STRUCTURES OF THE INCLUSION COMPLEXES OF SOME AROMA GLYCOSIDES WITH -CYCLODEXTRIN

Luciana Malpezzi, Andrea Mele and Giovanni Fronza

Dipartimento di Chimica, Politecnico di Milano, Via Mancinelli 7- 20131 Milano, Italy.
E-mail: Malpezzi@dept.chem.polimi.it

Keywords: -cyclodextrins, aroma glycosides, single crystal X-Ray diffraction.

Cyclodextrins (CDs), the macrocyclic oligosaccharides consisting of six, seven or eight a-(1-->4) linked glucose residues, present a torus-shaped structure with a rigid hydrophobic cavity, into which a wide variety of guest molecules of appropriate size can be hosted. The resulting host-guest (h-g) complexes are of interest in fundamental research, as well as in the pharmaceutical applications and in the food industries.

The present X-Ray single crystal analysis is part of an investigation about (h-g) complex of CDs with some aroma substances which may take advantage from modification of solubility, bioavailability and delivery properties induced by their inclusion in CDs. In particular, the complexes here reported are the b-CD complexes of the b-glucoside of (-)-menthol, (I), whose molecular and crystal structure has been completely characterised, and of neohesperidin dihydrochalcone, (II), whose structure is in the course of determination.

The compounds are 1:1 inclusion complexes; each of them was precipitated from an aqueous solution and then dissolved on heating at about 350K; the solutions were allowed to evaporate slowly over a several days period. To obtain single crystals suitable for the X-Ray diffraction, the last procedure was repeated several times for each compound.

Crystal data.-

(I): C₄₂H₇₀O₅₅ C₂₈H₃₆O₁₆ 10H₂O, orthorhombic, space group P2₁2₁2₁ , Z =4,
a = 15.328 (1), b = 20.012(1), c = 22.986(2) A, V = 7755.7(9)A³

(II): C₄₂H₇₀O₅₅ C₄₂H₇₀O₅₅, orthorhombic, space group C222₁, Z = 8,
a = 15.115 (3), b = 30.780(5), c = 41.589(6) A, V = 19349(7)A³

In (I) the guest molecule enters the b-CD cavity from the side of the secondary hydroxyl groups of b-CD with the (-)-menthol; while the glucosidic ring remains outside the host cavity. All hydroxyl groups of the guest are involved in hydrogen bonding with the host and with other three adjacent CD molecules, contributing to the stabilisation of the crystal structure.