Chiral recognition for the complexation dynamics of β-cyclodextrin with the enantiomers of 2-naphthyl-1-ethanol

Article abstract of DOI:10.1039/c3pp50298h

The focus of this study is to understand the origin of the chiral recognition for a host-guest system containing complexes with different stoichiometries. Each enantiomer of 2-naphthyl-1-ethanol forms two different 1:1 complexes with β-cyclodextrin, leading to the formation of three different 2:2 complexes. One of these 2:2 complexes leads to excimer emission of the guest. Fluorescence studies were employed to determine the binding isotherms for the 1:1 and 2:2 complexes. No chiral discrimination was directly observed for the formation of the 1:1 complexes, while higher equilibrium constants (29% from binding isotherms and 40% from kinetic studies) were observed for the formation of the 2:2 complexes with (R)-2-naphthyl-1-ethanol when compared to the formation of the 2:2 complexes formed from (S)-2-naphthyl-1-ethanol. The relaxation kinetics was studied using stopped-flow experiments. The formation of the 2:2 complexes was followed by detecting the excimer emission from one of the 2:2 complexes. The relaxation kinetics was faster for (S)-2-naphthyl-1- ethanol, where a higher dissociation rate constant, by 47%, was observed, suggesting that the chiral discrimination occurs because the interaction between two cyclodextrins is more favorable for the complexes containing (R)-2-naphthyl-1-ethanol when compared to (S)-2-naphthyl-1-ethanol. The same overall equilibrium constants were observed for the 1:1 complexes with both enantiomers showing that at a given cyclodextrin concentration the sum of the two types of 1:1 complexes is the same for both enantiomers. However, analysis of the binding isotherms indicates that the ratio between the two different 1:1 complexes for each enantiomer was different for (R)- and (S)-2-naphthyl-1- ethanol. The Royal Society of Chemistry and Owner Societies.

Full text of DOI:10.1039/c3pp50298h

Photochemical &
Photobiological Sciences
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PAPER
Chiral recognition for the complexation dynamics
of β-cyclodextrin with the enantiomers of
2-naphthyl-1-ethanol†
Cite this: Photochem. Photobiol.
Sci., 2014, 13, 358
Hao Tang,a Andria S. M. Sutherland,a Lana M. Osusky,a Yan Li,b Josef F. Holzwarthb
and Cornelia Bohne*a
The focus of this study is to understand the origin of the chiral recognition for a host–guest system
containing complexes with different stoichiometries. Each enantiomer of 2-naphthyl-1-ethanol forms
two different 1 : 1 complexes with β-cyclodextrin, leading to the formation of three different 2 : 2
complexes. One of these 2 : 2 complexes leads to excimer emission of the guest. Fluorescence studies
were employed to determine the binding isotherms for the 1 : 1 and 2 : 2 complexes. No chiral discrimi-
nation was directly observed for the formation of the 1 : 1 complexes, while higher equilibrium constants
(29% from binding isotherms and 40% from kinetic studies) were observed for the formation of the 2 : 2
complexes with (R)-2-naphthyl-1-ethanol when compared to the formation of the 2 : 2 complexes
formed from (S)-2-naphthyl-1-ethanol. The relaxation kinetics was studied using stopped-flow experi-
ments. The formation of the 2 : 2 complexes was followed by detecting the excimer emission from one of
the 2 : 2 complexes. The relaxation kinetics was faster for (S)-2-naphthyl-1-ethanol, where a higher
dissociation rate constant, by 47%, was observed, suggesting that the chiral discrimination occurs because
the interaction between two cyclodextrins is more favorable for the complexes containing
(R)-2-naphthyl-1-ethanol when compared to (S)-2-naphthyl-1-ethanol. The same overall equilibrium
constants were observed for the 1 : 1 complexes with both enantiomers showing that at a given cyclo-
dextrin concentration the sum of the two types of 1 : 1 complexes is the same for both enantiomers.
However, analysis of the binding isotherms indicates that the ratio between the two different 1 : 1
complexes for each enantiomer was different for (R)- and (S)-2-naphthyl-1-ethanol.
Received 29th August 2013,
Accepted 4th December 2013
DOI: 10.1039/c3pp50298h
www.rsc.org/pps
supramolecular systems provide the opportunity to study chir-
ality control in systems where recognition is imposed through
Introduction
A change in chirality is one of the smallest perturbations that non-covalent interactions.1–3 One of the ongoing challenges is
can be made to a chemical system and understanding how to obtain mechanistic information on how this discrimination
chiral discrimination is achieved is an ongoing quest. Supra- occurs for complex supramolecular systems beyond those with
molecular chemistry can impose chiral recognition through 1 : 1 guest–host stoichiometries. An example of such complex-
non-covalent interactions, which is advantageous when revers- ity is the use of serum albumins, which were exploited as
ible systems are desired. Photochemical reactions in hosts with several binding sites with different binding
affinities to which 2-anthracenecarboxylate binds as a guest.
The highest chiral discrimination for the photodimerization
reaction of 2-anthracenecarboxylate was obtained when pro-
chiral guests were bound to protein binding sites with moder-
aDepartment of Chemistry, University of Victoria, PO Box 3065, Victoria, BC,
Canada V8W 3V6. E-mail: cornelia.bohne@gmail.com; Fax: +1-250-721-7147;
Tel: +1-250-721-7151
bFritz-Haber-Institut, Department of Physical Chemistry Faradayweg 4-6,
ate affinity and where the guests were fairly immobile before
the photodimerization reaction occurred.4–7 This example
14195 Berlin, Germany. E-mail: holzwarth@fhi-berlin.mpg.de
†Electronic supplementary information (ESI) available: Determination of 1 : 1 shows that the guest was bound to sites with different pro-
equilibrium constants, decays for the triplet excited states of NpOH, analysis of
perties and that the desired property, chiral discrimination in
the binding isotherms for the 2 : 2 complexes, dependence of CEE with KEE
,
this case, was obtained from a site with moderate binding
affinity. Therefore, determination of systems in which one of
the enantiomers binds much more strongly to a single site,
such as in the case of the binding of a chiral derivative of
stopped-flow kinetics for the formation of EE, simulation of the stopped-flow
experiments, global analysis for the 2 : 2 formation kinetics, derivation of the
dependence of the overall 2 : 2 kinetics and the kinetics for EE formation. See
DOI: 10.1039/c3pp50298h
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pyrene to bovine serum albumin,8 may not be a sufficient and circular dichroism spectra but was not apparent in the
requirement to achieve a property that requires guest release determined equilibrium constants.19,22
such as in a bimolecular reaction, making it necessary to
investigate systems with multiple binding sites.
The enantiomers of NpOH form 1 : 1 and 2 : 2 (NpOH–β-CD)
complexes with β-CD with a higher equilibrium constant for
A different approach to complexity, described in this contri- the 2 : 2 complexes compared to the equilibrium constant for
bution, is to have one host and one guest where complexes the 1 : 1 complexes.19,22 Two types of 1 : 1 complexes were
with different guest–host stoichiometries are formed and formed where either the naphthyl (N) or the ethanol (E) moi-
where these complexes have different spectroscopic signatures. eties were included close to the narrower rim of β-CD which
Cyclodextrins (CDs) are cyclic glucose oligosacccharides contains the primary hydroxyl groups (Fig. 1). The presence of
that have
a hydrophobic cavity into which guests are N and E complexes leads to the formation of NN, NE and EE
included.9–12 The size of the cavity depends on the number of complexes (Fig. 1).22 In previous work with naphthalene/
glucose units in the ring, with the smallest being α-CD, fol- β-CD20 and α-terthiophene/γ-CD23 complexes the excimer emis-
lowed by β-CD and γ-CD. The majority of CD complexes have a sion of the guest was observed for the 2 : 2 complexes while no
1 : 1 stoichiometry, but higher order complexes with 1 : 1 : 1, triplet excited states were detected for the 2 : 2 complexes
1 : 2, 2 : 1 and 2 : 2 guest–host complexes can occur depending because the excited guest interacts readily with the second
on the relative size of the guest and the CD. Naphthalene and guest in the ground state leading to excimer emission. In the
its derivatives were shown to form higher order complexes case of the NpOH–β-CD system, excimer emission and long-
with all three CDs.13–20 When encapsulated by two CDs the lived triplet states were observed.19 This result suggested that
naphthalene excited states were located in a less polar environ- different 2 : 2 complexes were formed, one in which the
ment and were much better protected from quenching by naphthyl groups were in close proximity leading to the for-
molecules residing in the aqueous phase.19–21 The shape of mation of the excimer and other 2 : 2 complexes where the
the guest determined which stoichiometries were formed for naphthyl groups were kept separate leading to long-lived triplet
the CD complexes.17 Not all complexes containing two excited states. The NN and NE complexes placed the naphtha-
naphthalene guests led to the observation of excimer emission lene moieties at a distance leading to long-lived triplets that
because steric hindrance could impede the approach of the were well protected from quenchers in the aqueous phase.19
two guests inside the complex.18
For the EE complex the naphthalene moieties were in close
The current work is based on previous studies of the proximity leading to excimer emission of the guest dimer. The
binding of 2-naphthyl-1-ethanol (NpOH, Scheme 1) with formation of this dimer was consistent with the observation of
β-cyclodextrin (β-CD).19,22 These previous studies focused on a broadening of the excitation spectrum for NpOH when the
the determination of binding isotherms and the dynamics for excimer emission was monitored and the lack of a growth kine-
the 1 : 1 complexes,19 and on theoretical calculations where tics for the excimer emission.22 The calculated energies for the
predicted circular dichroism spectra were compared with NN, NE and EE complexes were close and a comparison of the
experimental ones.22 The current work focuses on kinetic calculated and the experimental circular dichroism spectra
studies for the formation of the 2 : 2 complexes for which only supported the presence of multiple 2 : 2 complexes.22
order of magnitude estimates were reported,19 and on the
possible mechanisms for chiral discrimination that previously
was observed only as differences in fluorescence intensities
Fig. 1 Formation of the two 1 : 1 complexes between NpOH and β-CD
Scheme 1 Structure of (R)-2-naphthyl-1-ethanol ((R)-NpOH) and
β-cyclodextrin (β-CD).
and the formation of the three 2 : 2 complexes from the reaction of the
E and N complexes.
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The dynamics for the 1 : 1 complexation of NpOH with β-CD Instrumentation
is fast with an association rate constant of 3 × 108 M−1 s−1 and
a dissociation rate constant of 2 × 105 s−1, while the dynamics
for the 2 : 2 complex is much slower and was estimated to
occur in the millisecond time domain.19 Higher excimer inten-
sities were observed for (R)-NpOH/β-CD than for (S)-NpOH/
β-CD, suggesting that chiral discrimination occurs but was not
apparent in the previous thermodynamic and kinetic studies
because of the relatively large experimental errors.19,22 The
objective of the current study was to investigate the origins of
the chiral discrimination for a host–guest system that contains
multiple complexes. The kinetics for the formation of the 2 : 2
complexes, for which chiral discrimination was observed in
the steady-state emission intensities, was followed by the kine-
tics of excimer emission, which is unique for the presence of
the EE complex. These kinetic data were combined with
binding isotherm studies at variable guest and host concen-
trations. The chiral discrimination is due to the faster dis-
sociation of the 2 : 2 complexes formed for (S)-NpOH, while no
chiral discrimination was observed for the equilibrium con-
stants (K11) of the 1 : 1 complexes.
UV-Vis spectra were recorded using a Varian Cary 1 spectro-
meter with samples kept at room temperature. A PTI QM-2
fluorimeter was employed to measure steady-state fluorescence
spectra. The excitation wavelength was set to 280 nm, and the
emission spectra were recorded between 300 nm and 500 nm.
The bandwidths for the excitation and emission monochroma-
tors were 2 nm. Samples contained in 10 mm × 10 mm quartz
cells were kept at a constant temperature of 10.0 0.1 °C using
a circulating water bath. Spectra were corrected by subtracting
a baseline spectrum for a solution containing all chemicals
with the exception of NpOH. This baseline spectrum corres-
ponds to the water Raman emission and a low impurity emis-
sion (300–375 nm) from β-CD.
Fluorescence decays and time-resolved emission spectra
(TRES) were measured using an Edinburgh Instruments
OB920 single photon counter. The excitation source was a
277 nm pulsed light emitting diode (EPLED280 – Edinburgh
Instruments). Decays were collected at 330 nm or 380 nm with
a monochromator bandwidth of 16 nm. A Ludox (Aldrich)
solution was used to collect the instrument response function
(IRF) by scattering the excitation light at 277 nm. The decays
were collected until at least 2000 counts were reached in the
channel with maximum intensity. Samples were held in
10 mm × 10 mm quartz cells kept at 10.0 0.1 °C for at least
15 min using a circulating water bath.
Experimental
Materials
Time-resolved emission spectra (TRES) were collected at
every 10 nm between 300 nm and 500 nm by keeping the col-
lection time constant at 5 min, which corresponded to a collec-
tion of ca. 20 000 counts in the channel of maximum intensity
for the maximum emission wavelength of 330 nm. Each decay
trace was integrated between specific time delays to construct
the TRES.
Fluorescence decays were fit to a sum of exponentials (eqn
(3)) using the Fast software (version 3, Edinburgh Instru-
ments), where τi corresponds to the lifetime of each species
and Ai is the associated pre-exponential factor. The calculated
fit was reconvoluted with the IRF and was compared to the
experimental data until the differences between the calculated
fit and the experimental data were minimized. Fits to indivi-
dual decays were deemed to be adequate when the residuals
were random and the χ2 values were between 0.9 and 1.2.26
The number of exponentials was increased until an adequate
fit was obtained. Global analysis (Fast software) was used
as an alternate analysis method when the emission decays
for the same sample were measured at two different
wavelengths.
β-CD was a generous gift from Cerestar (lots C6034-13 and
F6080-191), (R)- and (S)-NpOH (Fluka) and NaCl (BDH) were
used as received. The purities of (R)- and (S)-NpOH were
checked by measuring fluorescence decays of 10 µM aqueous
solutions for each NpOH. The decays were mono-exponential
indicating that no other emissive species were present. NaNO2
(Aldrich) was recrystallized from water. Deionized water
(Barnstead NANOpure, 17.8 MΩ cm−1) was employed for all
samples.
Solution preparation
NpOH (1 mM) and β-CD (10 mM) stock solutions were pre-
pared in 0.2 M NaCl aqueous solutions, since the magnitude
of the equilibrium constants were shown to depend on the
concentration of NaCl.21 NpOH–β-CD solutions with the
highest β-CD concentration (10 mM) were prepared by dissol-
ving β-CD in NpOH–NaCl solutions. Solutions with lower β-CD
concentrations were prepared by dilution with NpOH–NaCl
solutions. All solutions were shaken overnight. All samples
were aerated with the exception of the solutions for laser flash
photolysis experiments that were purged with N2O for 20 min.
N2O is known to trap solvated electrons24 formed from the
photoionization of organic molecules in aqueous solutions.25
The triplet excited states of NpOH were quenched by
n
h
i
X
Ai ꢀ et=τ
ð3Þ
i
It ¼ I0 ꢀ
i¼1
Laser flash photolysis (LFP) was employed to measure the
NaNO2. NaNO2 aqueous solutions (∼0.6 M) were prepared kinetics for the triplet excited states of NpOH.27,28 Samples
daily and were purged with N2O for 20 min. Appropriate were excited with a Quanta-Ray Lab 130-4 pulsed Nd:YAG laser
amounts of quencher stock solution were injected into 2.5 mL at 266 nm (≤20 mJ per pulse, Spectra Physics). The transient
of the sample solution using a gas tight syringe.
absorption changes at 420 nm were simultaneously detected
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on two oscilloscopes.28 This procedure was required to deter- 1 : 1 complexes followed by a slow reaction forming the 2 : 2
mine the kinetics at short and long times. The samples complexes.
contained in 10 mm × 10 mm quartz cells were maintained at
10.0 0.1 °C for at least 15 min using a modified cryostat
(Unisoku USP-203 cryostat) with 4 windows.29
Results
The decays were fit either to a monoexponential function
The various complexes in solution need to be known in order
to interpret the kinetic data. The experiments were performed
at 10 °C in order to sufficiently slow down the reaction for
kinetic experiments to be feasible with the stopped-flow
system. The changes in the fluorescence of NpOH in the pres-
ence of β-CD (Fig. 2) were qualitatively the same as observed
previously at 20 °C.19,22 At a low NpOH concentration (5 µM)
only the monomer emission of NpOH was observed in the
presence of β-CD and no differences in the emission intensi-
ties were observed for both enantiomers (inset Fig. 2). At this
concentration of NpOH only 1 : 1 NpOH–β-CD complexes are
formed. The same intensities observed for both enantiomers
suggest that either no chiral discrimination occurs for the for-
mation of the 1 : 1 complexes or that all 1 : 1 complexes formed
have the same emission efficiency for NpOH.
At a high NpOH concentration (150 µM) the formation of
the 2 : 2 complex was observed as a broad red-shifted excimer
emission (Fig. 2). The excimer emission intensity is related to
the formation of the EE complex, whereas the monomer emis-
sion includes the emissions from free NpOH, N, E, NN and NE
complexes. At a NpOH concentration of 150 µM the excimer
emission is higher for (R)-NpOH than for (S)-NpOH, while the
reverse trend was observed for the fluorescence intensities for
the emission from NpOH–β-CD complexes other than EE all of
which emit as a NpOH monomer. This result indicates a chiral
discrimination for the formation of the 2 : 2 complexes.
(kobs2 = 0 and a2 = 0, eqn (4)) or to the sum of two exponentials
(eqn (4)), where a1 and a2 are pre-exponential factors for pro-
cesses with rate constants kobs1 and kobs2. It is important to
note that the ai values in the case of laser flash photolysis
experiments correspond to absorbance values while in the
case of single photon counting experiments the Ai values corres-
pond to fractional values.
obs1 t
obs2 t
ΔA ¼ a0 þ a1eꢁk
þ a2eꢁk
ð4Þ
The data were fit using the Kaleidagraph software (v. 4.1,
Synergy Software). The values of kobs1 and kobs2 recovered
from the fit are very dependent on the value of the final
offset a0. Therefore, a0 was determined from a kinetic trace
collected at a sufficiently long collection time as a constant
absorbance value after the decay of all triplets. The kinetic
traces at two different collection times were determined
simultaneously.
A SX20 stopped-flow system from Applied Photophysics was
employed to measure the kinetics for the formation of the 2 : 2
NpOH–β-CD complexes. The dead time for mixing was 1 ms.
Samples were excited at 280 nm (9.3 nm bandwidth) and fluo-
rescence was collected with an interference filter with the
centre wavelength of 405 nm (FWHM of 38 nm, Melles Griot
ANDV7248, lot 13637). Two solutions were contained in separ-
ate syringes; one syringe with the NpOH solution and the
second one with the β-CD solution. These solutions were
mixed in a 1 : 1 ratio. Samples were kept at 10.0 0.1 °C using
a circulating water bath for at least 20 min before experiments
were performed. The following control experiments were per-
formed: (i) mixing of a 0.2 M NaCl solution with a 0.2 M NaCl
solution to determine the reading of the “zero intensity” for
the equipment, (ii) mixing of β-CD solution with 0.2 M NaCl
solution to determine the baseline value for the system in
the absence of fluorophore. The values for control experiments
(i) and (ii) are very similar and were less than 2% of the
lowest offset intensity measured for the system containing the
lowest concentrations of NpOH (50 µM) and β-CD (50 µM)
employed.
Characterization of the 1 : 1 NpOH–β-CD complexes
Excited-state lifetime measurements can be used to differen-
tiate fluorophores located in different environments. The life-
time for (R)- and (S)-NpOH in water measured at 330 nm was
25.4 0.3 ns, which was similar to the lifetimes measured at
The kinetic data were fit to a monoexponential function or
to a sum of exponentials using the Pro-data Viewer software
from Applied Photophysics. This fitting procedure was used to
determine how many relaxation processes were observed for a
particular set of data. A global analysis method using the Pro-
Kineticists II software from Applied Photophysics was
employed to simultaneously fit a set of data where an experi-
mental condition, e.g. the β-CD concentration was systemati-
cally varied. The global analysis method requires the
definition of a model for the sequence of reactions, which in
Fig. 2 Fluorescence spectra for (R)- and (S)-NpOH (150 µM) in the
absence (a, b – black; both spectra are superimposed) and presence of
10 mM β-CD (c, red (S)-NpOH; d, blue (R)-NpOH). The inset shows the
fluorescence spectra for (R)- and (S)-NpOH (5 µM) in the absence (black;
both spectra are superimposed) and presence of 10 mM β-CD (red
this study included a fast equilibrium for the formation of the (S)-NpOH and blue (R)-NpOH; both spectra are superimposed).
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20 °C (25 ns).19 The ratio between the NpOH and β-CD concen-
The observation of the same K11 values for both enantio-
trations was low and an impurity emission from β-CD was mers is in line with the same pre-exponential values observed
detectable in the lifetime measurements at the NpOH concen- for bound NpOH in the time-resolved emission studies. The
tration (5 µM) where only monomer emission was observed. fluorescence studies for the 1 : 1 NpOH complex showed that
The lifetime for this impurity emission (4.6
0.7 ns) was fluorescence cannot differentiate between the formation of the
shorter than the lifetimes for NpOH (>20 ns). The emission E and N complexes (Fig. 1). The presence of both these com-
decays in the presence of β-CD were adequately fit to a sum of plexes had previously been established in circular dichroism
three exponentials where the values for the impurity emission experiments combined with theoretical calculations.22 Unfor-
of β-CD (4.6 ns) and NpOH in water (25.4 ns) were fixed. The tunately circular dichroism experiments were not possible for
third lifetime recovered was 37.6 0.1 ns for (R)-NpOH and the determination of the equilibrium constant of E and N
38.1 0.1 ns for (S)-NpOH. These values were similar to the because of the poor signal-to-noise ratio at the low NpOH con-
34–38 ns lifetimes previously determined for the 1 : 1 centration required for these experiments. The lack of dis-
NpOH–β-CD complex at 20 °C.19,22 The longer lifetimes for the crimination between the E and N complexes in the binding
NpOH complexes are consistent with the increase in the studies suggests that their fluorescence properties, such as
steady-state fluorescence intensities observed for both NpOHs excited state lifetimes and emission quantum yields, are
in the presence of β-CD (inset Fig. 2).
similar. In this respect, it is important to note that time-
The pre-exponential factors for the fluorescence decays (Ai, resolved experiments are quite sensitive to differentiate
eqn (3)) are related to the relative concentrations of the fluoro- between similar lifetimes and the observation of only one life-
phores when the singlet excited state lifetimes are sufficiently time for the 1 : 1 complexes supports the suggestion that the
short so that interconversion between the excited species does complexes E and N cannot be differentiated by fluorescence.
not occur during their lifetimes. The lifetimes for NpOH in Therefore, experimentally it was only possible to determine
1 : 1 complexes with β-CD were shorter (<50 ns) than the relax- the value for K11, which corresponds to the sum of the indivi-
ation time for complex formation (310 ns), i.e. sum of the dual equilibrium constants for the formation of the E (KE) and
association and dissociation processes, at the highest β-CD N (KN) complexes.
concentration employed.19 Therefore, the Ai values can be
related to the relative concentrations for free and β-CD bound
Characterization of the 2 : 2 NpOH–β-CD complexes
NpOH. The same pre-exponential values were measured for The emission of the NpOH excimer was observed at high
both enantiomers. These values were 0.14 0.03 and 0.13
0.03 for free (R)- and (S)-NpOH, respectively and 0.86 0.03 complex was formed. The kinetics for the decay of the single
and 0.87 0.03 for (R)- and (S)-NpOH in the 1 : 1 complexes. excited states of NpOH were measured at 330 nm and 380 nm.
NpOH concentrations (150 µM) indicating that the 2 : 2 EE
The binding isotherms based on the changes in the fluo- The emission from NpOH free in water and from NpOH in E,
rescence intensity were fit numerically (see ESI† for details) N, EN and NN complexes predominate at 330 nm, while the
assuming a 1 : 1 binding stoichiometry between NpOH and emission from the excimer emission due to the EE complex
β-CD (Fig. 3). The parameters for the fit were the equilibrium predominates at 380 nm. The impurity emission from β-CD
constant for the 1 : 1 complex (K11, eqn (1)) and the relative was negligible at this NpOH concentration. No growth kinetics
emission intensity of the 1 : 1 complex (C11) compared to the was observed at 380 nm indicating that the excimer emission
intensity in water. The recovered K11 values from three indepen- was due to a NpOH dimer present before excitation, indicating
dent experiments were (1.10 0.03) × 103 M−1 and (1.08 0.03) × that both guest molecules are in close proximity in the EE
103 M−1 for (R)- and (S)-NpOH, respectively. The C11 values were complex.
1.74 0.01 and 1.78 0.02 for (R)- and (S)-NpOH, respectively.
The emission from the monomers and the excimer contri-
bute to the intensities at 330 nm and 380 nm. For this reason,
global analysis was employed in which the decays at both wave-
lengths were simultaneously fit to the sum of three exponen-
tials. No further lifetimes could be resolved. The shortest
lifetime, τ1, was fixed to 25.4 ns assigned to NpOH in water,
while the second lifetime, τ2, was fixed to 38 ns for the
monomer emission of NpOH complexed to β-CD. The third
lifetime recovered corresponds to the excimer emission and
was 72.7 0.3 ns for (R)-NpOH and 72.0 0.3 ns for (S)-NpOH.
The A3 values were significantly higher for (R)-NpOH than for
(S)-NpOH (Table 1) in line with the higher steady-state emis-
sion intensity for the excimer emission observed for the (R)-
enantiomer (Fig. 2). This result shows that for (R)-NpOH a
larger amount of the EE complex was formed when compared
to the 2 : 2 complexes for (S)-NpOH. The different intensities
could also have been related to different lifetimes for the
Fig. 3 Numerical fit for the dependence of the (R)-NpOH (5 µM)
monomer fluorescence intensity with the β-CD concentration assuming
the formation of a 1 : 1 complex.
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observed because the contribution from free NpOH was negli-
gible in the presence of 10 mM β-CD. Decays were collected
simultaneously for short and long time windows to ensure the
precise determination of the final absorbance values (a0) and
of the rate constants, kobs1 and kobs2 (Fig. S1 and S2, see ESI†
for details). The quenching plots (eqn (5)) for the short- and
long-lived species were linear (Fig. S3†), and the same quench-
ing rate constants were obtained for (R)- and (S)-NpOH. The
quenching rate constants for the short-lived species assigned
to the 1 : 1 complex ((8.8 0.2) × 108 M−1 s−1 for (R)-NpOH
and (8.4 0.2) × 108 M−1 s−1 for (S)-NpOH) were lower than for
the quenching in water, but were higher than the kq values for
the 2 : 2 complexes ((2.9 0.1) × 107 M−1 s−1 for (R)-NpOH and
(3.1 0.2) × 107 M−1 s−1 for (S)-NpOH). For each type of β-CD
complex, that is 1 : 1 or 2 : 2 complexes, the quenching rate
constants were the same for both enantiomers of NpOH,
showing that the accessibility of nitrite anions to NpOH inside
one or two β-CDs is not determined by the chirality of the
guest.
Table 1 Pre-exponential factors for the emission of NpOH in water (A1),
NpOH monomer emission in β-CD complexes (A2) and excimer emission
from the EE complex (A3) obtained from the global analysis for the
decays measured at 380 nm and 330 nma
NpOH
λem/nm
A1
A2
A3
R-
S-
R-
S-
380
380
330
330
0.17
0.21
0.22
0.09
0.11
0.38
0.70
0.86
0.72
0.42
0.08
0.05
a The errors for the Ai values are 0.03.
excimer emission of the two enantiomers, which was not the
case. In contrast, there is a higher fraction of the E, N, EN and
NN complexes formed for (S)-NpOH (see A2 values at both
wavelengths, Table 1). The relative concentrations of the E, N,
EN and NN complexes could not be determined from the time-
resolved fluorescence studies because all these complexes had
lifetimes close to 38 ns.
Complex EE is expected not to form triplet excited states
based on previous studies where the excimer emission from
guests in 2 : 2 CD complexes were observed.20,23 The formation
of excimer precludes intersystem crossing of the singlet
excited state to the triplet state in line with the lack of a growth
kinetics for the excimer emission. For this reason, studies on
the absorption of the triplet excited states of NpOH in the pres-
ence of β-CD is selective to differentiate between the E, N and
EN, NN complexes without interfering signals from the EE
complex. In the EN and NN complexes the naphthalene moi-
eties are too far apart to form excimers and excitation leads to
the formation of one of the NpOHs in the triplet state.22 Pre-
vious work19 also showed that triplet NpOH in the 2 : 2 com-
plexes was much better protected from quenchers in the
aqueous phase than triplet NpOH in 1 : 1 complexes. Laser
flash photolysis was employed where the samples were excited
at 266 nm to measure the kinetics of triplet NpOH at 420 nm
in the absence and presence of β-CD. Quenching experiments
were performed at 10 °C using nitrite anions as a quencher
(Q).30 The quenching plots for NpOH in water (eqn (5), where
kobs is the observed decay rate constant and k0 is the rate con-
stant in the absence of quencher) were linear leading to
quenching rate constants (kq) of (2.5 0.1) × 109 M−1 s−1 and
(2.6 0.1) × 109 M−1 s−1 for (R)- and (S)-NpOH, respectively.
These values were similar to the quenching rate constant for
triplet NpOH by nitrite ions of 3.1 × 109 M−1 s−1 determined in
water at 20 °C.31
The pre-exponential factors for the slow decay (eqn (4), a2)
were 0.020
0.001 for (R)-NpOH and 0.023
0.004 for
(S)-NpOH when solutions were excited that had the same
absorbance values at 266 nm. The a2 values correspond to the
sum of the concentrations of EN and NN complexes. The fact that
the same a2 values, within experimental error, were obtained
for (R)- and (S)-NpOH at a given β-CD concentration showed
that the amount of EN and NN complexes formed were similar
for both enantiomers of NpOH.
The dependence of the EE emission at 390 nm with the
concentration of β-CD was used to determine the equilibrium
constant for the 2 : 2 complexes. The monomer and excimer
emissions contribute to the intensity at 390 nm. The spectrum
for the excimer emission needs to be known in order to calcu-
late the contribution of the monomer emission at 390 nm.
TRES were collected for the emission of (S)- and (R)-NpOH
(Fig. S4 in the ESI†) in the presence of 10 mM β-CD. The
spectra at long delays correspond to the emission from the EE
complex because the lifetime for the excimer emission is
longer than the lifetime for the NpOH monomer emission.
The same spectra were obtained for (R)- and (S)-NpOH. The
fraction of the emission intensity from the monomer at
390 nm with respect to the intensity at 330 nm was calculated
using the NpOH emission spectrum in water, while the frac-
tion of the excimer emission at 330 nm with respect to the
intensity at 390 nm was calculated from the TRES. From these
values operational equations were derived for the corrected
monomer and excimer intensities at 330 nm and 390 nm,
respectively (eqn (S14)–(15) in the ESI†).
kobs ¼ k0 þ kq½Qꢂ
ð5Þ
The decay for triplet NpOH in the presence of 10 mM β-CD
The equilibria for the formation of the 1 : 1 and 2 : 2 com-
was adequately fit to the sum of two exponentials. This result plexes are coupled (Scheme 2) and all equilibria need to be
indicated that two populations of triplets were present that did considered when fitting the binding isotherm for the for-
not interconvert during the lifetime of these excited states mation of one of the products, which corresponds to the EE
(<100 µs). The short-lived triplet was previously assigned to complex in this study.
triplet NpOH in the 1 : 1 complex (E and N), while the long-
The binding isotherm was numerically fit considering the
lived triplet was assigned to triplet NpOH in the 2 : 2 com- equilibria shown in Scheme 2 by making the following
plexes (EN and NN).19,22 No triplet for NpOH free in water was assumptions (see ESI† for details): (i) the value for KE was
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with respect to the intensity for the monomer of NpOH in
water at the detection wavelength. The CEE values were infinity
when KE was zero and (1.43 0.03) × 104 and (0.72 0.01) ×
104 for the (R)- and (S)-enantiomers when KE was equal to K11.
Fits were attempted by fixing the CEE and KE for both enantio-
mers to the same values, but the fits were not adequate
suggesting that either CEE or KE values were different for the
two enantiomers. The singlet-state lifetimes for EE complex
were the same for both enantiomers, suggesting that the CEE
values for both enantiomers were similar. Therefore, the KE
values were different for (R)- and (S)-NpOH. The CEE value of
infinity has no physical meaning. KE cannot be zero since EE
complexes were observed. However, this limit was included to
show the continuous variation of KEE and CEE when KE was
varied. The maximum values of KEE of (7.8 0.3) × 103 M−1 for
Scheme 2
fixed for each fit to a value between 0 and K11 (1100 M−1),
which corresponds to the theoretical limits for this parameter,
and (ii) the values for KEE, KEN and KNN were considered to be
equal based on the analysis of the kinetic data for the for-
mation of the EE complex where only one relaxation process
was observed (see details below). The binding isotherm fit well
to this model (Fig. 4) and the quality of the fit was the same
irrespective of the KE values used. The KEE (inset Fig. 4) and
the relative emission efficiency (CEE, Fig. S5†) values varied in
a systematic manner as the KE values were changed showing
that these parameters were correlated. In all cases, the values
for KEE and CEE were higher for (R)-NpOH than for (S)-NpOH,
showing that a chiral discrimination occurs for the formation
of the 2 : 2 complexes.
(R)-NpOH and (6.0
0.3) × 103 M−1 for (S)-NpOH were
obtained when KE and KN were fixed to be equal. It is impor-
tant to note that the dependencies for KEE and CEE with KE for
(R)-NpOH never cross with the dependencies for (S)-NpOH.
Kinetics for EE complex formation
The kinetics for the formation of the EE complex was
measured using a stopped-flow set-up by mixing a solution of
NpOH with a solution containing β-CD. The fluorescence
intensity levelled off within 0.1 seconds (Fig. 5) indicating that
the reaction was completed within this timeframe. All kinetic
traces were adequately fit to a mono-exponential function
(Fig. S6†). At given NpOH and β-CD concentrations the
observed rate constant for the relaxation kinetics of (S)-NpOH
was always higher than for (R)-NpOH, showing that the relax-
ation kinetics for the (S)-enantiomer were faster.
The limiting values for KE are when this parameter is equal
to zero, equal to KN, or equal to K11. The KEE value for (R)-
NpOH was (5.8 0.2) × 103
M
−1, while it was (4.5 0.2) ×
103 M−1 for (S)-NpOH when KE was either fixed at zero or equal
to K11. The increase in the emission efficiency at 390 nm (CEE
The kinetic traces showed an initial offset value (Fig. 5).
Most of the offset is due to the presence of NpOH as a fluoro-
phore and therefore the offset was higher as the NpOH concen-
)
corresponds to the ratio of the intensity of the EE complex
tration was raised. However, at
a
constant NpOH
concentration, the offset was higher at the higher β-CD concen-
tration (for clarity see traces “e” in Fig. 5). This additional
increase in the presence of β-CD indicates that a process
occurred involving β-CD with a time constant faster than the
1 ms time-resolution of the stopped-flow system. This process
Fig. 4 Dependence of the corrected excimer emission intensity at
390 nm for (R)-NpOH (150 µM) with the β-CD concentration (solid
symbols; three independent experiments). The solid line corresponds
to the numerical fit of the data using fixed KE values between 0 and
1100 M−1. The residuals between the data and the fit are shown as open
symbols. The inset shows the variation of the KEE value as the fixed value
for KE was changed in the numerical fit for (R)-NpOH (blue circles) and
(S)-NpOH (red squares). The lines in the inset were included to guide
the eye.
Fig. 5 Kinetics for the formation of the EE complex of (R)-NpOH when
mixed with β-CD (black: 50 µM; red: 100 µM) at various concentrations
of (R)-NpOH: a, 50 µM; b, 100 µM; c, 150 µM; d, 200 µM and e, 300 µM.
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corresponds to the formation of the 1 : 1 NpOH–β-CD complex,
which was previously shown to occur in the microsecond time
domain.19 This process is observable because the NpOH
monomer emits to some extent at the detection wavelengths
(405 nm, FWHM = 38 nm) and the intensity for NpOH in the
1 : 1 complex is higher than in water (see above).
Table 2 Overall association and dissociation rate constants for the
formation of the 2 : 2 complexes of the enantiomers
NpOH
k2+2/105 M−1 s−1
k2−2/102 s−1
(R)-
(S)-
6.9 0.3
7.2 0.4
1.11 0.05
1.63 0.02
The amplitude of the kinetic traces (final offset intensities)
corresponds to the amount of EE complex formed. This ampli-
tude increases with the concentration of β-CD at constant
NpOH concentrations (Fig. 5: compare red and black traces for
each set) and also increases with the NpOH concentrations at
constant β-CD concentrations (Fig. 5: compare all black or all
red traces). These trends are consistent with the formation of a
2 : 2 complex where the concentration of NpOH and β-CD
needs to be high for the formation of the complex to occur.
The fast formation of the E and N complexes can be treated
as being in pre-equilibrium. The kinetics for the formation of
the EE complex is coupled to the kinetics for the formation of
the other 2 : 2 complexes (EN and NN). All kinetic traces fit
adequately to a mono-exponential function indicating that
only one relaxation process was observed. This result suggests
that the kinetics for the formation of all 2 : 2 complexes have
similar relaxation times. If one of the complexes was formed
faster, then it would transiently accumulate before redistribu-
tion to the other complexes occurred. Such a scenario would
lead to kinetics with more than one relaxation process. Simu-
lations were performed in which the association or dis-
sociation rate constants for the EE and the EN/NN complexes
were varied in order to establish how different the rate con-
stants had to be for the simulated data to lead to non-random
residuals when fit with the model used to analyze the experi-
mental data (Fig. S7, see ESI† for details). Association and/or
dissociation rate constants that differed by 20–25% would lead
to non-random residuals and to the detection of more than
one relaxation time. This simulation set the upper limit for the
difference in the association and dissociation rate constants
for the EE, EN and NN complexes, and consequently for differ-
ences in their equilibrium constants. Differences of ca. 25% in
the rate constants and consequently in the equilibrium con-
stants would not be discernable in the experiments.
residuals (Fig. S8†) were random showing that the model is
consistent with the experimental data. The k2+2 values were the
same for both enantiomers of NpOH, while a larger k2−2 value
was observed for (S)-NpOH when compared to (R)-NpOH
(Table 2). The faster dissociation of the 2 : 2 complexes for
(S)-NpOH is responsible for the faster relaxation kinetics and
the lower equilibrium constants observed for this enantiomer.
A relationship can be obtained between the rate constants
in Table 2 and the rate constants for the formation of the EE
complex (Scheme 2) when it is assumed that the equilibrium
constants for the formation of the three 2 : 2 complexes are the
same (eqn (15)–(18), see ESI† for derivation).
d½EEꢂ
2
¼ kþEE½Eꢂ ꢁ kꢁEE½EEꢂ
ð15Þ
ð16Þ
dt
ꢀ
ꢁ
2
KN
1 þ
d½EEꢂ
dt
KE
2
¼
k22½Eꢂ þ kꢁ22½EEꢂ
ꢀ
ꢀ
ꢁ ꢁ
þ
2
KN
KE
KN
KE
1 þ
þ
The relationship between the association and dissociation
rate constants for the EE complex and the experimentally
determined parameters k2+2 and k−22 are:
ꢀ
ꢁ
2
KN
1 þ
KE
kþEE
¼
k22
ð17Þ
ð18Þ
ꢀ
ꢀ
ꢁ ꢁ
þ
2
KN
KE
KN
KE
1 þ
þ
kEꢁE ¼ k2ꢁ2
The value for the dissociation rate constant is independent
of the magnitude of KE with respect to KN for the two enantio-
The kinetic data were fit using global analysis to a model in mers. Therefore, the higher dissociation rate constant for (S)-
which a pre-equilibrium is followed by the formation of 2 : 2 NpOH does not depend on the exact value for KE. The value for
complexes (Scheme 3). Global analysis was employed to fit kE+E is minimum when KE is equal to K11 ((6.9 0.3) × 105 M−1
simultaneously the kinetic data at different NpOH and β-CD s−1 for (R)-NpOH and (7.2 0.4) × 105 M−1 s−1 for (S)-NpOH),
concentrations. The K11 value was fixed to the value deter- while the upper limit is reached when KE and KN are equal
mined in the fluorescence binding isotherm studies. All ((9.2
0.4) × 105 M−1 s−1 for (R)-NpOH and (9.6
0.5) ×
105 M−1 s−1 for (S)-NpOH). The values for kE+E are the same
within experimental errors for both enantiomers for the same
KE value assumed. The values for the equilibrium constant for
the EE complex can be calculated from the association and dis-
sociation rate constants (eqn (11), Scheme 2) and these calcu-
lated KEE values are the same as those obtained from the
binding isotherms based on the variation of the steady-state
emission intensities (Table S1 in the ESI†). This equality
shows that the analysis of the kinetic study and the binding
studies are self-consistent.
Scheme 3
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relaxation kinetics for (S)-NpOH. The higher excimer intensity
indicates that for (R)-NpOH a higher concentration of EE was
Discussion
CDs have been used as chiral selectors in a variety of analytical formed than for (S)-NpOH. It is important to note that the
techniques.32–34 The potential of CDs as chiral selectors was observed rate constant of a relaxation process corresponds to
realized early on when these hosts were used for the partial the sum of the association and dissociation processes and
chiral discrimination of racemates.35,36 This discrimination either the association or the dissociation process (or both)
was attributed to differences in the efficiencies for the for- could be faster for (S)-NpOH when compared to (R)-NpOH.
mation of the enantiomer–CD complexes. Preferential precipi-
No chiral discrimination was observed for the values of K11,
tation of one enantiomer through formation of a 1 : 2 (guest– which corresponds to the sum of KE and KN. Only one singlet
CD) complex was shown to be due to a combination of differ- excited state lifetime was observed for the NpOHs in the 1 : 1
ential binding constants and differential inherent solubilities complexes and the intensity increase for the fluorescence and
of the enantiomers.37 The chiral discrimination of natural CDs the lengthening of the lifetimes for the 1 : 1 complexes were
is in general low,12,38 and the ability to discriminate between the same for both enantiomers. A similar result was previously
enantiomers of guests is attributed to binding of the guest observed for the binding of the enantiomers of 1-naphthyl-1-
within the cavity and its interactions with the rim of the ethylamine with β-CD as a 1 : 1 complex.50 The observation of
CDs.39,40 Chiral recognition is improved for modified CDs one lifetime for the 1 : 1 complexes indicates that E and N
because these modifications introduce asymmetries to the cannot be differentiated spectroscopically and that the
host.3,41 Chiral discrimination is also improved when higher environment for the chromophore in E and N is similar for
order CD complexes are formed.42 In general, CDs are poor in (R)-NpOH and (S)-NpOH. Therefore, the binding isotherms do
discriminating for guests with central chirality, and better dis- not provide information on the ratio between E and N for each
crimination is observed when the stereocenters are located one of the enantiomers of NpOH. Analysis of the binding iso-
axially or the guests are helical.43 Hydrogen bonding is not a therm for the formation of the EE complex (Fig. 4) showed that
pre-requisite, as chirality was achieved by asymmetric “twist- parameters KE and CEE are correlated (Fig. S5†). CEE corres-
ing” of the guests within the chiral CD. For example, circularly ponds to the relative emission efficiency for the EE complex.
polarized luminescence was observed for the excimer of pyrene CEE can be assumed to be the same for both enantiomers
in γ-CD.44 There is no direct relationship between the thermo- because the lifetimes for the excimer emission of (R)-NpOH
dynamic parameters for formation of 1 : 1 complexes and the and (S)-NpOH in the 2 : 2 complexes were the same. For a con-
degree of chiral recognition observed for different chiral stant CEE value, the value of KE for (R)-NpOH is always higher
guests.38
than for (S)-NpOH (Fig. S5). Since the K11 values are the same
Asymmetric photoreactions were achieved for various for both enantiomers (1100 M−1) the values of KE can be esti-
systems in CDs.2,3 Most examples are related to unimolecular mated to be between 600 and 700 M−1 for (R)-NpOH and 400
reactions in 1 : 1 complexes. Different outcomes were observed and 450 M−1 for (S)-NpOH. This analysis shows that for (R)-
when benzaldehyde was photolyzed in the solid state with NpOH there is a preference for the formation of the E complex
different CDs. For the 1 : 1 complex with β-CD an enantiomeric over the N complex.
excess for the chiral product was observed, while no chiral dis-
The kinetics for the formation of EE probed the formation
crimination was observed for the 2 : 1 (guest–CD) complex with of all 2 : 2 complexes because all equilibria are coupled. No
γ-CD.45 The rate of bimolecular photocyclizations reactions for growth kinetics was observed for the excimer emission
CD complexes containing two guests is frequently enhanced.2 suggesting that its formation occurred instantly upon exci-
In the case of anthracene derivatives the enhancement was tation in the same fashion as for the photodimerization reac-
observed for the 2 : 2 complexes with β-CD and 2 : 1 complexes tion of 2-anthracenecarboxylate in γ-CD.49 The structure of the
with γ-CD.46–48 Photodimerization of 2-anthracenecarboxylate EE complexes are likely to be similar for both NpOH enantio-
in γ-CD occurred immediately upon excitation leading to mod- mers, since the same lifetimes were observed for their excimer
erate enantiomeric excesses.49 These examples show that emissions. In addition, the structure for the EN and NN com-
higher order complexes containing more than one CD play a plexes are also similar because the same quenching rate con-
role in the chiral discrimination achieved with CDs.
stants were observed for the triplet excited states of the NpOHs
The objective of this study was to determine the origin for in these complexes. A comparable observation was reported
the chiral discrimination of a system with different host–guest for the binding of the enantiomers of ketoprofen with β-CD in
stoichiometries (1 : 1 vs. 2 : 2) and with different geometries for a 1 : 1 (guest–CD) complex where the binding constants and
the 1 : 1 and 2 : 2 complexes (E vs. N and EE vs. EN and NN). the decarboxylation kinetics of the excited states were the
The chiral discrimination observed for this system is modest, same and subtle structural changes were only observed by
but the importance of this study is to understand the mechan- circular dichroism and NMR experiments.51 These results
ism of the chiral discrimination for a complex system where suggest that subtle changes in the structure of the complexes
several host–guest complexes co-exist. The formation of these are not always detectable in photophysical experiments.
five complexes was established previously.19,22 The chiral dis-
crimination for the NpOH–β-CD system appears as a higher the (S)-enantiomer as the pre-exponential factor for the excimer
The concentration of EE was higher for (R)-NpOH than for
excimer emission intensity for (R)-NpOH and
a
faster emission was higher for the former guest. Consequently, the
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pre-exponential factor for the monomer emission, which sum of both complexes is the same. The dynamics for the for-
includes the EN and NN complexes, was higher for the mation of the three 2 : 2 complexes (EE, EN and NN) were
(S)-enantiomer. The same concentrations within experimental similar leading to one relaxation process being observed in the
errors were observed for the sum of EN and NN for both enan- kinetic studies. The faster relaxation kinetics observed for (S)-
tiomers from the analysis of the triplet excited state decays, NpOH is because the 2 : 2 complexes for this enantiomer dis-
but the errors in these experiments were too large, ca. 25%, to sociate faster. This result shows that the chiral discrimination
be conclusive. The kinetics for EE formation showed only one observed is primarily dictated by the stability of the 2 : 2 com-
relaxation time for both NpOH enantiomers. This result is con- plexes and not by the structural diversity of the 1 : 1 complexes.
sistent with similar dynamics for the formation of the three This system is an example where the formation of higher order
2 : 2 complexes (EE, EN and NN). Simulations showed that the complexes is essential for the observation of the chiral dis-
differences in the association and dissociation rate constants crimination of enantiomers.
for the various 2 : 2 complexes would have to be larger than
25% for more than one relaxation process to be observed in
the kinetics for the formation of EE. Based on this analysis we
assumed that the equilibrium for all 2 : 2 complexes were the
Acknowledgements
same. Therefore, the higher excimer intensity observed for (R)- The Natural Sciences and Engineering Council of Canada in
NpOH when compared to (S)-NpOH is because of a higher con- the form of Discovery (DG) and Research Tools and Instru-
centration of all 2 : 2 complexes.
ments (RTI) grants funded this work.
The dissociation rate constant for the 2 : 2 complexes of
(S)-NpOH was higher than for (R)-NpOH. It is important to
note that this result is independent of the KE value assumed.
The latter parameter only affects the magnitude of the associ-
ation rate constants. The values for the overall association rate
constants (k2+2) were the same for (R)-NpOH and (S)-NpOH,
suggesting that the association of the E and N complexes
leading to the formation of the 2 : 2 complexes is not respon-
sible for the chiral discrimination observed. The faster dis-
sociation rate constants for the 2 : 2 complexes with (S)-NpOH
suggest that these complexes are less stable than the ones
formed with (R)-NpOH. This difference could be related to the
alignment of the secondary hydroxyl groups at the rim of the
CDs that interact in the 2 : 2 complex. This alignment is more
favorable in the case of (R)-NpOH. The chiral recognition is
not primary related to the difference in the E and N complexes
because the overall equilibrium constant for the formation of
the 1 : 1 complexes is the same for both enantiomers. E is pre-
ferred for (R)-NpOH, while N is the preferred complex for (S)-
NpOH, but the sum of both is the same for each enantiomer.
This result maybe a reflection of the similar energies for these
two complexes.22
Notes and references
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Conclusions
The origin for the chiral discrimination in a system containing
two 1 : 1 and three 2 : 2 guest–host complexes was investigated
with the same guest and host for the NpOH–β-CD system. The
insights for this work is that for complex systems an interplay
of factors affects the chiral discrimination and that kinetic
studies can be instrumental to understand such systems. No
chiral discrimination was observed for equilibrium constants
of the 1 : 1 complexes between (R)- or (S)-NpOH with β-CD.
However, the analysis of the binding isotherm for the 1 : 1 and
2 : 2 complexes suggests that the formation of one of the 1 : 1
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complex for (R)-NpOH and N complex for (S)-NpOH), while the
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C. Bohne, Photophysical Studies on the Supramolecular
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2-Anthracenecarboxylate within Human Serum Albumin,
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8 C. V. Kumar, A. Buranaprapuk and H. C. Sze, Large Chiral
Discrimination of a Molecular Probe by Bovine Serum
Albumin, Chem. Commun., 2001, 297–298.
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