Properties of cationic monosubstituted tetraalkylammonium cyclodextrin derivatives - Their stability, complexation ability in solution or when deposited on solid anionic surface

Article abstract of DOI:10.3762/bjoc.11.20

The thermal stability of the monosubstituted cationic cyclodextrin (CD) derivatives PEMEDA-β-CD and PEMPDA-β-CD, which differ in their substituent linker length (ethylene and propylene, respectively), was studied via ¹H NMR experiments. PEMPDA-

Full text of DOI:10.3762/bjoc.11.20

Properties of cationic monosubstituted tetraalkylammonium
cyclodextrin derivatives – their stability, complexation ability
in solution or when deposited on solid anionic surface
Martin Popr1, Sergey K. Filippov2, Nikolai Matushkin2, Juraj Dian3 and Jindřich Jindřich*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 192–199.
1Department of Organic Chemistry, Faculty of Science, Charles
University in Prague, Hlavova 8, 128 40, Prague 2, Czech Republic,
2Institute of Macromolecular Chemistry AS CR, v. v. i., Heyrovskeho
nam. 22, 16206, Prague 6, Czech Republic and 3Department of
Chemical Physics and Optics, Faculty of Mathematics and Physics,
Charles University in Prague, Ke Karlovu 3, 121 16, Prague 2, Czech
Republic
doi:10.3762/bjoc.11.20
Received: 23 October 2014
Accepted: 14 January 2015
Published: 02 February 2015
This article is part of the Thematic Series "Superstructures with
cyclodextrins: Chemistry and applications II".
Email:
Jindřich Jindřich* - jindrich@natur.cuni.cz
Guest Editor: G. Wenz
* Corresponding author
© 2015 Popr et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
cyclodextrins; inclusion properties; solid surface; tetraalkylammonium
derivatives; thermal stability
Abstract
The thermal stability of the monosubstituted cationic cyclodextrin (CD) derivatives PEMEDA-β-CD and PEMPDA-β-CD, which
differ in their substituent linker length (ethylene and propylene, respectively), was studied via 1H NMR experiments. PEMPDA-β-
CD exhibited higher resistance towards the Hofmann degradation and was chosen as a more suitable host molecule for further
studies. Inclusion properties of PEMPDA-β-CD in solution with a series of simple aromatic guests (salicylic acid, p-methoxyphenol
and p-nitroaniline) were determined by isothermal titration calorimetry (ITC) and compared to the native β-CD. Permanently
charged cationic CD derivatives were successfully deposited on the anionic solid surface of polymeric Nafion® 117 membrane via
electrostatic interactions. Deposition kinetics and coverage of the surface were determined by ELSD. Finally, the ability of the CD
derivatives bound to the solid surface to encapsulate aromatic compounds from aqueous solution was measured by UV–vis spec-
troscopy. The obtained results are promising for future industrial applications of the monosubstituted β-CD derivatives, because the
preparation of cationic CD derivatives is applicable in large scale, without the need of chromatographic purification. Their ionic
deposition on a solid surface is simple, yet robust and a straightforward process as well.
Introduction
Cyclodextrins (CDs) are a very interesting group of natural D-glucopyranose units forming a cycle with the shape of a
macrocyclic carbohydrates, which were first described by hollow truncated cone. Naturally occurring CDs are α-, β- and
Villiers in 1891 [1]. They are composed of α-(1→4)-linked γ-CD with 6, 7 and 8 glucopyranose units, respectively. Their
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Beilstein J. Org. Chem. 2015, 11, 192–199.
inner cavity, which is relatively hydrophobic, creates a proper butanol isomers [25]. Multilayered assemblies of anionic
environment for encapsulation of a wide range of organic and sulfonated CD derivatives and cationic polyelectrolytes were
inorganic molecules [2]. Therefore CDs and their derivatives also prepared by a layer-by-layer strategy and used for the
are most often utilized in pharmaceutical industry for drug solu- construction of an optical sensor for litocholic acid [26]. Similar
bilization [3] and delivery [4].
approaches of ionic self-assembly with CDs were used for the
preparation of nanoparticles with external CD trigger [27], new
Recently, we have published an article on the synthesis of a material nanostructures [28], polyelectrolyte-surfactant
complete series of cationic monosubstituted tetraalkylammo- complexes which yield new types of solid mesomorphous ma-
nium CD derivatives with one, two or three permanent positive terials [29], or membranes with size-selective transport of
charges [5]. The described preparation is straightforward and aromatic compounds [30].
relatively high-yielding in contrast to other conventional
methods which usually suffer from low yields and the need of In this paper we report on a comprehensive study of the prop-
chromatographic purification of reaction mixtures in order to erties of the novel monosubstituted cationic CD derivative
obtain pure isomers [6]. This makes our approach expandable to PEMPDA-β-CD (PErMethylated PropyleneDiAmine substi-
large scale and potentially suitable for industrial applications. tuted). Its thermal stability is compared to the previously
Previously, it has been described, that cationic CD derivatives reported analogue PEMEDA-β-CD (PErMethylated Ethylene-
with permanent positive charge can be synthesized [7] and used DiAmine substituted). Its binding ability toward selected
as chiral selectors in capillary zone electrophoresis [8-12] and aromatic guests is compared to the native β-CD at different pH.
also as catalysts of chemical reactions [13-15].
Deposition of PEMPDA-β-CD on the surface of an anionic
Nafion® 117 membrane via ionic self-assembly was also
The main goal of our research is to explore the possibility of studied along with the determination of sorption kinetics and
binding of the monosubstituted cationic CD derivatives to an surface coverage. Finally, the ability of the PEMPDA-β-CD
anionic surface by simple electrostatic interactions. Such an bound to the solid surface to accommodate aromatic guests
assembly could be potentially used for transdermal transporta- from aqueous solution into the cavities was explored.
tion of encapsulated active compounds across the epidermis
Results and Discussion
with the possibility of controlling the rate of the transport by
Thermal stability of PEMEDA- and PEMPDA-
β-CD
modulating the CD carrier, which is bound on the surface. The
obtained results can be possibly used for designing a new
topical drug formulation such as “smart” plasters or bandages
capable of prolonged release of the antiseptic drug. There are
several examples of deposition of CD derivatives onto a solid
support in the literature. The most common immobilization
method is binding of various alkylthio-CDs onto a gold surface
[16-18]. This approach offers self-assembled monolayers
(SAMs) which can be utilized for derivatization of gold elec-
trodes for the construction of electrochemical sensors [19,20] or
even for the development of molecular printboards [21]. These
results imply that the structure of CD derivatives strongly influ-
ences the properties of the SAMs [16]. Similar assemblies/films
can be prepared on glass surface, by depositing different
CD-siloxanes [22,23].
It is generally known, that quaternary ammonium salts exhibit a
low stability in basic environment at elevated temperatures.
This thermal decomposition of quaternary ammonium
hydroxide to an olefin and an amine proceeds via an E2 mecha-
nism and is referred to as the Hofmann elimination [31]. Upon
preparation of the PEMEDA-β-CD diiodide, we experienced
some partial decomposition of our material while drying the
final product at 60 °C. We decided to look at the thermal
stability in more detail. An analog with longer propylene linker
(PEMPDA-β-CD diiodide), connecting the charged nitrogen
atoms, was prepared for this reason. A higher stability of this
derivative was presumed, because of the higher distance
between the charged nitrogen atoms. The preliminary experi-
mental setup consisted of heating the aqueous solution of both
isomers to 80 °C with 1 equivalent of NaOH. The decomposi-
tion process was conveniently monitored by TLC, and revealed
a higher stability of the PEMPDA-β-CD derivative. The TLC
after 20 h showed a spot of the starting compound, while the de-
gradation of PEMEDA-β-CD was already complete. We
managed to separate the degradation products on a column of
silica gel and identified them by ESIMS (Scheme 1). To deter-
mine the decomposition half-life of each isomer, we set up a
1H NMR experiment at elevated temperature (50 °C). Spectra of
Recently, some examples of anchoring CDs onto the solid
support via ionic interactions were reported. Most of them
describe deposition of cationic CD polymers, where the CD
units are randomly crosslinked by suitable reagents. Systems for
separation of gases, composed of cationic CD pyridinium
polymer and a Nafion® membrane were reported by Grossi and
coworkers [24]. The Kusumocahyo group studied separation
abilities of cationic CD copolymers deposited on an anionic
Nafion® membrane, which showed good selectivity toward
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Scheme 1: Thermal decomposition of PEMEDA- and PEMPDA-β-CD with the decomposition products as characterized by MS.
the sample solutions in D2O, with 20 equivalents of NaOH were
acquired every hour in a course of 36 hours. The decreasing
values of the integral intensity of CH3 protons of the substituent
were plotted against time and kinetic curves for each derivative
were obtained (Figure 1). Data from the plot clearly indicate
that PEMPDA-β-CD is the more stable derivative. Values of the
decomposition time constants were calculated by fitting the
experimental data by a monoexponential function and are 7.9 h
and 20.1 h for the PEMEDA-β-CD and PEMPDA-β-CD, res-
pectively. Most probably, the closer proximity of the two
cationic nitrogen sites, in the ethylene-linked substituent of the
PEMEDA-β-CD diiodide, causes the stronger electrostatic
repulsion which leads to a more rapid formation of the corres-
ponding olefin. Based on these results the PEMPDA-β-CD was
selected for further measurements.
Inclusion properties of PEMPDA-β-CD in
solution
Figure 1: Decomposition kinetics of PEMEDA- and PEMPDA-β-CD at
50 °C as determined by 1H NMR thermal experiments.
To evaluate the ability of the cationic derivative PEMPDA-β-
CD to form inclusion complexes in aqueous solution, we
measured the stability constants (Ks) with three model aromatic
guest molecules (salicylic acid – SAL, p-methoxyphenol – 1H NMR spectroscopy [34] requires relatively high concentra-
MEQ, p-nitroaniline – NIA) at three different pH values (2.50, tions of the guest and host molecules, which are often impos-
7.00, 10.00) (Scheme 2).
sible to achieve due to the poor solubility in aqueous media.
Conventional UV–vis spectroscopy [35] was found difficult to
The Ks values were obtained by isothermal titration micro- be applied as well, because we were not able to achieve zero
calorimetry (ITC) [32,33], which was selected as the most suit- absorbance of the host solution (aqueous PEMPDA-β-CD
able method for our purpose. Other methods were also tested, dichloride), which is required to carry out the titration of guest
but their usability was found to be lower than ITC. For example solution.
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Beilstein J. Org. Chem. 2015, 11, 192–199.
Scheme 2: Host and guest molecules employed in Ks determination in solution at different pH.
The Ks values for β-CD and PEMPDA-β-CD diiodide from ITC those obtained for the β-CD. Complexation of NIA with
measurements are summarized in Figure 2 and Figure 3 respect- PEMPDA-β-CD was not observed as well as binding of SAL
ively. The stoichiometry of all of the employed complexes was and MEQ at basic pH. In conclusion, we can state that the
found to be 1:1. The Ks calculated for β-CD are in agreement monosubstituted derivative PEMPDA-β-CD is able to form
with the literature [36-39]. The most stable complex of β-CD inclusion complexes with SAL and MEQ at acidic and neutral
was obtained with SAL at pH 2.50 (Ks = 469 ± 7 M−1), whilst pH, whose stabilities are superior to those of the native β-CD.
no association was detected with SAL and MEQ at pH 10.00.
NIA was complexed by β-CD only at pH 10.00 (Figure 2).
The collected data comply with the concept, that uncharged
neutral molecules fit the best into the CD’s lipophilic inner
cavity.
Figure 3: Stability constants for PEMPDA-β-CD with SAL, MEQ and
NIA obtained by ITC measurements.
Immobilization of PEMPDA-β-CD on anionic
surface via ionic self-assembly
In the next part of our research, we focused on the possibility of
Figure 2: Stability constants for β-CD with SAL, MEQ and NIA
obtained by ITC measurements.
deposition of the permanently charged PEMPDA-β-CD onto the
solid surface of a model anionic polymeric Nafion® 117
In the case of positively charged PEMPDA-β-CD, again the membrane via simple ionic interactions (Scheme 3). Nafion® is
highest Ks value was received for SAL at pH 2.50 (Figure 3). a sulfonated tetrafluoroethylene fluoropolymer-copolymer,
This may be explained by the contribution of the ion–dipole especially favorable for our purpose because of its defined
interaction between the positively charged substituent of the structure, with a known number of –SO3H groups (equivalent
host and the carboxylic group of SAL, similarly as described in weight – EW = 1100 g·mol−1) and the absence of aromatic
the literature [40]. Also the values of Ks of the PEMPDA-β-CD groups, which could be encapsulated by the CD cavity and
with MEQ in acidic and neutral solutions were higher than negatively influence the complexation properties of the
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Beilstein J. Org. Chem. 2015, 11, 192–199.
Scheme 3: Deposition of PEMPDA-β-CD onto solid surface (Nafion® 117).
deposited CD. Preliminary experiments were performed on a 0.03 mM, which was attributed to the signal of the nascent HI
strong cation exchange resin (Dowex® 50), which has the which is formed during the immobilization and its signal is
chemical constitution of sulfonated polystyrene. It showed high picked up by the ELSD detector. Overall 10 mol % of available
deposition of cationic PEMPDA-β-CD, but also of the native –SO3H groups were saturated by cationic CD derivative. These
β-CD. The results indicated that undesired inclusion of the results were confirmed by repeated experiments and compos-
styrene moieties in the CD cavity takes place, which blocks the ition of the assembly was reproducible, also when using
cavity from further complexation of guest molecules from solu- Nafion® 117 in the NH4+ cycle. The attempts to achieve a
tion. At this point we replaced Dowex® 50 for aliphatic higher degree of saturation of –SO3H groups by modifying the
Nafion® 117. A cut-out of the foil (100 mm2, 35 mg) in H+ deposition conditions – saturated solution of PEMPDA-β-CD,
cycle was stirred in aqueous solution of the PEMPDA-β-CD higher temperature, longer deposition times – were not
and provisional data were collected by monitoring the decrease successful.
of the concentration by TLC and gravimetry. TLC indicated
completion of the immobilization after 48 h and the amount of
deposited PEMPDA-β-CD diiodide determined by gravimetry
was 5.0 mg.
The amount of the immobilized PEMPDA-β-CD along with the
deposition kinetics was determined by monitoring of the
concentration decay by an evaporative light scattering detector
(ELSD). The initial concentration of PEMPDA-β-CD was
0.2 mM (5.0 mg dissolved in 16.5 mL H2O). Nafion® 117 H+
foil (100 mm2) was added to the solution and the deposition rate
was monitored by direct injection of the reaction mixture in the
ELSD input. Measured peak areas (area under curve – AUC)
were converted to the concentrations and plotted against time to
receive the deposition kinetics (Figure 4). From the log c vs t
plot it follows, that the deposition kinetics is governed by a two-
exponential process. The deposition time constants were calcu-
lated by fitting the experimental data by two exponential decay
functions. Two time constants – τ1 = 1.03 h and τ2 = 11.43 h
were obtained. The final residual equilibrium concentration was
Figure 4: Deposition kinetics of PEMPDA-β-CD onto Nafion® 117 as
obtained from ELSD detection of the decreasing concentration in the
solution.
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Beilstein J. Org. Chem. 2015, 11, 192–199.
The assembly exhibits high overall stability. Attempts for against the blank sample which consisted of Nafion® 117 with
desorption of the absorbed PEMPDA-β-CD were performed. no deposited PEMPDA-β-CD. This method proved to be very
The experiment consisted of stirring the Nafion® cut-outs with useful and simple tool for determination of the amount of
immobilized PEMPDA-β-CD (5 mg) overnight in different included guests in the cavities of CD anchored to the surface.
solutions which were selected as capable of disrupting the ionic
interactions between the support and CD (water, 5% and 10% For the sake of clarity we defined a new unit of measure, which
aqueous NH4OH, 5% and 10% aqueous NH4HCO3). The describes the extent of inclusion of the guest in Nafion®-bound
possible desorption of the deposited CD was monitored by PEMPDA-β-CD. ROC (ratio of occupied cavities) is defined as
TLC. No leaching of the deposited CD could be observed, a molar percentage of the cavities forming inclusion complex
except in the case of 10% aqueous NH4HCO3, where some (Equation 1). In other words, the final number describes the
insignificant traces of PEMPDA-β-CD were detected by TLC. percentage of the cavities saturated by the guest molecule.
Inclusion of model guest molecules from the
(1)
solution in the cavities of Nafion®-bound
PEMPDA-β-CD
The ability of the prepared ionic assembly, composed of the
cationic monosubstituted β-CD derivative anchored to the
anionic surface, to encapsulate suitable guest molecules from
the solution was finally studied. Three model guest molecules
(SAL, MEQ, NIA) were employed in the experiment, so the
acquired data can be compared with the results of inclusion
obtained in the solution. The cut-outs of Nafion® 117 in NH4+
cycle (100 mm2, 35 mg) with deposited PEMPDA-β-CD (5 mg)
were stirred in the solution of the guest for 20 h, to reach equi-
librium inclusion (Scheme 4). Subsequently the assembly was
washed with H2O (5 × 3 mL) to remove the unspecifically
bound guest and then the included guest was extracted from
cavities by MeOH (1 × 3 mL). The amount of complexed guest
was quantified by UV–vis spectroscopy of the MeOH extracts
Here n(extr. guest) is the number of moles of guest extracted by
MeOH from the Nafion®-bound PEMPDA-β-CD, n(extr. guest
blank) is the number of moles of guest extracted by MeOH
from the unmodified Nafion® and n(PEMPDA-β-CD) is the
number of moles of deposited PEMPDA-β-CD.
The results are summarized in Table 1. The highest ROC was
obtained for SAL (34.5%) which corresponds very well with the
data from the measurement of complexation in the solution.
ROC of MEQ (17.5%) is proportionally lower and also corre-
lates nicely with the data from solution. Maximal solubility of
NIA is lower than the ones of SAL and MEQ, for this reason we
had to use a lower concentration of the incubation solution.
Scheme 4: Deposition of three model guests into the cavities of immobilized PEMPDA-β-CD.
Table 1: Results of the inclusion of different guests on the Nafion® 117 and PEMPDA-β-CD ionic assembly at neutral pH.
Guest
c (guest) M
n (included guest) mol
n (CD on surface)
ROC (%)
SAL
SAL
MEQ
MEQ
NIA
1.40 × 10−2
1.40 × 10−3
1.40 × 10−2
1.40 × 10−3
3.20 × 10−3
3.20 × 10−4
1.14 × 10−6
1.05 × 10−7
5.76 × 10−7
4.55 × 10−8
3.40 × 10−7
1.40 × 10−8
3.30 × 10−6
3.30 × 10−6
3.30 × 10−6
3.30 × 10−6
3.30 × 10−6
3.30 × 10−6
34.5
3.2
17.5
1.4
10.3
0.4
NIA
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Beilstein J. Org. Chem. 2015, 11, 192–199.
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