CRYSTAL AND MOLECULAR-STRUCTURE OF THE IDAZOLYL)ETHYL)AMINO]CYCLOMALTOHEPTAOSE]COPPER(II) TERNARY COMPLEX WITH L-TRYPTOPHANATE - ROLE OF WEAK FORCES IN THE CHIRAL RECOGNITION PROCESS ASSISTED BY A METALLOCYCLODEXTRIN

The ternary copper(II) complex of 6-[(2-(4-imidazolyl)ethyl)amino]cycl omaltoheptaose (CDhm) and L-tryptophanate (L-TrpO(-)) was characterize d by ESR and X-ray diffraction. The solid state structure of [Cu(CDhm) (L-TrpO)](+) shows that the aromatic side chain of TrpO(-) is outside the cavity and that the two amino nitrogen atoms, one from the histami ne molecule and one from the amino acidate, are in a cis disposition. The two amino nitrogens, the imidazole nitrogen, and the carboxylate o xygen atoms form the base of a square pyramid, which surrounds the cop per(II) ion, a water molecule occupying an apical position. Atomic dis tances suggest for this complex that pi-pi and d-pi interactions could occur in the solid state. Morover, the [Cu(CDhm)(L-TrpO)](+) has a se lf-assembled structure in which a CDhm molecule behaves as host and as guest. The imidazole and the indole ring are directed into the cavity of an adjacent CDhm molecule from the wider cyclodextrin rim, thus fo rming a polymeric column structure. ESR spectra were run on the copper (II) ternary complexes with L- or D-tryptophanate and L- or D-alaninat e in frozen aqueous solution and on the former pair of enantiomers in the solid state, as well. While in the case of the ternary complex wit h L- or D-alaninate no differences are observed in their frozen soluti on spectra, in the case of complexes with TrpO(-) subtle differences a re found. These differences, which disappear when excess methanol is u sed, are ascribed to the presence of weak forces, such as hydrophobic or d-pi interactions.