes from aqueous solution with space-group symmetry P21 and lattice parameters: a = 20.253(8), b = 10.494(5), c = 16.892(6) A and β = 105.32(1)o, Z = 2; the apparent formular per asymmetric unit is C48H80O40·17H2O. The macrocycle is in an open conformation but displays significant deviations from ideal eight fold molecular symmetry. Of the 19 water molecules thus far located, four of which have occupancy factors of one half, 12 may be characterized as being in the torus of the cycloamylose.

THE cycloamyloses (CnA) are α-1, 4-linked cyclic oligomers of D-glucopyranose, which have attracted considerable attention as enzyme models1. These doughnut-shaped molecules have the primary hydroxyl groups from the 6-position of the glucose residues at one side of the torus, and the secondary hydroxyl groups from the 2- and 3-positions at the other. On the inside of the cavity there is a ring of CH groups, a ring of glycosidic oxygens, and a further ring of CH groups, resulting in a hydrophobic ether-like interior. The ability of the cycloamyloses to form stable complexes with a variety of organic compounds by inclusion within the hydrophobic cavity has prompted their use as models for the active sites of enzymes. Of particular interest is the observation that they can accelerate the release of phenols from a variety of aryl esters2 and of anilines from anilides3 by a reaction pathway similar to that observed for the hydrolytic enzyme α-chymotrypsin. Furthermore, a marked degree of substrate specificity is observed: thus, the cleavage of meta-substituted aryl acetates is accelerated more than that of their para-analogues2. It has been suggested that this specificity is due to the closer positioning of the nucleophilic secondary hydroxyl group of the cycloamylose to the ester carbonyl in the meta complex than in the para complex2. To test the validity of this hypothesis we have made an X-ray crystallographic study of a series of meta- and para-substituted acetanilides and report here on the 1:1 complex of C7 A with p-nitroacetanilide (PNA).