MOLECULAR RECOGNITION BY CYCLODEXTRINS - THE X-RAY STRUCTURE OF C-L-PHENYLALANYLAMINO-6(A)-DEOXY-BETA-CYCLODEXTRIN

The first crystallographic example of a P-cyclodextrin monosubstituted at O6 by a bulky group possessing two terminal apolar side chains, te rt-butoxycarbonyl-L-phenylalanylamino (Boc-L-Phe-NH), illustrates an u nusual cyclodextrin host/guest organisation. The structure is a novel packing of monomer units. Two symmetry independent Boc-L-Phe-NH-beta-C D molecules are stacked parallel to the two-fold screw axis forming tw o independent alternate ''endless screw head-to-tail'' channels. Both apolar side chains are at the exterior of the intramolecular cavity an d are fully enclosed within the channels: the phenyl ring of the amino residue lies above the parent macrocycle while the tert-butyl group i s intermolecularly included within a neighbouring molecule. Water mole cules Located outside the monomeric cavities over 34 sites, all but on e external to the channels, fill the intermolecular spaces and reinfor ce the cohesion of the structure.