Electrochemical and spectrophotometric determination of the formation constants of the ascorbic acid-β-cyclodextrin and dopamine-β- cyclodextrin inclusion complexes

Article abstract of DOI:10.1007/s10847-010-9818-0

From UV-vis spectrophotometry and cyclic voltammetry it is demonstrated the interaction of dopamine, DA, and ascorbic acid, AA, with β-cyclodextrin, βCD, in aqueous media at pH 3.0. The formation constants of the respective inclusion complexes were also determined. The spectrophotometry data fed into the data processing software SQUAD, allowed calculation of the said constants for the AA-βCD complex (H₂AA-βCD) giving a value of 3236 M^-1, while for the dopamine complex DA-βCD (H ₃DA+βCD) this was 5888 M^-1. From these results, the theoretical absorption spectra were generated which fitted quite well the experimental ones, thus indicating clearly that the constants are reliable. Moreover, from the cyclic voltammetry data, the stated constants were also calculated whereby that of the H₂AA-βCD gave a value of 3981 M^-1 while for the dopamine complex DA-βCD (H ₃DA+βCD) it was 4898 M^-1. It is noteworthy to say that these values were similar to those found through spectrophotometry.

Full text of DOI:10.1007/s10847-010-9818-0

J Incl Phenom Macrocycl Chem (2011) 69:91–99
DOI 10.1007/s10847-010-9818-0
ORIGINAL ARTICLE
Electrochemical and spectrophotometric determination
of the formation constants of the ascorbic acid-b-cyclodextrin
and dopamine-b-cyclodextrin inclusion complexes
´
M. Palomar-Pardave G. Alarcon-Angeles
•
•
´
M. T. Ramırez-Silva M. Romero-Romo
´
•
•
´
•
´
A. Rojas-Hernandez S. Corona-Avendan˜o
Received: 15 February 2010 / Accepted: 8 June 2010 / Published online: 30 June 2010
Ó Springer Science+Business Media B.V. 2010
Abstract From UV–vis spectrophotometry and cyclic
voltammetry it is demonstrated the interaction of dopa-
mine, DA, and ascorbic acid, AA, with b-cyclodextrin,
bCD, in aqueous media at pH 3.0. The formation constants
of the respective inclusion complexes were also deter-
mined. The spectrophotometry data fed into the data pro-
cessing software SQUAD, allowed calculation of the said
constants for the AA–bCD complex (H2AA–bCD) giving a
value of 3236 M-1, while for the dopamine complex DA–
bCD (H3DA?bCD) this was 5888 M-1. From these
results, the theoretical absorption spectra were generated
which fitted quite well the experimental ones, thus indi-
cating clearly that the constants are reliable. Moreover,
from the cyclic voltammetry data, the stated constants were
also calculated whereby that of the H2AA–bCD gave a
value of 3981 M-1 while for the dopamine complex DA–
bCD (H3DA?bCD) it was 4898 M-1. It is noteworthy to
say that these values were similar to those found through
spectrophotometry.
Introduction
Cyclodextrins are crystalline compounds comprised of
torus-like macro-rings of glucopyranose units, in particular
the b-cyclodextrin, bCD, has seven glucose units. Various
3D structures result after joining the unit molecules that
differ in their chemical and physical properties such as
size, functionality or solubility, many of which are very
interesting from a pharmaceutical point of view.
Derived from their peculiar molecular structure, these
molecules display the chemical tendency to form inclusion
complexes with other ionic species: these complexes are of
great interest in practical applications and in fundamental
research, because, when adequately manipulated, they
provide information on non-covalent intermolecular forces.
In this way, the bCD has been applied to develop chemical
sensors for biologically interesting analytes, such as
dopamine, DA, which is in its own right, an essentially
important natural molecule, containing a catechol nucleus
and an amine group on a chain with two-carbon atoms:
meta or para to the phenolic hydroxyl groups, that are
bioenergetic amines. Plainly said, dopamine plays a basic
role as neurotransmitter in the central nervous system.
Therefore, the DA is naturally expected to be used in
diverse pharmaceuticals to treat some diseases [1]. How-
ever, its determination may be hindered by the presence of
some interferents, of which the ascorbic acid, AA, exhibits
an oxidation potential close to that of the DA, thus pro-
voking an overlap of the voltammetric signals. Naturally,
this has given rise to an increased interest to know the
possible interactions between the complexes AA–bCD and
DA–bCD.
Keywords Dopamine ꢀ Ascorbic acid ꢀ b-cyclodextrin ꢀ
Inclusion complex ꢀ UV–vis spectrophotometry ꢀ
Cyclic voltammetry
´
´
´
M. Palomar-Pardave ꢀ G. Alarcon-Angeles ꢀ M. Romero-Romo
ꢀ S. Corona-Avendan
˜o (&)
´
Departamento de Materiales, Universidad Autonoma
Metropolitana Azcapotzalco, Av. San Pablo #180,
´ ´
Col. Reynosa-Tamaulipas, C.P. 02200 Mexico, DF, Mexico
e-mail: sylvia_cav@yahoo.com.mx
´
´
M. T. Ramırez-Silva (&) ꢀ A. Rojas-Hernandez
´
Departamento de Quımica, Universidad Autonoma
Metropolitana Iztapalapa, Av. San Rafael Atlixco #186, Col.
´
Fukuda et al. [2], have reported that the a-amino acid 3,4-
dihydroxyphenylalanine (DOPA) derivatives can be strongly
included in the a-CD cavity. From this information, Fragoso
´
Vicentina, C.P. 09340 Mexico, DF, Mexico
e-mail: mtrs218@xanum.uam.mx
´
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J Incl Phenom Macrocycl Chem (2011) 69:91–99
et al. [3], developed a novel strategy to detect DA in the
presence of AA, at pH 7.0, that consists on the modification
of gold electrodes with a mixed bCD and thioctic acid (TA)
monolayer. The claim was that this provides the electrode
with a molecular recognition motif (the CD) in addition to
the electrostatic attraction of the TA. Moreover, Fragoso
et al. [3] assumed, from literature data [3, 4], that CD forms
stronger inclusion complexes with DA (KDA * 2000 M-1
[2]) than with the AA (KAA = 130 M-1 [4]), thus its pres-
ence in the monolayer in combination with TA increases the
selectivity and sensitivity for the determination of the neu-
rotransmitter by inducing a co-operative effect based on
simultaneous electrostatic and host–guest interactions.
However, even when this is clearly a good idea, the values of
the apparent constant (K) that Fragoso et al. [3], mentioned
are wrong since as one could see in the original references [2,
4] the apparent KAA value is 2900 200 M-1 at pH 6.9 [4]
and KDA for DA is not reported in reference [2].
Autolab PGSTAT 30. A carbon paste electrode, CPE, was
prepared from graphite powder [6, 7] (Johnson Matthey
1 lm, 99.9%) and it was used as working electrode. A BAS
MW-1032 Pt wire was used as counter electrode, and
saturated Ag/AgCl was the reference also from BAS
MF-2052, to which all potentials herein reported should
be referred. The spectrophotometric behaviour was deter-
mined with the aid of a UV–vis Perkin Elmer Lambda 20
spectrophotometer.
Results and discussion
Determination of the AA’s acidity constants
In order to understand and establish the complexation
equilibria between the bCD, the DA and the AA at
pH = 3, it is necessary to find out the acidity constants,
pKa, of each species. In the DA’s case these have calcu-
lated by means of UV–vis spectrophotometry and SQUAD,
a computational software [8], which is fed with absorbance
data and a chemical model that considers the possible
chemical equilibria as basic input; this has already been
reported by our group [9]. Therefore, the same method
shall be used to calculate the AA’s pKa. Figure 1 shows the
absorption spectra of AA 0.1 mM at different pH. The
spectral behaviour of the AA revealed two isosbestic points
at 220 and 290 nm, which are associated to three acid–base
species.
On another respect, Gao et al. [5], reported the study of
the inclusion complexes of catecholamines (dopamine and
adrenaline) with bCD by cyclic voltammetry. The experi-
mental results indicate that both catecholamines can form
1:1 inclusion complexes with bCD in aqueous solutions.
Therefore, the importance granted to knowing this sort
of interactions, is the root cause for the objectives of this
work, whereby, apart from calculating the acidity constants
of the AA (Ka), we report also the complexation constants
(Kf) between the DA–bCD and the AA–bCD, obtained
through two different techniques, spectrophotometry UV–
vis and cyclic voltammetry.
The spectral data obtained as a function of pH were
used to assess the acidity constants by means of SQUAD
[8]. The software was fed with 40 pH values within the
200–350 nm wavelength range (k). The global formation
constants calculated through this method are shown in
Table 1.
Experimental
Reagents and chemicals
All DA, AA, NaCl, HCl and b-CD solutions were prepared
from Merck analytic grade reagents, where the HCl (37%)
and H3PO4 acids were used to adjust the pH to 3.0. All
solutions were prepared with deionised water Type I hav-
ing 18.2 MX resistivity, free from organic matter obtained
from a US Filter Pure-Lab Plus UV. Thus, the solutions
prepared were deareated with nitrogen and freshly prepared
prior to each determination. Also, they were protected from
the incidence of light even during the performance of the
experiments.
2.5
2
1.5
1
0.5
0
200
220
240
260
280
300
320
Instrumentation
λ
/ nm
Fig. 1 Absorption spectral family of the AA 0.1 mM, for different
pH values (—) 1.50, ( ) 2.94, ( ) 3.36, ( ) 7.37, ( ) 10.2, (
10.76, ( ) 10.98, (——) 11.14, ( ) 11.20, (d) 11.31, (h) 11.75, (_)
The DA’s and AA’s electrochemical behaviour were char-
acterized through cyclic voltammetry, CV. All determina-
tions were carried out by means of a potentiostat-galvanostat
)
14.48
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J Incl Phenom Macrocycl Chem (2011) 69:91–99
93
undertaken, the results of which are shown in Fig. 3a,
where an absorption peak with its maximum at
k = 242 nm can be noted having also a small absorption
increment as the bCD concentration increases in the sys-
tem. This trend can be seen clearly in Fig. 3b, that plots the
absorbance at this wavelength as a function of the bCD
concentration in the system.
Table 1 Global formation constants of the AA evaluated by means
of spectrophotometry, see Fig. 1 and SQUAD [8]
Equilibrium
log b
H? ? AA2- , HAA-
2H? ? AA2- , H2AA
11.36 0.01
15.22 0.02
The standard deviation for this data group was 9.42 9 10-2
With the spectra reported in Fig. 3, the inclusion con-
stant for Scheme 1 can be calculated using SQUAD [8], for
which the most favourable equilibrium considered for the
formation of the inclusion complex as discussed above for
this pH, is the complex H2AA–bCD. Table 2 shows the
results obtained for the global formation constant, log b,
and for formation, log Kf.
Determination of the inclusion constant of the AA
with bCD
UV–vis spectrophotometry
From the values of the constants shown in Table 1 and
´
following the methodology proposed by Rojas-Hernandez
The molar absorptivity coefficients (e) for the H2AA y
H2AA–bCD species at each wave length (k), calculated
from the experimental spectra, see Fig. 3, and the SQUAD
software are reported in Table 3. It is important to mention
that bCD does not absorb in the wave length considered.
In order to gather evidence on the precision of the
experimental values obtained through this method, a com-
parison of the experimental absorption spectra at different
pbCD (-log[bCD]) with the theoretical ones generated
from the data refined of the formation constants and of the
molar absorptivity coefficients, see below, was carried out.
et al. [10–13], a predominance zones diagram can be
constructed, like that shown in Fig. 2, from which it
becomes clear that at pH 3, AA is the neutral predominant
species, termed H2AA; therefore, the inclusion reaction of
AA and bCD will be as indicated in Scheme 1 with a 1:1
interaction ratio.
To calculate the inclusion constant for Scheme 1, a
spectrophotometry study of the neutral species H2AA at
different bCD concentrations and at constant pH = 3 was
Theoretical absorption spectra
O
O
O
From the formation constants of the species, see Tables 1
O-
O-
OH
OH
OH
O-
and 2 and Eqs. 1 and 2 their respective molar fractions can
O
O
O
be obtained
1
HO
HO
HO
HO
HO
HO
fspecie
¼
ð1Þ
ð2Þ
1 þ 10ðK ꢁpbCDÞ
f
ꢀ
ꢁ
1
H2AA
HAA-
AA2-
fcomplex ¼ 10ðK ꢁpbCDÞ
f
1 þ 10K ꢁpbCD
f
pH
4.1
11.9
Using the previous fractions and the overall
concentration (Ct), the concentration of each species
(Cspecie) and (Ccomplex) is obtained from Eqs. 3 and 4.
Fig. 2 Predominance zones diagram for the AA species and its
structures as a function of pH
O
O
OH
O
OH
O
+
OH
HO
HO
OH
HO
HO
Scheme 1 Schematic reaction between the neutral AA species (H2AA) and the bCD
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J Incl Phenom Macrocycl Chem (2011) 69:91–99
Fig. 3 a Spectral behaviour of
AA 0.1 mM at pH 3 with
different bCD concentrations. b
Variation of the absorbance
maximum at k = 242 nm as a
function of the bCD
1.5
1.6
1.2
0.8
0.4
0
(b)
(a)
1.4
1.3
concentration
200
220
240
260
280
300
0
0.05
0.1
0.15
0.2
λ
/ nm
[
β
-CD]/ mM
Table 2 Global formation constant of the complex H2AA–bCD
evaluated by means of experimental spectrophotometry data and
SQUAD [8]
Table 3 Molar absorptivity coefficients (e) for the H2AA and H2AA–
bCD species as a function of wave length (k) calculated from the
SQUAD software
Equilibrium
k/nm
10-3
e
H AA=Mꢁ1cmꢁ1
10-3 eH AA
b
CD=Mꢁ1cmꢁ1
2
2
log b
200
204
208
212
216
220
224
228
232
236
240
244
248
252
256
260
264
268
272
276
280
284
288
292
296
300
3.39
2.50
2.16
2.37
3.00
3.93
5.08
6.36
7.68
8.85
9.63
9.81
9.28
8.11
6.51
4.81
3.30
2.16
1.38
0.87
0.55
0.34
0.21
0.12
0.07
0.03
6.82
4.77
3.67
3.55
4.17
5.25
6.57
8.02
9.54
10.95
11.98
12.37
11.92
10.74
9.06
7.20
5.48
4.05
2.94
2.06
1.34
0.81
0.41
0.16
0.00
0.00
2H? ? AA2- ? bCD , H2AAbCD
H2AA ? bCD , H2AAbCD
18.73 0.04
log Kf
3.51 0.05
The standard deviation for this data group was 9.46 9 10-3
Cspecie ¼ fspecieðCtÞ
Ccomplex ¼ fcomplexðCtÞ
ð3Þ
ð4Þ
From these concentrations and the molar absorptivity
coefficients, see Table 3, the theoretical absorption at each
wave length (k) value, Aktheoretical, can be obtained using
Eq. 5.
Atkheoretical ¼ ekspecieðCspeceieÞ þ ekcomplexðCcomplex
Þ
ð5Þ
Figure 4 shows the comparison of the theoretical absorp-
tion spectrum and that obtained experimentally, for
pbCD = 3.7. Both spectra are indeed similar, which
strongly suggests that the refining procedure of the calcu-
lated constant was appropriate.
With the respective Kf values and Eqs. 1 and 2 it is
possible to construct the species distribution diagram
shown in Fig. 5, for the complexes formed in aqueous
solution as a function of pbCD.
Cyclic voltammetry
starting in the anodic direction; all cases depicted an oxi-
dation peak, though when the potential was reversed there
were no reduction peaks, which suggests that the AA
exhibits an irreversible oxidation process. As the bCD
concentration increases, this oxidation peak is shifted to
greater potentials till at the bCD concentration 0.9 mM it
remains constant; this trend can be more clearly appreci-
ated in Fig. 6b.
Cyclic voltammetry, CV, experiments were also recorded
in the system CPE/0.1 M NaCl, 0.1 mM AA at pH 3 with
different bCD concentrations to corroborate the calculated
constant of the inclusion complex H2AA–bCD estimated
by means of UV–vis spectrophotometry. Figure 6a shows
the CV’s of the system CPE/0.1 M NaCl, 0.1 mM AA at
pH 3 with different bCD concentrations, carried out,
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95
6.20
6.00
5.80
5.60
5.40
5.20
5.00
1.6
1.2
0.8
0.4
0.0
200
220
240
260
280
300
4000
5000
(ip,AA2- ip )/[
6000
7000
A2M-1
8000
9000
λ
/ nm
2
β
CD]/
μ
Fig. 4 Comparison of the theoretical (Solid line) spectrum generated
using Eq. 5 and the date in Table 3 with the experimental absorption
spectrum (Open square) obtained in the system: AA 0.1 mM at pH 3
with [bCD] = 0.2 mM, see Fig. 3a
Fig. 7 Plot i2p versus ip2(R) - i2p/[bCD], the current values were
obtained from the voltammograms shown in Fig. 6a. The line was
obtained from linear fitting of the experimental results
Evans and coworkers [14, 15] using diffusion coefficients
1
instead of currents, and by Dang et al. [16] using currents
Kd
i2p ¼
ði2pðRÞ ꢁ i2pÞ þ ip2ðRꢁbCDÞ
ð6Þ
H2AA
0.8
H2AA-βCD
½bCDꢂ
0.6
0.4
0.2
0
where ip is the observed peak current; ip(R) and ip(R-bCD)
are the peaks currents of the guest-free bCD and of the
inclusion complex, respectively. Both ip(R) and ip(R-bCD)
can be determined from the experiments. The inclusion
dissociation constant, Kd, value can be obtained from the
slope of the linear plot i2p versus ip2(R) - ip2/[bCD] and from
it the value of the inclusion formation constant, Kf, since
Kf = 1/Kd.
Figure 7 shows the plot ofi2p versus ip2(R) - ip2/[bCD] where
the equation of the straight line is given by i2p/
lA2 = (0.0002478 0.0000062) M-1 (i2p,DA - i2p/[bCD])/
lA2M-1 ? (4.152 0.036) lA2 with a correlation coeffi-
cient of 0.99. From the slope of the linear fit the following was
obtained Kd = 0.0002478 0.0000062, which gives pKd =
3.60 0.03; this value is very close to that found by means of
UV–vis spectroscopy.
1
2
3
4
5
6
pβCD
Fig. 5 Distribution diagram of the ascorbic acid species as a function
of pbCD
With these data it becomes possible to calculate the
inclusion constant using the method described by Gao et al.
[5], using Eq. 6. The procedure was proposed earlier by
0.64
(a)
(b)
2.5
2.0
1.5
1.0
0.5
0.0
0.6
0.56
0.52
0.2
0.4
0.6
0.8
1
1.2
0
0.0002 0.0004 0.0006 0.0008 0.001 0.0012
E / V
β
[ CD] / M
Fig. 6 a Cyclic voltammograms recorded in the system CPE/NaCl 0.1 M, AA 0.1 mM at pH 3 for different bCD additions (-) 0.2, ( ) 0.4, (
0.6, ( ) 0.8 and (d) 1 mM, at 100 mVs-1 scan rate. b Peak potential variations as a function of the bCD concentration
)
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HO
HO
NH2
NH2
NH2
Fig. 8 Predominance zones
diagram for the DA species and
its structures as a function of pH
NH3+
HO
HO
HO
O
O
O
H3DA+
HDA-
DA2-
H2DA
pH
10.58
12.07
9.04
2
Determination of the DA–bCD inclusion constant
(a)
UV–vis spectroscopy
1.6
Following the same method as described above, and con-
sidering the predominance zones diagram shown in Fig. 8,
proposed by Corona et al. [17], it can be established that
the inclusion reaction between the DA and the bCD is as
shown in Scheme 2, where the DA is fully protonated,
H3DA?, as the solution’s pH is 3.
1.2
0.8
0.4
0
The calculation of the formation constant of the inclu-
sion complex H3DA–bCD is made varying the bCD con-
centration in a DA 0.28 mM solution at pH 3 recording the
UV–vis absorption spectrum for each bCD concentration.
Figure 9 shows two absorption spectra obtained for the DA
0.28 mM and for the DA with bCD 2.5 mM. These spectra
exhibit the DA characteristic band located at 279 nm that
increased its absorbance as the bCD content increased in
the system, see Fig. 9b. This increment can be associated
with formation of the inclusion complex H3DA–bCD. The
absorbance change as a function of the molar relation
between the [bCD]/[DA], showed that the stoichiometry of
the complex H3DA–bCD is 1:1.
221
241
261
(nm)
281
λ
0.8
(b)
0.78
0.76
0.74
With the data from the absorption spectra plotted as a
function of the bCD concentration at pH 3 and from
SQUAD [8], the reaction constants for Scheme 2 were
calculated. The acidity constants of the DA were set con-
´
sidering the pKa reported by Sanchez-Rivera et al. [9]. A
0.0006
0.0011
0.0016
CD]/ M
0.0021
0.0026
value of log Kf = 3.77 0.16 was refined by SQUAD as
the complexation constant of the H3DA–bCD inclusion
complex. This value is greater than that found for the AA,
thus the complex H3DA–bCD is more stable than H2AA–
bCD.
[
β
Fig. 9 a Spectral behaviour of the DA 0.28 mM (open triangles) and
DA with bCD 2.5 mM (solid line) at pH 3. b Absorbance variation at
k = 279 nm as a function of the bCD concentration
HO
HO
NH3+
HO
HO
NH3+
p
+
Scheme 2 Schematic reaction between the protonated species of the DA (H3DA?) and bCD
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J Incl Phenom Macrocycl Chem (2011) 69:91–99
97
From the value of the H3DA–bCD inclusion complex
constant, the molar absorptivity coefficients reported in
Table 4 and Eqs. 1–5, the theoretical spectrum of the
system 0.28 M DA with bCD 0.9 mM was determined.
Figure 10 shows, as example, a comparison between
spectra, the theoretical and the experimental one obtained
at pbCD = 3. It becomes straightforward that the proce-
dure is quite satisfactory for each and every wavelength
considered, thus underlining the good quality of the fitting
for all constants calculated by SQUAD [8].
1.6
1.2
0.8
0.4
0.0
With the respective Kf values and Eqs. 1 and 2, the
distribution diagram shown in Fig. 11 was constructed.
214
234
254
/ nm
274
294
λ
Cyclic voltammetry
Fig. 10 Comparison of the theoretical (solid line) spectrum gener-
ated using Eq. 5 and the date in Table 4 with the experimental
absorption spectrum (open square) obtained in the system: 0.28 M
DA with [bCD] = 1 mM at pH 3
In order to carry out this study, bCD additions were made
to a DA solution until the bCD concentration was 20 times
greater than the DA concentration, to insure formation of
the complex. The CV’s from this study are shown in
Fig. 12, at two bCD concentrations. The shift of anodic and
cathodic peak’s potential in the presence of bCD, is asso-
ciated to formation of the inclusion complex, though it
becomes evident that the said shift is greater for the
cathodic peak.
1
0.8
0.6
Figure 13 shows the potential variation for the oxidation
(a) and reduction (b) peaks, respectively, for both of which
it becomes noticeable that oxidation and reduction are
favored, overall making the process more reversible.
+
H3DA-βCD
H3DA
0.4
0.2
0
Table 4 Molar absorptivity coefficients (e) for the H3DA and
H3DAbCD species as a function of wave length (k) calculated from
the SQUAD software
2
2.5
3
3.5
4
4.5
5
p
β
CD
k/nm
10-3
e
H DA=Mꢁ1cmꢁ1
10-3 eH DA
b
CD=Mꢁ1cmꢁ1
3
3
Fig. 11 Distribution diagram of the dopamine species as a function
of pbCD
230
234
238
242
246
250
254
258
262
266
270
274
278
282
286
290
294
300
3.44
2.04
1.00
0.48
0.30
0.28
0.37
0.56
0.86
1.28
1.79
2.27
2.57
2.47
1.98
1.00
0.26
0.03
3.85
2.36
1.22
0.63
0.40
0.36
0.44
0.63
0.93
1.37
1.89
2.39
2.70
2.61
2.10
1.09
0.31
0.04
4
3
2
1
0
-1
-2
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
E / V
Fig. 12 Cyclic voltammograms recorded in the system CPE/NaCl
0.1 M, 0.1 DA mM at pH 3 for different bCD additions (solid line) 0
and (open circle) 0.2 mM, at 100 mVs-1 scan rate
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Fig. 13 Trends displayed by
the peak potentials: a oxidation,
0.69
0.67
0.65
0.63
(a)
(b)
0.28
Eox and b reduction, Ered
obtained from the cyclic
,
0.26
0.24
0.22
0.2
voltammograms shown in
Fig. 12
0.0000
0.0002
-CD] / M
0.0004
0
0.0002
-CD] / M
0.0004
[
β
[
β
Conclusions
11.4
11.3
11.2
11.1
11
The formation of inclusion complexes between H3DA–
bCD and the H2AA–bCD was demonstrated by spectro-
photometry and electrochemical means, in aqueous media
at pH 3. The values found through both methods were very
similar, which emphasizes the reliability of the complex-
ation constants derived. Further, the theoretical spectra
represent the experimental behavior closely. The difference
between the pKf values shows that the H2AA–bCD com-
plex is more stable than the H3DA–bCD. From the
respective Kf values, distribution diagrams for these com-
plexes were constructed as a fuction of pbCD.
10.9
10.8
10.7
10.6
-3500
-3000
-2500
(ip,DA2- ip )/[
-2000
-1500
-1000
-500
2
β
-CD]/
μ
A2M-1
Fig. 14 Trend of the ip2 versus (ip2(R)- ip2)/[bCD] obtained from the
data of the voltammograms shown in Fig. 12
Acknowledgments SCA 58250 and GAA 184930 express their
gratitude to CONACyT for their postdoctoral and Ph.D., grants.
MTRS thanks CONACyT for support through project 82932 and SCA
for project 80305. Also GAA, SCA, MRR, MPP and MTRS gratefully
thank the SNI for the distinction of their membership and the stipend
received. MPP and MRR wish to thank the Departamento de Mate-
riales, UAM-A, for the financial support given through projects
(2261203, 2261204, 2261205).
In order to assess the complexation constant with these
data, the method described above [5, 14–16], was followed;
Fig. 14 shows the plot of i2p versus (ip2(R) - ip2)/[bCD] where
the expression for the resulting straight line is given by
i2p/lA2 = -(0.00020 0.00003) M-1 (i2p,DA - i2p/[bCD])/
lA2M-1 ? (10.6 0.09) lA2 with a correlation coeffi-
cient of 0.99. From the slope it results Kd = 0.00020
0.00003, from which one gets pKd = 3.70 0.15; this
value is quite close to that found by means of UV–vis
spectroscopy.
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