Crystal structure of cyclomaltoheptaose (β-cyclodextrin) complexes with p-aminobenzoic acid and o-aminobenzoic acid

Article abstract of DOI:10.1016/j.carres.2008.06.023

Crystal structures of cyclomaltoheptaose (β-cyclodextrin) complexes with p-aminobenzoic acid and o-aminobenzoic acid have been determined by single-crystal X-ray diffraction. The space group of the β-cyclodextrin-p-aminobenzoic acid complex is P2₁ with a host:guest stoichiometry of 1:1, and that of the β-cyclodextrin-o-aminobenzoic acid complex is P1 with a stoichiometry of 2:3. The different structures of the guest molecules lead to the different molecular packing structures of the two complexes. Intermolecular hydrogen-bond interactions are the main force that stabilize the supramolecular systems. In both crystals, there are water molecules located near the cavity rims and in interstices between molecules of β-cyclodextrin participating in formation of intermolecular hydrogen bonds.

Full text of DOI:10.1016/j.carres.2008.06.023

Carbohydrate Research 343 (2008) 2504–2508
Contents lists available at ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Note
Crystal structure of cyclomaltoheptaose (b-cyclodextrin) complexes
with p-aminobenzoic acid and o-aminobenzoic acid
*
Yimin Zhang , Shanbao Yu, Feng Bao
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 June 2008
Received in revised form 21 June 2008
Accepted 26 June 2008
Available online 4 July 2008
Crystal structures of cyclomaltoheptaose (b-cyclodextrin) complexes with p-aminobenzoic acid and
o-aminobenzoic acid have been determined by single-crystal X-ray diffraction. The space group of the
b-cyclodextrin–p-aminobenzoic acid complex is P21 with a host:guest stoichiometry of 1:1, and that of
the b-cyclodextrin–o-aminobenzoic acid complex is P1 with a stoichiometry of 2:3. The different struc-
tures of the guest molecules lead to the different molecular packing structures of the two complexes.
Intermolecular hydrogen-bond interactions are the main force that stabilize the supramolecular systems.
In both crystals, there are water molecules located near the cavity rims and in interstices between
molecules of b-cyclodextrin participating in formation of intermolecular hydrogen bonds.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
b-Cyclodextrin
Crystal structures
Inclusion complex
Hydrogen bonds
Cyclomaltoheptaose (b-cyclodextrin, b-CD) is a macrocyclic oligo-
saccharide consisting of seven -(1?4)-linked
-glucose units.1 It
Structure description of b-CD: All glucose residues are in the nor-
mal 4C1 chair conformation, and the overall b-cyclodextrin mole-
cule has an approximate 7-fold axis. The geometric parameters
for the b-CD molecules are listed in Table 1.
a
D
forms a torus-like macrocycle with the primary 6-OH groups on
the narrow side and the secondary 2-OH and 3-OH hydroxyl
groups at the wide side.2 CDs are well known for their ability to
form inclusion complexes with a variety of guest molecules fitting
partially or completely into the host CD cavity as shown by crystal-
lographic results.3–5 Several factors are recognized to be responsi-
ble for the formation of inclusion complexes of cyclodextrins (CDs)
with suitable guest molecules: van der Waals forces, and hydrogen
bonds, as well as hydrophobic and dipole–dipole interactions.6
Single-crystal X-ray diffraction studies of host–guest (H–G)
complexes of CDs provide detailed information on the role of
such weak interactions for establishing host–guest supramolecular
assemblies,7 molecular recognition, and stereodifferentiation.6,8
In this work, in order to understand how the nature of the guest
molecule determines the host–guest ratio and the crystal packing,
we have carried out an investigation of p-aminobenzoic acid and
o-aminobenzoic acid as guest molecules. The structures of the
host and guest molecules are given in Figure 1: Gn denotes the
nth glucosidic residue of the b-CD.
The glycosidic O4n atoms lie in a plane within 0.28 Å, the
O4nÁ Á ÁO4(n + 1) distances vary between 4.235 and 4.494 Å, and
the values of the angles between the glucosidic oxygen atoms
O4(n À 1)Á Á ÁO4nÁ Á ÁO4(n + 1) do not differ significantly from
128.57° (between 124.47° and 130.42°), the ideal value for an angle
of a regular heptagon, denoting that the cavity is only slightly dis-
torted due to inclusion. The annular shape of the b-CD is stabilized
by the interglucose hydrogen bonds connecting the secondary
hydroxyl groups O3n and O2(n + 1) of neighboring glucosidic units
(Table 1) (average O3nÁ Á ÁO2(n + 1) distance 2.864 Å, range 2.773–
2.951 Å) similar to other b-CD complexes.2,5,9,10 The orientation
of the C6–O6 bond is described by torsion angles C4n–C5n–C6n–
O6n and O5n–C5n–C6n–O6n, listed in Table 1. Most of the primary
hydroxyl groups (major disordered sites) have the gauche–gauche
orientation (mean torsion angles C4n–C5n–C6n–O6n and O5n–
C5n–C6n–O6n 55.00 and À66.41°, respectively) and point out of
the cavity, except for glucose residue G6 (labeled in Fig. 2a), where
a gauche–trans orientation is adopted and O-6 points toward the
cavity. Five water molecules are disposed outside the cavity at bor-
ders of the toroid rims and in interstices between b-CD molecules.
Table 2 lists the hydrogen bonds of the b-CD A with the guest and
water molecules.
Structure of the inclusion complexes: In the b-CD–p-aminobenzo-
ic acid complex, b-CD forms a 1:1 complex with p-aminobenzoic
acid as shown in Figure 2a. b-CD and o-aminobenzoic acid form a
2:3 host–guest inclusion complex (Fig. 2b). Two crystallographi-
cally independent b-CD molecules per unit cell form head-to-head
dimers by means of intermolecular hydrogen bonds.
Table 1 lists the geometrical parameters for the two indepen-
dent b-CD molecules B and C that comprise the complex. Average
bond lengths and angles of B and C show that the monomers
have similar geometries which compares well with other b-CD
* Corresponding author.
E-mail addresses: zhangym001@263.net, yushanbao4047@163.com (Y. Zhang).
0008-6215/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2008.06.023

Y. Zhang et al. / Carbohydrate Research 343 (2008) 2504–2508
2505
C5n–C6n–O6n 56.72° and À63.67°, respectively). However, the
O6n–C6n bonds within the glucose residue G7 of molecule C (la-
beled in Fig. 2b) adopt a gauche–trans orientation and point toward
of the cavity. All the other primary hydroxyl groups are in the
gauche–gauche orientation with the mean torsion angles C4n–
C5n–C6n–O6n and O5n–C5n–C6n–O6n at 57.64° and À62.68°,
respectively.
The two b-CDs B and C form a head-to-head dimer in which the
two O-4 planes make an angle of 20.10° and the corresponding dis-
tance from center to center of the two O-4 planes is 7.084 Å. The
glycosidic O4n atoms of b-CD B lie in a plane within 0.03 Å, the
O4nÁ Á ÁO4(n + 1) distance varies between 4.282 and 4.423 Å, and
the O4(n À 1)Á Á ÁO4nÁ Á ÁO4(n + 1) angles range from 125.35° to
131.52°. This gives the cavity a slightly elliptical shape that could
be related to the shape of the guest molecule. Hydrogen bonds
O3n-HÁ Á ÁO2(n + 1) between neighboring glucose residue (2.698–
2.775 Å; Table 1) stabilize and maintain the rigidity and ‘round-
ness’ of b-CD B.
As shown in Table 1, the b-CD molecule C also possesses a near-
sevenfold symmetry. The maximal deviation of the O4n atoms to
their least-squares plane is less than 0.02 Å. The distances
O4nÁ Á ÁO4(n + 1) range from 4.272 to 4.497 Å, and the angles
O4(n À 1)Á Á ÁO4nÁ Á ÁO4(n + 1) range from 122.62° to134.87°. The
annular geometry of b-CD C is stabilized by interglucose O3n–
HÁ Á ÁO2(n + 1) hydrogen bonds with OÁ Á ÁO distances 2.703–
2.803 Å. There are 23 water molecules per asymmetric unit, dis-
tributed over 28 sites. All the hydrogen bonds between the two
b-CDs molecules B and C and of the two b-CDs with the guest mol-
ecule and water molecules are listed in Table 3.
Geometry of the guest molecule: In the b-CD–p-aminobenzoic
acid complex, the guest molecule is deeply included into the cavity
of the b-CD, with the amino group located on the wide site of the
cavity and the carboxyl at the narrow side as shown in Figure 2a.
Its molecular plane makes an angle of 72.30° with the O-4 plane
made by the seven O4n atoms of the b-CD, and the mass center
of the aromatic ring is 1.643 Å below the mass center of the O-4
plane.
Figure 1. The structures of b-cyclodextrin, p-aminobenzoic acid and o-amino-
benzoic acid.
complexes.11,12 All O6n–C6n bonds of molecule B are directed
away from the macrocyclic cavity, adopting the gauche–gauche
conformation (mean torsion angles C4n–C5n–C6n–O6n and O5n–
Host–guest interaction plays a crucial role in determining the
orientation of a guest molecule in the cavity.13 There is no conven-
tional hydrogen bond observed between b-CD and its guest mole-
Figure 2. (a) X-ray structure of the 1:1 host–guest inclusion complex between b-CD and p-aminobenzoic acid. (b) The crystal structure of the 2:3 host–guest inclusion
complex between b-CD and o-aminobenzoic acid.

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Y. Zhang et al. / Carbohydrate Research 343 (2008) 2504–2508
Table 1
b-CD macrocycle characteristics
Residue
Da (Å)
ub (°)
dc (Å)
Dd (Å)
Torsion angle (°) C4n–C5n–C6n–O6n
O5n–C5n–C6n–O6n
Molecule A
G1
G2
G3
G4
G5
G6
G7
4.494
4.410
4.263
4.403
4.480
4.235
4.307
130.42
129.82
127.53
124.63
133.19
128.60
124.47
0.18
À0.16
À0.10
0.21
2.951
2.877
2.887
2.804
2.773
2.879
2.886
55.58 (À158.13)
49.51 (179.22)
60.22
57.26
51.67
À68.23 (89.34)
À71.69 (61.60)
À60.55
À63.59
0.05
À69.85
À0.28
À178.58
61.86
0.10
55.75
À64.56
Molecule B
G1
G2
G3
G4
G5
G6
G7
4.331
4.282
4.423
4.351
4.290
4.366
4.420
128.97
125.35
129.88
131.52
125.49
128.02
130.73
0.01
À0.01
À0.02
0.03
2.724
2.775
2.759
2.770
2.698
2.706
2.752
61.64
53.64
53.21
53.37
57.24
56.57
61.38
À59.20
À67.94
À65.95
À66.80
À62.53
À65.19
À58.05
0.00
À0.03
0.02
Molecule C
G1
G2
G3
G4
G5
G6
G7
4.310
4.497
4.331
4.272
4.369
4.483
4.281
122.62
132.57
132.04
123.79
126.71
134.87
127.38
À0.01
0.02
2.803
2.787
2.802
2.746
2.703
2.771
2.792
54.57
57.85
54.59
57.24
56.67
64.92
À175.94
À66.43
À61.56
À67.27
À61.96
À64.78
À54.08
63.27
À0.01
0.00
0.00
0.02
À0.01
a
Distance between atoms O4nÁ Á ÁO4(n + 1).
b
c
Angles between atoms O4(n À 1)Á Á ÁO4nÁ Á ÁO4(n + 1).
Deviations (Å) from the least-squares optimum plane of the seven O4n atoms.
Intramolecular hydrogen-bond distance between O3nÁ Á ÁO2(n + 1).
d
b-CD complex.14 The angles between the aromatic rings of guest
molecule b and guests a and c are 45.60° and 66.50°, respectively.
There is no conventional hydrogen bond formed between guest a
and c and the two host molecules. Obviously, van der Waals con-
tacts are the main forces to keep the guests a and c in position.
However, a number of hydrogen bonds that contribute to stabilize
the dimeric b-CD complex formed among the guest, water
Table 2
Hydrogen bonds of the b-CD A with the guest molecule and water molecules
D–H
d(HÁ Á ÁA) (Å)
\DHA (°)
d(DÁ Á ÁA) (Å)
A
Between b-CD and water molecules
O17–H17
O25–H25
1.985
2.006
153.50
163.45
2.743
2.801
O38W [Àx + 1,
y + 1/2, Àz + 1]
O40W [Àx + 1,
y + 11/2, z + 1]
O41W [x, y + 1, z]
O39W [Àx + 1,
y + 1/2, Àz + 1]
O27–H27
O32–H32
2.317
1.965
115.99
152.26
2.776
2.718
Table 3
Hydrogen bonds between the two b-CDs molecules B and C and of the two b-CDs with
the guest molecule and water molecules
Between b-CD and guest molecules
N1–H1A 2.178 147.25
2.965
O8 [x, y, z + 1]
D–H
d(HÁ Á ÁA) (Å)
\DHA (°)
d(DÁ Á ÁA) (Å)
A
Between b-CD molecules and water molecules
O10–H10
O12–H12
O15–H15
O32–H32
O35–H35
O40–H40A
O42–H42
O45–H45
O65–H65
O65–H65
O68–H68
O70–H70
1.900
1.935
1.878
1.825
1.881
1.920
1.865
1.874
2.109
1.954
2.048
2.293
167.29
152.27
158.35
171.14
168.24
167.61
163.12
174.77
136.39
162.87
152.55
107.71
2.725
2.707
2.677
2.657
2.709
2.746
2.680
2.711
2.778
2.768
2.820
2.671
O77W [x, y, z]
cule. However, the amino of p-aminobenzoic acid forms hydrogen
bond, which is listed in Table 2, with the adjacent b-CD 3-OH
groups. Therefore, the guest is held in the cavity mainly by van
der Waals contacts and hydrogen bonds.
O92W [x + 1, y, z + 1]
O78W [x + 1, y, z]
O88W [x, y, z + 1]
O84W [x, y, z]
O93W [x + 1, y, z + 1]
O99W [x + 1, y, z + 1]
O83W [x + 1, y, z + 1]
O103W [x, y, z + 1]
O85W [x, y, z + 1]
O97W [x + 1, y + 1, z]
O81W [x + 1, y, z + 1]
In the b-CD–o-aminobenzoic acid complex, there are three
guest molecules (a, b, and c) per asymmetric unit (Fig. 2b). The dis-
tances between the centers of the two aromatic rings of a and c and
the centers of the O-4 planes of the two b-CDs B and C are 0.969
and 1.422 Å, respectively. That is to say the two guest molecules
(a and c) are only shallowly encapsulated in the two hydrophobic
b-CD cavities, and the two guest molecules are incorporated into
the b-CD cavities with the carboxyl and amino groups emerging
from the primary faces of the b-CD dimer because of the steric hin-
drance of the two groups. Guests a and c make angles of 52.24° and
72.05° with O-4 planes of the two b-CD molecules B and C, respec-
tively. The corresponding distances from center to center of the
two aromatic rings and the angle between the two aromatic rings
are 9.440 Å and 21.70°. The third guest molecules b, situated in two
directions with occupancy factors of 0.72 and 0.28, are found in the
middle of the dimer approximately perpendicular to its 7-fold axis.
A similar motif has been described in the literature for the dimeric
Between b-CD molecules and guest molecules
O8–H8
O76–H76A
2.473
2.233
126.44
163.20
3.047
3.047
O76 [x, y, z]
O8 [x, y, z]
Between two b-CD molecules
O8–H8
2.344
2.152
2.413
2.017
2.316
2.411
2.457
2.183
113.49
173.12
99.80
161.63
115.85
121.26
126.68
127.94
2.787
2.988
2.687
2.827
2.787
2.937
3.034
2.779
O48 [x, y, z]
O38 [x, y, z]
O38 [x, y, z]
O13 [x, y, z]
O8 [x, y, z]
O7 [x, y, z]
O33 [x, y, z]
O28 [x, y, z]
O17–H17
O18–H18
O43–H43
O48–H48
O48–H48
O57–H57
O63–H63

Y. Zhang et al. / Carbohydrate Research 343 (2008) 2504–2508
2507
Figure 3. (a) Crystal packing of the complex b-CD–p-aminobenzoic acid. (b) Crystal packing of the complex b-CD–o-aminobenzoic acid. All water molecules are omitted.
molecules and O6 atoms of the two b-CDs are listed in Table 3. For
example, a hydrogen bond forms between the guest molecule a
and a water molecule (d[H1AÁ Á ÁO86] = 2.344 Å, d[N1Á Á ÁO86] = 2.968 Å,
\N1–H1AÁ Á ÁO86=128.01°).
shows that the guest molecule along with the co-crystallized water
molecules plays an important role in the formation of the complex
and the crystal packing. The p-aminobenzoic acid molecules are
deeply included in the b-CD cavities, while the o-aminobenzoic
acid molecules are less deeply included because of the severe steric
hindrance of the latter. The structure of the b-CD-o-aminobenzoic
acid complex revealed a 2:3 stoichiometry, with two o-aminobenz-
oic acid molecules included in the cavities of a b-CD dimer and a
third o-aminobenzoic acid molecule at the interface between two
layers of b-CD dimers. It is this third o-aminobenzoic acid molecule
that provides the major source for the stabilization of the complex
and the crystal packing.
Crystal packing: Crystal structures of the b-CD inclusion
complexes are classified into three types which are cage-type,
channel-type, and layer-type according to the host–guest interac-
tions.15 Figure 3a shows that the b-CD–p-aminobenzoic acid com-
plex packs in ‘herring-bone’ style, and both sides of each
macrocycle cavity are blocked by glucose residues of neighboring
molecules leading to a typical cage structure. Numerous hydrogen
bonds of type O–HÁ Á ÁO and N–HÁ Á ÁO link neighboring cyclodextrin
rings either directly or via bridges of water molecules.
In the b-CD–o-aminobenzoic acid complex, as shown in Figure
3b, the two b-CD molecules are arranged in a channel-packing
mode approximately parallel to the crystal c-axis, frequently
observed in the b-CD complexes.15 Two b-CD monomers assemble
through hydrogen bonds of type O–HÁ Á ÁO listed in Table 4 to form
the b-CD dimer. A summary of intermolecular hydrogen bonds that
are crucial for packing is given in Table 4, excluding the hydrogen
bonds in which water molecules are donors.
1. Experimental
Crystallization: p-Aminobenzoic acid (obtained from Guangfu
Fine Chemical Graduate School of Tianjin) and b-CD (purchased
from the Kewei Company of Tianjin University) in a 1:1 molar ratio
were fully dissolved separately in ethanol and water, and then the
two solutions were mixed and stirred at 70 °C for two hours. The
mixture was filtered at this temperature, stored at room tempera-
ture, and crystals were obtained after 7 days. Single crystals of the
b-CD–o-aminobenzoic acid complex were obtained by a similar
method.
In summary, analysis of the structure of the b-CD–p-amino-
benzoic acid and b-CD–o-aminobenzoic acid inclusion complexes
X-ray diffraction experiments: A single crystal of each complex
was mounted in a capillary tube in the presence of a small amount
of mother liquor. X-ray diffraction experiments were carried out
Table 4
Intermolecular hydrogen bonds
D–H
d(HÁ Á ÁA) (Å)
\DHA (°)
d(DÁ Á ÁA) (Å)
A
using
a SMART CCD (Bruker) with Mo Ka radiation (k =
Between channels
O5–H5
0.71073 Å) operating at 50 kV, 30 mA. A total of 20415 (b-CD–p-
aminobenzoic acid complex) and 22114 (b-CD–o-aminobenzoic
acid complex) reflections were measured in the h-range 2.05–
27.83° (b-CD–p-aminobenzoic acid complex) and 1.29–25.00° (b-
CD–o-aminobenzoic acid complex), respectively. Data reduction
was carried out with the program SAINT, including semiempirical
absorption correction with SADABS. Crystal data collection and
refinement details are listed in Table 5.
1.881
172.93
175.39
166.13
147.75
172.43
118.26
176.17
2.715
2.869
2.719
2.743
2.901
2.781
2.728
O70 [x, y + 1, z]
O20–H20
O27–H27
O30–H30
O50–H50
O57–H57
O60–H60
2.029
1.896
1.996
2.067
2.283
1.888
O35 [x + 1, y + 1, z + 1]
O47 [x + 1, y + 1, z + 1]
O10 [x, y + 1, z + 1]
O65 [x, y, z + 1]
O17 [x, y + 1, z + 1]
O40 [x, y + 1, z + 1]
Between guest molecules
O74–H74A
O72–H72
1.708
1.866
175.75
163.44
2.546
2.681
O71 [x + 1, y + 1, z + 1]
O73 [x + 1, y + 1, z + 1]
Structure solution and refinement: The structures of the two
complexes were determined by direct methods and refined using

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Y. Zhang et al. / Carbohydrate Research 343 (2008) 2504–2508
Table 5
Crystallographic data
b-CD–p-aminobenzoic acid
b-CD–o-aminobenzoic acid
Chemical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C
42H70O35ÁC7H7NO2Á5H2O
2C42H70O35Á3C7H7NO2Á23H2O
1362.20
294(2)
0.71073
Monoclinic
P21
15.2196(10)
3095.74
113(2)
0.71070
Triclinic
P1
15.249(2)
b (Å)
10.2279(7)
15.2998(16)
c (Å)
20.9321(14)
17.853(3)
a
(°)
90
99.5650(10)
b (°)
110.9270(10)
113.421(5)
h (°)
90
3043.4(4)
101.931(2)
3593.7(9)
Volume (Å3)
Z
2
1
Calculated density (mg/m3)
Absorption coefficient (mmÀ1
F(000)
1.486
0.132
1448
1.430
0.128
1650
)
Crystal size (mm)
Theta range for data collection (°)
Limiting indices
Reflections collected/unique
R (int)
0.32 Â 0.28 Â 0.22
0.22 Â 0.20 Â 0.18
2.05–27.83
1.29–25.00
À19 6 h 6 19, À13 6 k 6 13, À16 6 l 6 27
20415/14052
0.0105
À10 6 h 6 18, À18 6 k 6 18, À21 6 l 6 20
22114/12576
0.0391
Data/restraints/parameters
Goodness-of-fit on F2
Refinement method
14052/33/828
1.050
12576/184/2008
1.027
Full-matrix least-squares on F2
R1 = 0.0702, wR2 = 0.2009
R1 = 0.0805, wR2 = 0.2153
0.9716 and 0.9591
Semi-empirical from equivalents
0.904 and À0.568
Full-matrix least-squares on F2
R1 = 0.0715, wR2 = 0.1876
R1 = 0.0803, wR2 = 0.1994
0.9773 and 0.9723
Semi-empirical from equivalents
1.151 and À0.497
Final R indices [F2 > 2
R indices (all data)
r
(F2)]
Max. and min. transmission
Absorption correction
Largest difference peak and hole (e/Å3)
full-matrix least-squares based on F2 with the program SHELX, based
on 828 parameters and 33 restraints (b-CD–p-aminobenzoic acid
complex) and 2008 parameters and 184 restraints (b-CD–o-amino-
benzoic acid complex), respectively. The refinement converged at
R1 = 0.0702 and 0.0805 (b-CD–p-aminobenzoic acid complex) and
R1 = 0.0715 and 0.0803 (b-CD–o-aminobenzoic acid complex) for
Acknowledgment
We thank the X-ray crystallography laboratory at Nankai Uni-
versity, Tianjin, China, for data collection.
References
observed (F2 > 2
r
(F2)) and all reflections, respectively. Water
difference Fourier map. All
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molecules were located on
a
non-hydrogen atoms were refined anisotropically. Except for
those hydrogen atoms attached to the water molecules, all the
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Supplementary data
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Tables of atomic coordinates, bond lengths, and bond angles
have been deposited with the Cambridge Crystallographic Data
Center, CCDC Nos. 689669 for the b-CD–p-aminobenzoic acid com-
plex and 689670 for the b-CD–o-aminobenzoic acid complex.
These data may be obtained free of charge, on request, from The
Director, Cambridge Crystallographic Data Center, 12 Union Road,
Cambridge, CB2lEZ, UK (fax: + 44-1223-336033; e-mail: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
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