Crystal form III of β-cyclodextrin–ethanol inclusion complex: layer-type structure with dimeric motif

• Published in: Carbohydrate research Vol. 343; no. 13; pp. 2285 - 2291
• Main Authors: Aree, Thammarat, Chaichit, Narongsak
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier Ltd 08-09-2008
• Subjects: Amino Acid Motifs; beta-Cyclodextrins - chemistry; Binding Sites; Carbohydrate Conformation; Crystal structure; Crystallization; Crystallography - methods; Crystallography, X-Ray - methods; Dimerization; Ethanol; Ethanol - chemistry; Hydrogen Bonding; Inclusion complex; Molecular Conformation; Molecular Structure; Pseudopolymorphism; Solvents - chemistry; Temperature; X-Ray Diffraction; β-Cyclodextrin; β-Cyclodextrin; Ethanol; Pseudopolymorphism; Hydrogen bonding; Inclusion complex; Crystal structure
• ISSN: 0008-6215; 1873-426X
• DOI: 10.1016/j.carres.2008.04.028

Pseudopolymorphism in the β-CD–EtOH inclusion complex is depicted as a distinction in crystal packing modes.
The crystal form
III of the β-cyclodextrin (β-CD)–ethanol inclusion complex [2(C
6H
10O
5)
7·1.5C
2H
5OH·19H
2O] belongs to the triclinic space group
P1 with unit cell constants:
a
=
15.430(1),
b
=
15.455(1),
c
=
17.996(1)
Å,
α
=
99.30(1)°,
β
=
113.18(1)°,
γ
=
103.04(1)°. β-CD forms dimers comprising two identical monomers that adopt a ‘round’ conformation stabilized by intramolecular, interglucose O-3(
n)⋯O-2(
n
+
1) hydrogen bonds. The two β-CD monomers of form
III are isostructural to that of form
I in the monoclinic space group
P2
1 [Steiner, T.; Mason, S. A.; Saenger, W.
J. Am. Chem. Soc.
1991,
113, 5676–5687], but exhibit a striking difference from that of form
II in the monoclinic space group
C2 [Aree, T.; Chaichit, N.
Carbohydr. Res.
2003,
338, 1581–1589]. The small guest EtOH molecule orients differently in the large β-CD cavity. In form
III, two disordered EtOH molecules are embedded in the β-CD-dimer cavity. A half occupied EtOH molecule (#1) is located above the O-4 plane of β-CD #1, whereas another doubly disordered EtOH molecule (#2, #3) is situated at about the middle of the β-CD-dimer cavity. The three EtOH sites are maintained in positions by making van der Waals contacts to each other and to the surrounding water sites and β-CD O-3–H group. The EtOH molecules disordered (occupancy 0.3) above the β-CD O-4 plane in form
I and fully occupied beneath the O-4 plane in form
II are strongly held in positions by hydrogen bonding with the surrounding water site and β-CD O-6–H, O-3–H groups. Occurrence of the β-CD dimer as a structural motif of channel-type packing (form
II) and layer-type packing (form
III) is attributed to the higher tendency for self aggregation under the moderate acidic conditions. At weak acidic conditions, β-CD prefers a herringbone mode (form
I).