Molecular recognition by cyclodextrins: the x-ray structure of 6A-boc-L-phenylalanylamino-6A-deoxy-β-cyclodextrin

The first crystallographic example of a β-cyclodextrin monosubstituted at O6 by a bulky group possessing two terminal apolar side chains, tert-butoxycarbonyl-L-phenylalanylamino (Boc-L-Phe-NH), illustrates an unusual cyclodextrin host/guest organisation. The structure is a novel packing of monomer units. Two symmetry independent Boc-L-Phe-NH-β-CD molecules are stacked parallel to the two-fold screw axis forming two independent alternate ‚endless screw head-to-tail’ channels. Both apolar side chains are at the exterior of the intramolecular cavity and are fully enclosed within the channels: the phenyl ring of the amino residue lies above the parent macrocycle while the tert-butyl group is intermolecularly included within a neighbouring molecule. Water molecules located outside the monomeric cavities over 34 sites, all but one external to the channels, fill the intermolecular spaces and reinforce the cohesion of the structure.