Alizarin yellow-modified β-cyclodextrin as a guest-responsive absorption change sensor

Article abstract of DOI:10.1021/ac960727z

Alizarin yellow-modified β-cyclodextrin (ACD), in which alizarin yellow is linked to β-cyclodextrin via an ethylenediamine spacer, was synthesized as a new absorption change indicator for molecules. Alizarin yellow is a pH indicator mat exhibits absorption peaks at 360 and 480 nm in the neutral region and in the alkaline region, respectively, with pK_a = 10.98 for an equilibrium between two forms. ACD has two parts to be deprotonated: one is the phenolic hydroxyl group of alizarin yellow residue and the other is the secondary amine group of the spacer. ACD exhibits pH dependency very different from that of alizarin yellow. We obtained two pK_a values, 4.88 (pK_a1) and 8.89 (pK_a2), for ACD by pH titration of its absorption intensity. The pK_a1 and pK_a2 values were suggested to be the pK_a values of the phenolic hydroxyl group of alizarin yellow residue and the secondary amine group, respectively. Upon addition of guest species, the pK_a1 and pK_a2 values shifted to 5.11 and 7.56, respectively, indicating a larger shift in the pK_a for the amine group than for the hydroxyl group. The guest-induced pK_a shift in the alkaline region suggests that deprotonation of the amine group of ACD occurs when the alizarin yellow moiety is excluded from the cyclodextrin cavity associated with guest accommodation and exposed to an alkaline environment The sensitivities of this host to various guests were examined by absorption changes at 475 nm in pH 8.3 phosphate buffer, and the order of the sensitivities was found to be adamantane derivatives > borneol > bile acids. This order is not parallel with that of the binding constants, suggesting that the structural features of the host-guest complexes are important. All these results demonstrate that ACD can be used as an effective chemosensor for molecules.

Full text of DOI:10.1021/ac960727z

Anal. Chem. 1997, 69, 659-663
Alizarin Yellow-Modified â-Cyclodextrin as a
Guest-Responsive Absorption Change Sensor
Taiyo Aoyagi, Asao Nakamura, Hiroshi Ikeda, Tsukasa Ikeda, Hisakazu Mihara, and Akihiko Ueno*
Department of Bioengineering, Faculty of Bioscience and Biotechnology, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
Alizarin yellow-modified â-cyclodextrin (ACD), in which
alizarin yellow is linked to â-cyclodextrin via an ethylene-
diamine spacer, was synthesized as a new absorption
change indicator for molecules. Alizarin yellow is a pH
indicator that exhibits absorption peaks at 3 6 0 and 4 8 0
nm in the neutral region and in the alkaline region,
respectively, with pKa ) 10.98 for an equilibrium between
two forms. ACD has two parts to be deprotonated: one
is the phenolic hydroxyl group of alizarin yellow residue
and the other is the secondary amine group of the spacer.
ACD exhibits pH dependency very different from that of
Figure 1. Structures of alizarin yellow and ACD.
charides consisting of six or more members of
alizarin yellow. We obtained two pKa values, 4 .8 8 (pKa1
)
and 8 .8 9 (pKa2 ), for ACD by pH titration of its absorption
intensity. The pKa1 and pKa2 values were suggested to
be the pKa values of the phenolic hydroxyl group of alizarin
yellow residue and the secondary amine group, respec-
tively. Upon addition of guest species, the pKa1 and pKa2
values shifted to 5 .1 1 and 7 .5 6 , respectively, indicating
a larger shift in the pKa for the amine group than for the
hydroxyl group. The guest-induced pKa shift in the
alkaline region suggests that deprotonation of the amine
group of ACD occurs when the alizarin yellow moiety is
excluded from the cyclodextrin cavity associated with
guest accommodation and exposed to an alkaline environ-
ment. The sensitivities of this host to various guests were
examined by absorption changes at 4 7 5 nm in pH 8 .3
phosphate buffer, and the order of the sensitivities was
found to be adamantane derivatives > borneol > bile
acids. This order is not parallel with that of the binding
constants, suggesting that the structural features of the
host-guest complexes are important. All these results
dem onstrate that ACD can be used as an effective
chemosensor for molecules.
D
-(+)-glucopyra-
nose units and form inclusion complexes with a variety of organic
compounds in aqueous solution,6-8 may be useful for constructing
such molecule-sensing systems.9-15 Recently, we have shown that
methyl red-modified cyclodextrin (MRCD) exhibits a guest-
responsive color change, and it works as a chemosensor for
detecting organic species. Since the color change of MRCD
occurs only in the acidic solution, we have attempted to find other
CD systems that work in solutions of different pH regions, and
previously we prepared p-nitrophenol-modified CD as the sensor
that exhibits color change in neutral solution.16-19 Here, we report
an alizarin yellow-modified â-CD (ACD) as one example of such
CD systems, since ACD changes its absorption in alkaline solution
(Figure 1). In ACD, the alizarin yellow moiety is covalently linked
to â-CD through an ethylenediamine linkage, and we found that
the guest-responsive absorption change arises not from the simple
dissociation of the hydroxyl group of alizarin yellow moiety but
from the protonation-deprotonation equilibrium shift of the amine
group of the ethylenediamine part.
(6) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; Springer-Verlag: New
York, 1977.
During the past decade, much effort has been devoted to
developing analysis systems with which chemical and biological
compounds can be detected without any chemical modification
of analytes. Absorption and fluorescence changes of appropriately
designed hosts associated with guest binding are important in
this respect.1-5 Cyclodextrins (CDs), which are cyclic oligosac-
(7) Wenz, G. Angew. Chem., Int. Ed. Engl. 1 9 9 4 , 33, 803.
(8) Szejtli, J. Cyclodextrin Technology; Kluwer: Dordrecht, 1988.
(9) Hamasaki, K.; Ikeda, H.; Nakamura, A.; Ueno, A.; Suzuki, I.; Osa, T. J. Am.
Chem. Soc. 1 9 9 3 , 115, 5035.
(10) Hamasaki, K.; Ueno, A.; Toda, F. J. Chem. Soc., Chem. Commun. 1 9 9 3 ,
331.
(11) Hamasaki, K.; Ueno, A.; Suzuki, I.; Toda, F. Bull. Chem. Soc. Jpn. 1 9 9 4 ,
67, 516.
(12) Ueno, A.; Suzuki, I.; Osa, T. Anal. Chem. 1 9 9 0 , 62, 2461.
(13) Ueno, A.; Chen, Q.; Suzuki, I.; Osa, T. Anal. Chem. 1 9 9 2 , 64, 1650.
(14) Ueno, A.; Minato, S.; Osa, T. Anal. Chem. 1 9 9 2 , 64, 1154.
(15) Ueno, A.; Minato, S.; Osa, T. Anal. Chem. 1 9 9 2 , 64, 2562.
(16) Ueno, A.; Kuwabara, T.; Nakamura, A.; Toda, F. Nature 1 9 9 2 , 356, 136.
(17) Ueno, A. Adv. Mater. 1 9 9 3 , 5, 132.
(1) Czarnik, A., Ed. Fluorescent Chemosensors for Ion and Molecule Recognition;
ACS Symposium Series 538; American Chemical Society: Washington, DC,
1992.
(2) James, T. D.; Sandanayake, K. R. A. S.; Shinkai, S. Supramol. Chem. 1 9 9 5 ,
6, 141.
(3) Vance, D. H.; Czarnik, A. W. J. Am. Chem. Soc. 1 9 9 4 , 116, 9397.
(4) Aoki, I.; Kawahara, Y.; Sakaki, T.; Harada, T.; Shinkai, S. Bull. Chem. Soc.
Jpn. 1 9 9 3 , 66, 927.
(18) Kuwabara, T.; Matsushita, A.; Nakamura, A.; Ueno, A.; Toda, F. Chem. Lett.
1 9 9 3 , 2081.
(19) Kuwabara, T.; Nakamura, A.; Ueno, A.; Toda, F. J. Phys. Chem. 1 9 9 4 , 98,
6297.
(5) Cramer, F.; Saenger, W.; Spatz, H.-Ch. J. Am. Chem. Soc. 1 9 6 7 , 89, 14.
S0003-2700(96)00727-5 CCC: $14.00 © 1997 American Chemical Society
Analytical Chemistry, Vol. 69, No. 4, February 15, 1997 659

Figure 2. Two-step deprotonation equilibria of ACD.
EXPERIMENTAL SECTION
measurements, the temperature of the cell was kept at 25 °C, and
the ACD concentration was 0.05 mM. The pH of the solution
was measured with a pH meter (TOA HM60-S), calibrated at room
temperature with pH standard solutions of pH 4.01 ( 0.01, 6.86
( 0.01, and 9.22 ( 0.01. Hydrochloric acid was used to set the
pH in the acid region, and sodium hydroxide solution was used
in the alkaline region. Phosphate buffer of pH 8.3 was used for
measurements of sensitivity parameters and binding constants.
In this system, there are two equilibria involving three species,
I, II, and III, as shown in Figure 2. The acid-base dissociation
constants, Kal and Ka2, are defined by the following equations:
Materials. â-CD was kindly donated by Nihon Shokuhin Kako
Co., Ltd., and was used as received. All the guest compounds
were purchased from Tokyo Kasei and were used without further
purification.
Synthesis. (i) 6 -Deoxy-6 -[(2 -aminoethyl)amino]-â-CD
(CDen). The synthesis of 6-O-6-(p-tolylsulfonyl)-â-CD (Ts-CD)
was previously reported.20 Ts-CD (1.5 g, 1.16 mmol) was dissolved
in N,N-dimethylformamide (DMF, 8 mL), and this solution was
added dropwise to a mixed solution of ethylenediamine (15 mL,
0.224 mol) and DMF (10 mL). The resultant solution was stirred
for 2.5 h at 65 °C. The reaction mixture was then poured into
acetone (700 mL) to form a precipitate, and the precipitate was
collected by filtration. The precipitate was dissolved in water (15
mL), and this solution was poured into acetone (500 mL) to form
precipitate again. The precipitate was collected by filtration and
dissolved in water (20 mL). After filtration, ion-exchange column
chromatography on Sephadex CM-25 with 1N-NH3 aqueous
solution afforded the fractions that contain 6-deoxy-6-[(2-amino-
ethyl)amino]-â-CD (CDen). The fractions were concentrated on
a rotary evaporator and then lyophilized to give the white powder
of CDen (890 mg, 0.756 mmol, 65%).
Kal ) [II][H+]/ [I]
(1)
(2)
Ka2 ) [III][H+]/ [II]
Letting ꢀ+ and ꢀ- be the molar extinction coefficients of totally
protonated (I) and deprotonated (III) forms of ACD, respectively,
and letting ꢀ0 be the molar extinction coefficient of ACD when
the amino group is protonated and the phenolic hydroxyl group
is deprotonated (II), the absorption intensity of our sensory
system is
(ii) Alizarin Yellow-Modified â-CD (ACD). A solution of
CDen (200 mg, 0.170 mmol), alizarin yellow (60 mg, 2.09 mmol),
dicyclohexylcarbodiimide (DCC, 53 mg, 2.57 mmol), and 1-hy-
droxybenzotriazole (HOBt, 34 mg, 2.52 mmol) in DMF (8 mL)
was stirred for 3 h at 0 °C and subsequently for 10 h at room
temperature. Insoluble materials were removed by filtration. The
solution was poured into acetone (300 mL), and the resulting
precipitate was collected by filtration. The precipitate was dis-
solved in dimethyl sulfoxide (5 mL) and reprecipitated by pouring
into acetone (300 mL). This reprecipitation from acetone was
repeated until no alizarin yellow was observed by TLC. The
product was collected and dried in vacuo at 80 °C to give the red
powder of ACD (90 mg, 0.062 mmol, 37%). 1H NMR (500 MHz,
DMSO-d6): δ 2.90-4.00 (m), 4.50-4.75 (m, 6H, O6H), 4.80-5.00
(m, 7H, C1H), 5.60-5.90 (m, 14H, O2H, O3H), 6.50-6.60 (s, 1H,
aromatic), 7.74-7.82 (d, 1H, aromatic), 7.84-7.92 (d, 2H, aro-
matic), 8.29-8.36 (d, 2H, aromatic), 8.41-8.45 (d, 1H, aromatic).
Anal. Calcd for C57H83O38N5‚9H2O: C, 42.62; H, 6.32; N, 4.36.
Found: C, 42.90; H, 6.13; N, 3.99.
Iobs ) ꢀ+[I] + ꢀ0[II] + ꢀ-[III]
(3)
where Iobs is the absorbance at 500 nm under the conditions of
path length of 1 cm.
Here, the total ACD concentration, [ACD]0, is the sum of the
concentrations of the three species,
[ACD]0 ) [I] + [II] + [III]
(4)
If the absorbances for I, II, and III are abbreviated as I+, I0, and
I-, respectively, the absorbances are expressed as
ꢀ+[ACD]0 ) I+
ꢀ0[ACD]0 ) I0
ꢀ-[ACD]0 ) I-
(5)
(6)
(7)
Measurements. Absorption spectra were recorded on a
Shimadzu UV-3100 spectrophotometer. During the spectroscopic
(20) Ueno, A.; Moriwaki, F.; Osa, T.; Hamada, F.; Murai, K. Chem. Pharm. Bull.
1 9 8 6 , 14, 438.
From eqs 1-7, Iobs is expressed by the following equation:
660 Analytical Chemistry, Vol. 69, No. 4, February 15, 1997

I
- + ([H+]/ Ka2)I0 + ([H+]2/ Ka1Ka2)I+
1 + ([H+]/ Ka2) + ([H+]2/ Ka1Ka2)
Iobs
)
(8)
The pKa values were estimated by curve-fitting analysis using eq
8.
The binding constants were estimated as follows. Equation 9
describes the complexation equilibrium, where ACD, G, and
ACD + G a ACD-G
(9)
Figure 3. Absorption spectra of alizarin yellow (150 µM) at various
pH values.
ACD-G represent free host, free guest, and host-guest complex,
respectively. This means that ACD forms only a 1:1 complex with
a guest. The binding constant of ACD for a guest, K, is estimated
by fitting the following equation to the data obtained:19
∆I
) (K[G]0 + 1 + K[ACD]0 -
∆Imax
(K[G] + 1 + K[ACD] )2 - 4K2[ACD] [G] )/ 2K[ACD]
x
0
0
0
0
0
(10)
where [G]0 represents the initial concentration of guest, ∆I
represents the difference of absorption intensity at 475 nm for
ACD alone and ACD in the presence of guest, and ∆Imax is its
value when ACD exists totally as the inclusion complex.
Figure 4. Absorption spectra of ACD (50 µM) at various pH values.
RESULTS AND DISCUSSION
pH Dependence in Absorption Spectra of ACD. Alizarin
yellow, which is a pH indicator, is known to change its color from
yellow to red when the solution becomes basic. This color change
is associated with the dissociation of the phenolic hydroxyl group
in the alkaline solution, forming a phenolate form.21-23 Figure 3
shows the absorption spectra of alizarin yellow at various pH
values. The absorption intensity at 360 nm decreases, while that
at 500 nm increases with increasing pH of the solution. This
observation reflects the fact that the phenol type, which exists in
the acid or in neutral solution, shows yellow color with an
absorption around 360 nm, while the phenolate form, which exists
in the alkaline solution, shows red color with an absorption around
500 nm. All the spectra exhibit isosbestic points (280, 315, and
420 nm), demonstrating that alizarin yellow is in equilibrium
between the two forms, and the analysis of the pH dependency
of its absorption at 500 nm gave pKa ) 10.98.
Figure 4 shows the absorption spectra of ACD measured at
various pH values. The spectral variation associated with the
change in the pH value seems essentially the same as that of
alizarin yellow. However, the pH values where the spectral
variation occurs are quite different, as shown by the absorption
around 500 nm in the acidic region. Another observation is that
isosbestic points are not clear, suggesting the existence of two
equilibria. It is surprising that the absorption around 500 nm of
ACD appears even in the acidic region below pH 6.0. Therefore,
it is obvious that there exists a somewhat different deprotonation
scheme, which cannot be seen for alizarin yellow itself. Consider-
Figure 5. pH dependency of the absorption intensities of ACD alone
(O, 50 µM) and in the presence of 1-adamantanol (0, 1 mM) at 500
nm.
ing that the phenol and phenolate forms show absorptions around
360 and 500 nm, respectively, the absorption change of ACD
occurring in the acidic region should be caused by the dissociation
of the hydroxyl group, and that in the alkaline region must be
caused by the deprotonation of the amine group. Figure 2 shows
the scheme of this equilibrium for ACD, in which the species I,
II, and III are included and Ka1 and Ka2 represent dissociation
constants. Ka1 and Ka2 are the values for the phenolic hydroxyl
group of alizarin yellow residue and the secondary amine group
located near the CD cavity, respectively. Figure 5 shows the
absorption intensities of ACD at 500 nm as a function of pH for
ACD alone and in the presence of 1-adamantanol as a guest. From
the data of ACD alone, pKa1 and pKa2 were estimated to be 4.88
and 8.92, respectively, by fitting the theoretical equation to the
data obtained. The result indicates that there is a large difference
in the pKa values of the phenolic hydroxyl group between alizarin
yellow (10.98) and ACD (4.88). This may be related to their
structural differences. Alizarin yellow has a carboxyl group at
the ortho position of the hydroxyl group, but the carboxyl group
was changed to the amide bond in ACD. This structural difference
(21) Issa, I. M.; Issa, R. M.; Mahmoud, M. R.; Temerik, Y. M. Z. Phys. Chem.
Leipzig 1 9 7 3 , 253, 289.
(22) Issa, I. M.; Issa, R. M. M. R.; Temerik, Y. M.; Mahmoud, M. R. Electrochim.
Acta 1 9 7 3 , 18, 139.
(23) Gustavo Gonzalez, A.; Angeles Herrador, M.; Agustin, G. A. Anal. Chim.
Acta 1 9 9 1 , 246, 429.
Analytical Chemistry, Vol. 69, No. 4, February 15, 1997 661

Table 2. Sensitivity and Binding Parameters of ACD
for Various Guestsa
K
C0.01
∆Imax (µM)
guest
∆I/ I0 (M-1
)
(1) chenodeoxycholic acid (CDCA) 0.091
9180 0.102
16.8
7.0
234.1
112.8
8.0
(2) ursodeoxycholic acid (UDCA)
(3) cholic acid (CA)
(4) deoxycholic acid (DCA)
(5) hyodeoxycholic acid (HDCA)
(6) 1-adamantanol (1-ADA)
(7) 2-adamantanol
0.099 93200 0.088
0.021
0.035
0.094 66600 0.084
0.192 8160 0.214
0.226 10300 0.242
682 0.074
1360 0.079
8.3
6.3
(8) 1-adamantanecarboxylic acid
(9) (+)-menthol
0.184
0.052
0.051
0.035
0.032
0.136
0.134
0.005
0.029
0.029
0.027
9240 0.189
1010 0.138
918 0.142
460 0.168
393 0.178
2530 0.205
2450 0.204
8.7
81.3
86.0
141.3
154.1
22.7
23.5
(10) (-)-menthol
(11) (+)-camphor
Figure 6. Absorption spectra of ACD (50 µM), alone and in the
presence of 1-adamantanol (1-ADA), at pH 8.3.
(12) (-) camphor
(13) (+)-borneol
(14) (-)-borneol
(15) cyclohexanol
266 0.037 1408.0
Table 1. pKa Values of ACD Alone, ACD in the
Presence of 1-Adamantanol (1-ADA), and Alizarin
Yellowa
(16) cyclooctanol
(17) nerol
(18) geraniol
364 0.179
289 0.199
178 0.296
165.6
185.5
198.2
pKa1
pKa2
a
Measured at 25 °C at pH 8.3. ∆I, guest-induced increase of
absorption intensity of ACD; I0, absorption intensity of ACD alone (λ
) 475 nm). The concentration of ACD was 50 µM. The concentration
of guests was 500 µM. K is binding constant. ∆Imax is the ∆I value
when ACD exists as a complex. C0.01 is the guest concentration where
ACD gives 0.01 as the ∆I value.
ACD
ACD + 1-ADA
alizarin yellow
4.88
5.13
8.92
7.59
10.98b
a
[ACD], 50 µM; [1-ADA], 1 mM; and [alizarin yellow], 150 µM.
b pKa 10.69 in ref 24.
guests, chenodeoxycholic acid (CDCA), ursodeoxycholic acid
(UDCA), deoxycholic acid (DCA), and hyodeoxycholic acid
(HDCA) are isomers with one hydroxyl group attached to different
carbons or with one hydroxyl group attached to the same carbon
but stereochemically inverted. Cholic acid (CA) has one more
hydroxyl group than these steroids. The order of K for the
steroidal compounds is UDCA > HDCA > CDCA > DCA > CA,
and it is remarkable that there exist large differences among the
isomeric steroids UDCA, HDCA, CDCA, and DCA, ranging from
93 200 M-1 for UDCA to 1360 M-1 for DCA. CA has one
at the ortho position may be the main reason for the large
difference in pKa between the two compounds. Figure 6 shows
the absorption variations of ACD in pH 8.3 phosphate buffer
induced by 1-adamantanol, which is well known to fit the cavity
of the â-CD. There occur progressive red shifts in the band
maximum and increases in the absorption intensity with increasing
concentrations of 1-adamantanol. Isosbestic points are seen at
295, 340, and 440 nm. The pH dependency of the absorption at
500 nm in the presence of 1-adamantanol (1 mM) is shown in
Figure 5. From the analysis of the pH dependency, pKa1 and pKa2
were estimated to be 5.13 and 7.59, respectively. The result
indicates that pKa shifts with the guest addition, from 4.88 to 5.13
for pKa1 and from 8.92 to 7.59 for pKa2 (Table 1). The shift in
pKa1 means that the dissociation of the phenolic hydroxyl group
is depressed, and that in pKa2 means that deprotonation of the
amine group is facilitated in ACD. This may arise from the
environmental change associated with the conformational change
caused by the guest inclusion. The effect of the guest on pKa is
larger for pKa2 than for pKa1, indicating that the amine group is
more influenced than the hydroxyl group by the guest. As is the
case for 1-adamantanol addition to the ACD system, we observed
remarkable differences in the absorption intensity around 500 nm
upon addition of various guests. Therefore, we used ACD as a
new absorption change sensor for detecting molecules.
additional hydroxyl group and gives a smaller K value of 682 M-1
,
reflecting its less hydrophobic nature. The three adamantane
derivatives give the K values of about 10 000 M-1, these large
values being consistent with the well-known fact that adamantane
derivatives are good guests for â-CD. The order of K values for
monoterpenes is borneol > menthol > camphor > nerol >
geraniol, ranging from 2530 M-1 for (+)-borneol to 178 M-1 for
geraniol, with a negligible or slight preference of (+)-enantiomer
for the former three monoterpenes and cis preference of 1.62 for
the latter two open-chain monoterpenes. The K values were also
examined for cyclooctanol and cyclohexanol as guests, and the
result indicates that cyclooctanol is a better guest than cyclohex-
anol.
Sensitivity and Selectivity. The sensitivity of this system is
shown by ∆I/ I0, where ∆I ) I - I0 and I0 and I are the absorption
intensities at 475 nm of ACD in the absence and presence of a
guest, respectively, measured under the conditions of pH 8.3
phosphate buffer.
The data of ACD (0.05 mM) for 18 guests (0.5 mM) are
summarized in Table 2. For steroidal guests, the order of ∆I/ I0
for steroidal guests is UDCA (0.099) > HDCA (0.094) > CDCA
(0.091) > DCA (0.035) > CA (0.021), parallel to the order of their
binding constants. The order for monoterpenes is also parallel
to the order of their binding constants. However, detailed
examination of the data reveals that the ∆I/ I0 values are not totally
Binding Constants. The binding constants (K) of ACD were
determined from the guest-induced absorption variations at 475
nm for 18 guests based on 1:1 complexation stoichiometry. The
K values for these guests are summarized in Table 2, together
with sensitivity parameters. The selected guests are five steroids,
three adamantane derivatives, six cyclic terpenes, two acylic
terpenes, and two cyclic alkanols. Their structures are shown in
Figure 7 with abbreviations for several guests. Among steroid
(24) Yoshida, N.; Fujimoto, M. Bull. Chem. Soc. Jpn. 1 9 8 2 , 55, 1039.
662 Analytical Chemistry, Vol. 69, No. 4, February 15, 1997

Figure 7. Structures of guests used in this study.
related to the structures of the complexes. One possible explana-
tion is that steroidal guests may not be inserted enough into the
cavity, resulting in partial exclusion of the alizarin yellow moiety
from the cavity. Figure 8 also indicates the concentration ranges
for the guests to be detected. The limit of detection may be
regarded as the guest concentration to be detected over the noise
levels of the instruments used. Here, we use the guest concentra-
tion C0.01, which is the guest concentration that gives an absorption
change of 0.01, as the detection limit, and the values are listed in
Table 2. The C0.01 values are inversely related to the binding
constants, showing UDCA < HDCA < CDCA < DCA < CA for
steroids, borneol < menthol < camphor < nerol for monoterpe-
nes, and cyclooctanol < cyclohexanol for alkanols. The values
of C0.01 for adamantane derivatives are similar to each other,
reaching the level of UDCA. Figure 8 indicates that the ability of
ACD for detecting a particular compound from a mixture of several
substances is limited, but in some cases, such as a mixture of
UDCA, cyclooctanol, cyclohexanol, nerol, and geraniol, UDCA
would be detected by this host selectively. All these data
demonstrate that ACD has remarkable utility as a pH-indicator-
based chemosensor. On this basis, many spectroscopically inert
compounds may be detected from the absorption changes of the
host. Sensitivity and selectivity could be improved by appropriate
designing of the dye moiety, and we are now exploring various
types of dye-modified CD chemosensors.
Figure 8. Absorbance of ACD as a function of guest concentration.
9, CDCA; b, UDCA; 2, CA; [, DCA; 0, HDCA; O, 1-ADA; 4, (+)-
borneol; ], cyclohexanol; and ×, cyclooctanol.
parallel to the binding constants. For example, the ∆I/ I0 value
of 2-adamantanol (0.226) is much larger than that of UDCA
(0.099), in spite of the fact that the binding constant of UDCA
(93 200 M-1) is 9-fold larger than that of 2-adamantanol (103 000
M-1). It is noted that the values give no information on the limit
of detection of the guests. Therefore, we have attempted to get
whole views on the sensitivity and selectivity employing the curve-
fitting data used for determining binding constants. Figure 8
shows the guest-induced absorption changes against the guest
concentration for nine guests, including five steroidal guests. This
figure indicates that these guests give various saturation values.
The values are shown as ∆Imax in Table 2, and it is surprising
that the guest compounds that give larger ∆Imax values are not
those that give large binding constants. All steroidal guests give
smaller ∆Imax values, in contrast to the fact that small guests such
as nerol and geraniol give larger values. This result should be
ACKNOWLEDGMENT
We are grateful to Nihon Shokuhin Kako Co., Ltd., for the
generous gift of â-CD. This work was supported by a Grant-in-
Aid for Scientific Research from the Ministry of Education,
Science, Sports and Culture of Japan.
Received for review July 23, 1996. Accepted November
25, 1996.X
AC960727Z
X Abstract published in Advance ACS Abstracts, January 1, 1997.
Analytical Chemistry, Vol. 69, No. 4, February 15, 1997 663