Influence of temperature, pressure, and pH on the group-inclusion equilibrium in water soluble calixarenes

Article abstract of DOI:10.1246/bcsj.80.1932

Previously, we demonstrated, for the first time, that guest probes having more than one bulky group could produce a pair of bidirectional inclusion complexes with water soluble calixarene (Chem. Lett. 2006, 35, 772). The use of unique free radical probes and electron spin resonance has made this finding possible. In this report, we investigated thermodynamic and pH-dependent properties of the group-inclusion complex formation, i.e., the effects of temperature, pressure and the medium pH on group inclusion by p-sulfonatocalixarenes were determined. Temperature-dependence study indicated that enthalpy portions in ΔG play dominant roles in the group-in complexation. Pressure dependence experiments allowed us to calculate the reaction volumes, i.e., intrinsic volume changes related to inclusion into the calixarene cavity, which ranged from -5.1 to -16.3 cm³ mol ^-1. Data on pH dependence clarified the role of oxyanions in the entrant group of calixarenes. It should be emphasized that pressure-dependent study provided unique information on the structure of the group-inclusion complex.

Full text of DOI:10.1246/bcsj.80.1932

1932 Bull. Chem. Soc. Jpn. Vol. 80, No. 10, 1932–1938 (2007)
Ó 2007 The Chemical Society of Japan
Influence of Temperature, Pressure, and pH on the
Group-Inclusion Equilibrium in Water Soluble Calixarenes
ꢀ1
Yoshimi Sueishi, Mieko Negi,1 and Yashige Kotake2;3
1Department of Chemistry, Faculty of Science, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530
2Free Radical Biology and Aging Research Program, Oklahoma Medical Research Foundation,
Oklahoma City, OK 73104, U.S.A.
3Institute for Advanced Energy, Kyoto University, Uji, Kyoto 611-0011
Received April 23, 2007; E-mail: ysueishi@cc.okayama-u.ac.jp
Previously, we demonstrated, for the first time, that guest probes having more than one bulky group could produce
a pair of bidirectional inclusion complexes with water soluble calixarene (Chem. Lett. 2006, 35, 772). The use of unique
free radical probes and electron spin resonance has made this finding possible. In this report, we investigated thermo-
dynamic and pH-dependent properties of the group-inclusion complex formation, i.e., the effects of temperature, pres-
sure and the medium pH on group inclusion by p-sulfonatocalixarenes were determined. Temperature-dependence study
indicated that enthalpy portions in ꢀG play dominant roles in the group-in complexation. Pressure dependence experi-
ments allowed us to calculate the reaction volumes, i.e., intrinsic volume changes related to inclusion into the calixarene
cavity, which ranged from ꢁ5:1 to ꢁ16:3 cm3 molꢁ1. Data on pH dependence clarified the role of oxyanions in the
entrant group of calixarenes. It should be emphasized that pressure-dependent study provided unique information on
the structure of the group-inclusion complex.
Calixarenes are macrocycle oligomers of p-substituted phe-
nolic residues bridged by methylene groups and useful host
molecules that can be modified with a wide variety of func-
tional groups at the phenolic oxygens (lower rim) and the para
positions (upper rim).1,2 The modification of the macrocycle
by sulfonate groups give rise to water-soluble calixarenes,
which are useful in the study of the molecular recognition of
organic guests in water,3 that may mimic substrate–enzyme
binding. Therefore, we believe it is important to characterize
guest–host interactions in p-sulfonatocalixarene inclusion
complex in aqueous solution. Depending on the kind of guest
molecule, several intermolecular interactions for inclusion by
calixarenes have been proposed, including (1) ionic, (2) hydro-
phobic, (3) van der Waals, (4) ꢀ–ꢀ, (5) cation–ꢀ, and (6) hy-
drogen bonding.1,2 However, in previous studies, the inclusion
data has based on the premise that once the guest–host combi-
nation is selected the structure of the complex is unique.
Studies on the structure of inclusion complex of the cyclic
oligomer of glucose cyclodextrin indicated that a cyclodextrin
cavity can form group-inclusion complex, each bulky group
in the guest molecule is included into the cavity. Recently,
we have shown that the same happens in calixarene inclusion
complex, i.e., calixarene can form more than one inclusion
complex from a single guest–host combination. Previously,
there have been many reports on inclusion complexation of or-
ganic molecules with calixarenes; however, these concern in-
clusion of a guest molecule as a whole. Few previous NMR
studies on calixarene inclusion complex assume the presence
of bidirectional (bimodal) inclusion complexes, where the
guest molecule and calixarene produce bidirectional isomeric
inclusion complexes.4–7 However, NMR techniques have been
unable to separate included/non-included species,4–7 because
NMR has a long timescale (>103 Hz). In contrast, ESR has
a shorter timescale (ꢂ106 Hz) than NMR and could overcome
fast exchange difficulties. Obviously, one drawback of ESR is
that it requires the use of a free radical probe. By synthesizing
unique free radical probes, we have shown that calixarene
forms spectroscopically separable group-inclusion complex.8
Nitroxide free radical probe having more than one bulky func-
tional group was used in that study. For instance, tert-butyl
group and phenyl group in the probe each forms independent
complex (i.e., tert-butyl-in and phenyl-in complex (Fig. 1))
with water-soluble p-sulfonatocalix[8]arene. These isomeric
complexes show distinctive ESR spectrum. In that report, we
have determined the equilibrium properties of group-inclusion
complex of p-sulfonatocalix[8]arene based on pressure depend-
ence of equilibrium constants of the group-inclusion complex
which were calculated from the ESR spectra.
In this study, we employed ꢁ-substituted 2,4,6-trimethoxy-
benzyl(tert-butyl)-nitroxides as free radical guests and record-
ed their ESR spectra under varying temperature, pressure and
pH. Equilibrium constants (binding constants) of the complex
were calculated by using computer spectrum simulation. In or-
der to obtain further information concerning group-recognition
of water-soluble calixarenes, we conducted pressure-, tempera-
ture-, and pH-dependence studies on the group-recognition
properties of p-sulfonatocalixarenes. Quantitative information
on free energy change, volume change, and ionic contribution
was obtained by changing temperature, pressure, and pH, re-
spectively. We concluded that unlike cyclodextrin group-in
complex,9 dominant interaction, which defines the calixarene
group-in complex, is the van der Waals interaction.
Published on the web October 12, 2007; doi:10.1246/bcsj.80.1932

Y. Sueishi et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 10 (2007) 1933
tert-Butyl-in
Phenyl-in
Fig. 1. Ball and stick structures of tert-butyl-in and phenyl-in complex of p-sulfonatocalix[8]arene.
-
SO3
-
-O3S
-O3S
-O3S
-
SO3
SO3
SO3-
-O3S
-OS
SO3-
SO3-
-
-O3S
SO3
3
-
SO3
O
O
O
H
O
H
O
H
O
O
H
O
O
H
O H
O
O
H
H
H
H
O
O
H
H
H
H
Calix-S6
Calix-S8
CH3
O
CH3
Probe
1:
R
CH3
CH3
C N CH3
O
H
Phenyl-
H3C
O
2:
Cyclohexyl-
Cyclopentyl-
O
R
3:
Nitroxide Probe
Fig. 2. Structures of p-sulfonatocalixarenes and nitroxide probes.
0.3 s; field scan rate, 0.31 mT minꢁ1. Computer spectral simulation
was carried out using the WIN-RAD program (Radical Research
Inc., Hino Japan). For the calculation of equilibrium constants
using Eq. 2, the concentration ratio of the inclusion complex and
free radical species was determined as follows: 1) recorded first-
derivative ESR signal was reproduced by adjusting relative inten-
sity of the two species, 2) simulated ESR signal of each species
was independently double-integrated using the integration routine
in the software, and 3) the ratio of the two numbers was calculated
and adopted as the relative concentration for the calculation of
equilibrium constants in Eq. 2.
Experimental
Materials and Reagents. ꢁ-Substituted 2,4,6-trimethoxyben-
zyl(tert-butyl)nitroxides (Fig. 2) were synthesized by the reaction
of N-(tert-butyl)-2,4,6-trimethoxyaniline N-oxide, purchased from
Aldrich Chemical Co., (Milwaukee, WI, U.S.A.), with Grignard
reagents.10 p-Sulfonatocalix[n]arenes (Calix-Sn: n ¼ 6, Calix-S6;
n ¼ 8, Calix-S8 (Fig. 2)) were obtained from Dojin chemicals
(Kumamoto, Japan) and used as received. Water was purified by
distillation.
ESR Measurements. A phosphate buffer (pH 6.9 and 12.5,
ionic strength I ¼ 0:05), prepared from phosphate salts, was used
as solvent. The concentrations of nitroxide probes were selected
small (<2 ꢃ 10ꢁ4 mol dmꢁ3) to avoid exchange broadening in
ESR spectra. pH of the buffer was adjusted by adding diluted so-
lution of either Na2HPO4 or NaOH (I ¼ 0:05). Sample tempera-
ture was adjusted by using JEOL NM-PVT variable temperature
unit. Our high-pressure ESR system has been described else-
where.9 The probe solution was loaded into a thick wall quartz
capillary tubing (i.d. 1 mm, o.d. 6 mm). After applying pressure,
the ESR signals were recorded in a JEOL FE3XG X-band spec-
trometer (Akishima, Japan). The spectrometer settings for ESR
measurements were as follow: microwave power, 5 mW; field
modulation amplitude, 0.032 mT at 100 kHz; time constant,
Results and Discussion
Group-in Complex Formation with Calix-Sn. Figure 3a
shows the ESR spectrum of nitroxide probe 1 upon addition of
Calix-S8, exhibiting well-separated ESR peaks. The ESR spec-
trum could be reproduced with computer spectral simulation
by adjusting the relative intensity of the ESR spectra of free
(Fig. 3e) and two radical species (Figs. 3c and 3d). In the pre-
vious paper, based on the characteristic hyperfine splitting con-
stants (hfsc) of group-in complexes, we assigned the two radi-
cal species to tert-butyl-in and phenyl-in complexes, where
either a tert-butyl or a phenyl group is included in the calix-

1934 Bull. Chem. Soc. Jpn. Vol. 80, No. 10 (2007)
Group-Inclusion Equilibrium in Water Soluble Calixarene
(a)
(c)
(d)
Simulated
Simulated
(b)
Simulated
1 mT
(e)
(f)
Simulated
(h)
Simulated
(g)
Simulated
Simulated
Fig. 3. (a) ESR spectrum of probe 1 ([1] = 1:0 ꢃ 10ꢁ4 mol dmꢁ3) containing Calix-S8 ([Calix-S8] = 9:7 ꢃ 10ꢁ3 mol dmꢁ3): ꢀ:
free (non-included) radical species, : tert-butyl-in complex, and : phenyl-in complex. Computer-simulated spectra: (b) for
ESR spectrum (a) in 1-Calix-S8 solution; (c) for tert-butyl-in; (d) for phenyl-in; and (e) for free radical species. The relative
spin concentration ratio of group-in complexes: (c) [tert-butyl-in]/[free probe] = 1.27; and (d) [phenyl-in]/[free probe] = 0.433.
ESR spectra of probe 2 and 3 containing Calix-Sn and simulated spectra: (f) [2] = 2:0 ꢃ 10ꢁ4 mol dmꢁ3 and [Calix-S8] = 5:6 ꢃ
10ꢁ3 mol dmꢁ3; (g) [2] = 2:0 ꢃ 10ꢁ4 mol dmꢁ3 and [Calix-S6] = 1:5 ꢃ 10ꢁ2 mol dmꢁ3; and (h) [3] = 2:5 ꢃ 10ꢁ4 mol dmꢁ3 and
[Calix-S6] = 3:2 ꢃ 10ꢁ2 mol dmꢁ3
.
Table 1. The Association Constants K and Thermodynamic Parameters for Group-in Complexes of 1–3 at pH 6.9 in Water
K1 and K2/molꢁ1 dm3
290 K 300 K
44:6 ꢄ 0:8 33:5 ꢄ 0:6
ꢀH
/kJ molꢁ1
ꢀS
/J Kꢁ1 molꢁ1 /kJ molꢁ1
TꢀSc)
Probe Calixarenes Assignment
aN/mT aH/mT
1
1
2
2
3
Calix-S8
Calix-S8
Calix-S8
Calix-S6
Calix-S6
phenyl-in
1.659
1.691
1.674
1.679
1.685
0.619
1.270
0.448
0.428
0.786
ꢁ18:7 ꢄ 0:7 ꢁ33:0 ꢄ 2:4
ꢁ15:8 ꢄ 0:8 ꢁ14:2 ꢄ 2:8
ꢁ9:6
ꢁ4:1
ꢁ5:7
ꢁ7:4
ꢁ10:0
tert-butyl-in
cyclohexyl-in
cyclohexyl-in
cyclopentyl-in
131 ꢄ 3
105 ꢄ 3
58:6 ꢄ 1:8 43:1 ꢄ 1:4a) ꢁ15:5 ꢄ 1:1 ꢁ19:6 ꢄ 4:1
46:1 ꢄ 1:5 31:3 ꢄ 1:0a) ꢁ16:6 ꢄ 0:9 ꢁ25:6 ꢄ 3:2
12:2 ꢄ 0:4 10:3 ꢄ 0:3b) ꢁ16:3 ꢄ 0:3 ꢁ34:5 ꢄ 1:1
a) At 305 K. b) At 298 K. c) At 290 K.
arene cavity (see Fig. 1).8 The hfsc values of the tert-butyl-in
and phenyl-in complexes of probe 1 were estimated from com-
puter spectral simulation by combining three sets of triple-
doublet spectra (Fig. 3b) and are listed in Table 1. The ESR
spectrum of the group-in complex of probe 1 with Calix-S6 did
not separate from the free species (data not shown). Possible
reasons for this are: (1) Calix-S6 may not form the stable
group-in complexes with probe 1, and (2) the ESR spectra of
the possible group-in complexes may have heavily overlapped
with free species.
Analogs of nitroxide probes having various alkyl or cyclo-
alkyl groups (ethyl, propyl, butyl, cyclopentyl, and cyclohexyl)

Y. Sueishi et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 10 (2007) 1935
-
SO3
-OO3SS
-
-O3S
SO3
CH3
-O3S
-
CH3
SO3
-O S
-
O
3
O33S
SO
SO3
O
H
C
N
O
O
O
H
H3C O
H3C
H3C
O
H
O
H
O
CH3
H
O H
+
O
O
H
H
H
K1
K2
CH3
CH3
CH3
O
CH3
O
-O3S
-O3S
O
-O3S
-O3S
-
-
SO3
O
SO
CH3
H
C
-O3S
-
-
-O3S
H
CH33
CH
N
SO3
SO3
-O3S
H C
-
3
-
-
O
-
O
O3S
-
CN
SO
O
CH3
SO
3
3
O S
3
O3S
H3C3
H3C
O
O
K3
O
O
H
O
O
H
O
O
O
H
H
O
H
O
H
O
H
H
H
O
O
O
H
O
O
H
H
H
H
H
H
tert-Butyl-in
Fig. 4. The reaction scheme for group-in inclusion of nitroxide probe 1 with Calix-S8.
Phenyl-in
were synthesized and tested for the group-in complex forma-
tion with Calix-S8 and -S6. For straight chain alkyl groups,
in the ESR spectra group-in complexes were not found. How-
ever, probes 2 and 3 that contain cycloalkyl groups formed
cycloalkyl-in complexes (Figs. 3f–3h). These ESR spectra
were readily reproduced with computer spectrum simulation
(Fig. 3). The hfsc values calculated from the spectrum simula-
tion cycloalkyl-in complexes with Calix-S6 and -S8 are listed
in Table 1.
2
1
Inclusion equilibrium constants were calculated based on
the relative concentration of the complex and free species,
which were also determined with spectral simulation. The
equilibrium based on the reaction scheme for the group-in
complex formation of probe 1–3 with calixarenes shown in
Fig. 4 is expressed as follows:
0
0.005
0.010
0.015
[Calix-S8]0 / mol dm-3
Fig. 5. Relative abundance of group-in complex and free
radical probe 1, plotted as a function of the initial concen-
tration of Calix-S8: tert-butyl-in ( ) and phenyl-in (
complex formations.
½P-Calix-Snꢅ
K ¼
;
ð1Þ
)
½Probeꢅ½Calix-Snꢅ
where P-Calix-Sn denotes the group-in complex. Under the
condition of [Calix-Sn] ꢆ [probe], Eq. 1 can be rewritten as
follows:
equilibrium (K) for group-in complexation. Figure 6 shows
the ESR spectra of probe 1 and 2 in the presence of excess
Calix-S8 at various temperatures. With an increase in the tem-
perature, the ESR signal intensities of group-in complexes
(tert-butyl-in and ꢁ-substituent-in) decreased, and those of free
(non-complexed) probes increased. Figure 7 shows the plots
of ln K (inclusion equilibrium constant) against Tꢁ1 (a van’t
Hoff plot). From the slope and intercept of the plots, thermo-
dynamic parameters (ꢀH and ꢀS) were estimated and are
given in the right-hand side of Table 1. All of the values of
ꢀH are similar, and the values of ꢀS were negative in the
½P-Calix-Snꢅ
½Probeꢅ
¼ K½Calix-Snꢅ0;
ð2Þ
where [Calix-Sn]0 denotes the initial concentration of calixar-
enes. Thus, the inclusion equilibrium constants (K) can be cal-
culated from plots of the relative abundance of free probe and
group-in complex against the initial concentration of calixar-
enes. In Fig. 5, a plot of [P-Calix-S8]/[Probe] vs. [Calix-S8]0
for probe 1 gave a straight line with the slope K, which passed
through the origin. Equilibrium constants (K1 and K2) obtained
for the group-in complexes of probe 1–3 with Calix-Sn are
listed in Table 1.
range of ꢁ14– ꢁ35 J Kꢁ1 molꢁ1
.
The comparison of ꢀH with TꢀS is informative for the in-
clusion mechanism. The relatively large negative values of
ꢀH for the group-in complexation as compared to TꢀS are
indicative that group-in complex formation is favored by
Temperature Dependence of Group-in Inclusion. We
have examined the temperature dependences of the inclusion

1936 Bull. Chem. Soc. Jpn. Vol. 80, No. 10 (2007)
Group-Inclusion Equilibrium in Water Soluble Calixarene
(a)
at 283 K
5
4
at 290 K
3
4
at 298 K
1 mT
(b)
3
2
at 290 K
at 298 K
at 305 K
3.3
3.4
3.5
-1 / 10-3K-1
3.6
T
Fig. 7. Plots of ln K against Tꢁ1 for group-in equilibrium
of probes 1–3. tert-Butyl-in ( ) and phenyl-in ( ) com-
plex formation of probe 1 with Calix-S8. Cyclohexyl-in
complex formation of probe 2 with Calix-S8 ( ) and
Calix-S6 ( ). ( ) cyclopentyl-in complex formation of
probe 3 with Calix-S6.
various pressures are listed in Table 2. The K values increased
as a function of increasing external pressure.
Pressure dependence data can be analyzed with following
formulations. The change in the volume accompanied by the
group-in complex formation (reaction volume ꢀV) was calcu-
lated according to the following equations:
Fig. 6. ESR spectra of probes 1 and 2 in the presence of
Calix-S8 at various temperatures: (a) [1] = 1:0 ꢃ 10ꢁ4
mol dmꢁ3 and [Calix-S8] = 9:7 ꢃ 10ꢁ3 mol dmꢁ3; and (b)
[2] = 2:5 ꢃ 10ꢁ4 mol dmꢁ3 and [Calix-S8] = 5:6 ꢃ 10ꢁ3
mol dmꢁ3. Open arrows and closed arrows are assigned
to the group-in complexes and free species, respectively.
ln K ¼ aP þ b;
ð3Þ
ð4Þ
ꢀ
ꢁ
enthalpic contributions and disfavored by entropic contribu-
tions. The major contribution to enthalpy of inclusion most
likely comes from the van der Waals interactions, because
probes 1–3 are neutral and electrostatic interaction can be
ignored. In the study of the complexation mechanism of p-
sulfonatocalixarenes with dipeptides or tripeptides, Douteau-
Guevel et al. have suggested that the binding process for the
alkyl chain is controlled by the favorable enthalpy resulting
from the inclusion of the apolar part of the guest into the hy-
drophobic cavity of host through van der Waals interactions.11
This is in agreement with our observation for the apolar group
inclusion by p-sulfonatocalixarenes.
@ ln K
ꢀV ¼ ꢁRT
ꢁꢂT RT;
@P
T
where ꢂT is the thermal compressibility of the solvent12 and
ꢂT ¼ 4:43 ꢃ 10ꢁ5 barꢁ1. In the group-in complex formation of
calixarenes, all probes showed negative ꢀV values (Table 2).
ꢀV that is accompanied by chemical reaction can be divided
into two terms: 1) an intrinsic volume change, and 2) a volume
change arising from reorganization of the solvent molecules.
Similarly, the volumetric contributions during formation of
an inclusion complex with calixarenes can be expressed by
two discrete volume changes:
Effects of Pressure on Group-in Inclusion.
Figure 8
ꢀV ¼ ꢀVinclu þ ꢀVdesolv
;
ð5Þ
shows ESR spectra of probes 1–3 in the presence of Calix-S8
and -S6 at high pressures. When a static pressure of 490 bar
(1 bar = 1 ꢃ 105 Pa) was applied to the solution, the ESR
spectral intensities of group-in complexes increased. These
ESR spectra were reproduced using computer simulation by
adjusting relative abundance of the two or three radical spe-
cies, but keeping the hfs constants the same. Inclusion equilib-
rium constants (K) for the group-in complex of probes 1–3 at
where ꢀVinclu denotes volume changes related to inclusion of
guest molecules in the calixarene cavity (ꢀVinclu < 0), and
ꢀVdesolv denotes accompanying desolvation around the guest
and host molecules (ꢀVdesolv > 0). ꢀVdesolv for inclusion was
assumed to be small, because probe 1–3 are neutral. Therefore,
the reaction volume observed for group-in complexation corre-
sponds to the change in the intrinsic volume. Unlike in the case

Y. Sueishi et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 10 (2007) 1937
at 1 bar
at 490 bar
(a)
(b)
at 490 bar
at 1 bar
(c)
(d)
at 1 bar
at 490 bar
at 490 bar
at 1 bar
Fig. 8. ESR spectra of probes 1–3 in the presence of excess Calix-Sn under 1 and 490 bar at 290 K. (a) [1] = 2:5 ꢃ 10ꢁ4 mol dmꢁ3
and [Calix-S8] = 8:0 ꢃ 10ꢁ3 mol dmꢁ3. (b) [2] = 2:5 ꢃ 10ꢁ4 mol dmꢁ3 and [Calix-S8] = 7:0 ꢃ 10ꢁ3 mol dmꢁ3. (c) [2] = 2:0 ꢃ
10ꢁ4 mol dmꢁ3 and [Calix-S6] = 1:5 ꢃ 10ꢁ2 mol dmꢁ3. (d) [3] = 2:5 ꢃ 10ꢁ4 mol dmꢁ3 and [Calix-S6] = 3:2 ꢃ 10ꢁ2 mol dmꢁ3
.
Open arrows and closed arrows are assigned to the group-in complexes and free radical species, respectively.
Table 2. The Association Constants K under External Pressures and Reaction Volumes for Group-in Complexes of 1–3
at 290 K
K1 and K2/molꢁ1 dm3
ꢀV
Probe Calixarenes Assignment
pH
/cm3 molꢁ1
1 bar
98 bar
294 bar
490 bar
1
1
2
2
2
2
3
Calix-S8
Calix-S8
Calix-S8
Calix-S8
Calix-S6
Calix-S6
Calix-S6
phenyl-in
6.9 44:6 ꢄ 0:8 48:7 ꢄ 0:8 55:6 ꢄ 1:0 60:9 ꢄ 1:1 ꢁ16:3 ꢄ 2:0
tert-butyl-in
cyclohexyl-in
cyclohexyl-in
cyclohexyl-in
cyclohexyl-in
cyclopentyl-in
6.9
6.9 58:6 ꢄ 1:8 59:7 ꢄ 1:5 61:5 ꢄ 1:6 63:6 ꢄ 1:9
131 ꢄ 3
135 ꢄ 3
148 ꢄ 5
156 ꢄ 4
ꢁ10:0 ꢄ 1:5
ꢁ5:1 ꢄ 1:1
12.5 34:8 ꢄ 1:5 36:5 ꢄ 1:2 39:4 ꢄ 1:5 44:6 ꢄ 1:7 ꢁ13:0 ꢄ 1:7
6.9 46:1 ꢄ 1:5 47:0 ꢄ 1:5 48:6 ꢄ 1:1 50:3 ꢄ 1:5
12.5 31:4 ꢄ 1:3 33:4 ꢄ 1:3 35:9 ꢄ 1:8 40:0 ꢄ 1:5 ꢁ12:5 ꢄ 1:6
6.9 12:2 ꢄ 0:4 12:5 ꢄ 0:5 13:6 ꢄ 0:5 14:2 ꢄ 0:6 ꢁ9:1 ꢄ 1:5
ꢁ5:4 ꢄ 0:9
of cyclodextrin inclusion complex, ꢀV (ꢂꢀVinclu) for probes 2
and 3 with Calix-S6 and -S8 (ꢁ5– ꢁ9 cm3 molꢁ1, at pH 6.9)
are insensitive to the size of guests.9 The large negative ꢀV
for the phenyl-in complex formation of probe 1 by Calix-S8
indicates deep penetration from the phenyl group side into
the Calix-S8 cavity, which may also be responsible for the
large negative value of ꢀS. This explains the small aN value
of the phenyl-in complex, because aN in nitroxide radicals be-
comes larger in a polar environment.13 In the group-in com-
plex of probe 1, Calix-S8 favors the inclusion from the tert-
butyl side than the phenyl side, probably because the inclusion
from the tert-butyl side forces N–O group to interact with
polar sulfonato groups, resulting in the increase in aN
(Table 1). The small negative ꢀV for the tert-butyl-in com-
plex formation as compared with that of the phenyl-in complex
indicates shallow inclusion from the tert-butyl side, which may
be also responsible for the small negative value of ꢀS.
Based on the reaction scheme in Fig. 4, the inclusion equi-
librium between tert-butyl-in and phenyl-in complexes of
probe 1 can be expressed as:
½tert-butyl-inꢅ
½phenyl-inꢅ
K3 ¼
;
ð6Þ
where K3 denotes the equilibrium constant between group-in
complexes. The value of K3 was calculated to be 2.9 at 290
K using the data in Table 2. The volume change ꢀV3 of the
group-in complexes (tert-butyl-in and phenyl-in) of probe 1
with Calix-S8 was evaluated as follows:
ꢀV3 ¼ ꢀVtert-butyl-in ꢁ ꢀVphenyl-in ¼ 6:3 cm3 molꢁ1: ð7Þ

1938 Bull. Chem. Soc. Jpn. Vol. 80, No. 10 (2007)
Group-Inclusion Equilibrium in Water Soluble Calixarene
The pH dependence in hydrogen hfs (aH) occurs probably
because the group inclusion of the group causes a slightly dif-
ferent magnitude of torsion at the N–ꢁ-C bond.16 In the group
inclusion of probe 2 with ꢃ-cyclodextrin (CD), the aH value
(¼0:48 mT) for the cyclohexyl-in complex was larger (¼0:28
mT) than ꢄ-CD with a larger cavity.9 The increase in the aH
values for group-in complexes of calixarene with an increase
in the pH suggests that the size of the Calix-Sn cavity diameter
(upper rim of sulfonato group side) decreases in highly basic
media, resulting in deeper encapsulation into the Calix-Sn
cavity. We speculate that the decrease in the portal size of
Calix-Sn is responsible for the smaller K values. Deep inclu-
sion causes the larger negative intrinsic volume change, which
is indicated by large negative ꢀV’s.
Free
(a)
Complex
pH 12.5
pH 6.9
Complex
Free
(b)
Fig. 9. ESR spectra of probes 2 in the presence of excess
Calix-S8 at pH (a) 12.5 and (b) 6.9: [2] = 1:5 ꢃ 10ꢁ4
mol dmꢁ3 and [Calix-S8] = 7:45 ꢃ 10ꢁ3 mol dmꢁ3
.
In conclusion, we determined basic thermodynamic and pH
dependent properties of newly found group-inclusion complex
of calixarene. In particular, the high-pressure studies provided
useful insights for the functional group-recognition of calix-
arenes.
This ꢀV3 value shows that with an increasing external pres-
sure, the group inclusion equilibrium shifts to the phenyl-in
complex side.
pH Dependence of Group-in Inclusion. Shinkai et al.
have reported that the pKa values of the OH groups in Calix-
S6 and -S8 are pK1 < 1, pK2 ¼ 3:0, pK3 ¼ 4:0, pK4 > 11 for
Calix-S6 and pK1 ¼ 3:69, pK2 ¼ 4:34, pK3 ¼ 8:04 for Calix-
S8.14,15 Therefore, in Calix-S6 and -S8, three and two of the
OH groups are dissociated at the neutral pH region (pH 6.9),
respectively. We determined the effects of the H-dissociated
oxyanions (lower rim depicted in Fig. 2) on the group-inclu-
sion of p-sulfonatocalixarenes by varying the pH of the sol-
vent. In very acidic medium, such as pH 2.0, the instability
of probes 1–3 hampered the experiment. In very basic medium
(pH 12.5), a group-in complex of probe 1 was not observed,
but those of probes 2 and 3 were observed. Therefore, we per-
formed the ESR measurements of group-in complexes of probe
2 with Calix-S6 and -S8 at pH 12.5.
References
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C. D. Gutsche, in Monographs in Suparmolecular Chemis-
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Figure 9 show the ESR spectrum of probes 2 in the presence
of excess Calix-S8 at pH 12.5 and 6.9. The ESR spectral inten-
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jꢀV ðꢂ ꢁ13 cm3 molꢁ1Þj at pH 12.5 was large as compared
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(pH 12.5), four H-dissociated oxyanions are situated on the
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