Enantioselective recognition of dicarboxylic acid guests based on an allosteric effect of a chiral double-decker porphyrin which changes the stoichiometry upon the guest binding

Article abstract of DOI:10.1080/10610278.2010.510191

A double-decker porphyrin 1R bearing one chiral substituent per porphyrin ring was synthesised. The host 1R showed highly cooperative binding towards chiral guest 2R with stoichiometry of the complex (1:2) and its stepwise association constants (K1 and K2

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Enantioselective recognition of dicarboxylic acid guests
based on an allosteric effect of a chiral double-decker
porphyrin which changes the stoichiometry upon the
guest binding
Tomohiro Ikeda a b , Kazuki Sada b , Seiji Shinkai b c & Masayuki Takeuchi a
a Macromolecules Group, National Institute for Materials Science, Organic Nanomaterials
Centre, Tsukuba, 305-0047, Japan
b Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu
University, Fukuoka, 819-0395, Japan
c Institute of Systems, Information Technologies and Nanotechnologies, Fukuoka, 814-0012,
Japan
Published online: 31 Aug 2010.
To cite this article: Tomohiro Ikeda , Kazuki Sada , Seiji Shinkai & Masayuki Takeuchi (2011) Enantioselective recognition of
dicarboxylic acid guests based on an allosteric effect of a chiral double-decker porphyrin which changes the stoichiometry
upon the guest binding, Supramolecular Chemistry, 23:01-02, 59-64, DOI: 10.1080/10610278.2010.510191
To link to this article: http://dx.doi.org/10.1080/10610278.2010.510191
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Supramolecular Chemistry
Vol. 23, Nos. 1–2, January–February 2011, 59–64
Enantioselective recognition of dicarboxylic acid guests based on an allosteric effect of a chiral
double-decker porphyrin which changes the stoichiometry upon the guest binding
Tomohiro Ikedaa,b, Kazuki Sadab, Seiji Shinkaib,c and Masayuki Takeuchia*
aMacromolecules Group, National Institute for Materials Science, Organic Nanomaterials Centre, Tsukuba 305-0047, Japan;
bDepartment of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan; cInstitute
of Systems, Information Technologies and Nanotechnologies, Fukuoka 814-0012, Japan
(Received 3 June 2010; final version received 12 July 2010)
A double-decker porphyrin 1R bearing one chiral substituent per porphyrin ring was synthesised. The host 1R showed highly
cooperative binding towards chiral guest 2R with stoichiometry of the complex (1:2) and its stepwise association constants
(K1 and K2) were evaluated to be log K1 ¼ 1.6 and log K2 ¼ 4.1, respectively. 1R also recognised 2S cooperatively and
formed into a 1:3 complex in log K1 ¼ 1.9, log K2 ¼ 1.2 and log K3 ¼ 3.8. When we use racemic 2, 1R exhibits
unconventional enantioselectivity only towards 2R in a specific concentration region. It not only showed a chiral screening
but also changed the stoichiometry depending on the guest chirality. These high enantioselectivity and stoichiometric
modulation are effected by proper incorporation of the structural information of two enantiomers into the host and utilisation
of multiple equilibrium characteristic of positive allosterism.
Keywords: host–guest chemistry; double-decker porphyrin; enantioselective recognition; allosteric effect
The design of synthetic allosteric systems is of great
significance not only to regulate the complexation ability
or the catalytic activity of receptors but also to attain high
selectivity in a nonlinear fashion (1, 2). Among them, the
design of positive homotropic allosteric systems is the
most attractive but most difficult one because the guest-
binding information in a subunit should be passed to all
other subunits in unison (3). The pivotal feature of positive
homotropic allosterism is a nonlinear sigmoidal response
to outside information, for example, the effector and/or
substrate concentration, which then generates bistable
OFF/ON states (4). It is expected, because of the multi-
step cooperative binding mode, that the nonlinear
sigmoidal response (binding isotherm) would generate
high selectivity and specificity towards the effectors and/or
substrates for the precise processing of molecular
information in such allosteric systems.
we designed DDPy7R and demonstrated unconventional
selectivity for 2R over 2S (Figure 1) (7). Herein, we
designed a new host (1R), in which each porphyrin has one
covalently bound 2R (Figure 1). Because of bridging two
porphyrins by two intramolecular hydrogen bonds, one
may expect the higher ‘chiral memory effect’ on the 2R vs.
2S selectivity. 1R was synthesised from Ce(IV) double-
decker porphyrin of 5-(4-aminophenyl)-10,15,20-tripyr-
idylporphyrin according to Scheme 1 and identified by 1H
NMR and MALDI-TOF-MS spectra (Figures S1 and S2 in
Supplementary Material, available online).
Initially, we evaluated the conformation of 1R in the
absence of guest in order to confirm the existence of
intramolecular hydrogen bonding. The presence of
intramolecular hydrogen bonds between the carboxylic
acid groups and the pyridyl groups can be easily estimated
from the rotational freedom of the two porphyrin rings.
The shape of the circular dichroism (CD) signals observed
for 1R was similar to that observed for DDPy8· (2R)
complex (4, 5). As shown in Figure 2, 1R gave the positive
CD around 310 nm, which was attributable to the chirality
around the pyridyl groups (5). On the other hand, the CD
signal around 310 nm disappeared by the addition of
excess triethylamine (Figure 2). These observations are
consistent with the intramolecular hydrogen bonding in 1R
(7). The similarity in the CD spectral shape between 1R
and DDPy8· (2R) complex implies that 1R has been
pre-organised so that it can readily accept 2R guest.
We previously reported that a cerium(IV) bis(porphyr-
inato) double-decker complex, DDPy8, exhibits positive
homotropic allosterism on binding cyclohexane-1R,2R-
dicarboxylic acid (2R) through the hydrogen bonds to form
the 1:4 DDPy8 · (2R)4 complex (Figure 1) (4). This is
because the binding of the first 2R suppresses the rotation
of two porphyrin rings to create three geometrically
equivalent binding sites (5, 6). When the structural
information of 2R was input into such an allosteric host
through a covalent bond, the conformation should be pre-
organised complementary to 2R. To confirm the hypothesis,
*Corresponding author. Email: takeuchi.masayuki@nims.go.jp
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2011 Taylor & Francis
DOI: 10.1080/10610278.2010.510191
http://www.informaworld.com

60
T. Ikeda et al.
Figure 1. Chemical structures of hosts DDPy8, DDPy7R, 1R and chiral guests 2.
N
N
N
H
NHAc
CHO
N
i), ii)
iii)
N
N
++
H
N
N
H
CHO
N
H2N
3
N
N
N
N
N
N
N
N
N
N
N
N
N
H
O
H
O
iv)
N
N
N
N
N
N
N
N
Ce
Ce
O
O
*
O
O
O
N
N
N
*
*
N
H
H2N
N
H
NH2
*
*
O
*
O
N
N
N
N
RR-CHDA
anhydride
4
1R
Scheme 1. Reagent and conditions: (i) propionic acid, reflux, 5 h; (ii) TFA water, reflux, 2 h (15% in total step); (iii) Ce(acac)3 · 3H2O,
p-t-BuPhNH2, 1,2,4-trichlorobenzene, reflux, 6 h (24%) and (iv) 2R anhydride, dichloromethane, rt, 1 h (quant.).
The theoretical examination of the 1R structure also
supports that the proposed structure is a reasonable one
(vide infra).
which were estimated to be 2.0 for the 1R · (2R) complex
and 2.8 for the 1R · (2S) complex (Figure 3(d) and (e)).
Generally, nH is defined as the value related to a degree of
cooperativity in a recognition process and the higher value
of nH (.1.0) means the higher degree of cooperativity. It is
also known that the nH value provides the information
about the stoichiometry of the complex. The nH value for
1R · (2R) was 2.0, indicating that the complex consists of
1:2 host–guest ratio. To further corroborate this
stoichiometry, we prepare the Job plot using the CD
intensity at 310 nm. The peak appearing at
[1R]/([1R] þ [2R]) ¼ 0.33 supports the view that the 1R
host binds two guests cooperatively (Figure S3 in
Supplementary Material, available online).
We evaluated the formation of the 1R · 2 complexes in a
tetrachloroethane (TCE)–tetrahydrofuran (THF) 30:1
(v/v) mixed solvent at 298 K using the CD spectral change
induced upon the successive addition of 2 (Figure 3(a) and
(b)). The CD intensity at 310 nm increased with the
increase in the 2R concentration with tight isosbestic
points, whereas it decreased with the increase in the 2S
concentration. First, these guest-binding events were
analysed using the Hill equation for plots of the CD
intensity at 310 nm (Figure 3(c)) (8). From the slope of the
linear Hill plots, we obtained Hill coefficient nH values

Supramolecular Chemistry
61
rotational conformers although the most stable one is that
illustrated in Figure 4(a), bearing two intramolecular
COOHZPy hydrogen bonds and two pairs of Py–Py
guest-binding sites. The process where several conformers
are optimised into the most stable one by the guest binding
would be reflected by the sigmoidal curvature. On the
other hand, the nH value for the 1R · (2S) complex was
estimated to be 2.8, indicating that the complex consists of
1:3 host–guest ratio, and the three binding sites are
supplied for the 2S binding.1 The stepwise analysis of the
three binding steps provided log K1 ¼ 1.9, log K2 ¼ 1.2,
log K3 ¼ 3.8 and log Ktotal ¼ 6.9 (correlation coefficient,
R ¼ 0.998). The 1:3 molar ratio calls one possible host
conformation to our mind that the intramolecular
hydrogen bonds are formed between the pendent amide
(CONH) groups, which eventually results in three pairs of
Py–Py guest-binding sites suitable for 2S (Figure 4(c)).2
The molecular modelling studies of 1R · (2R)2 complex and
1R · (2S)3 complex provided a few clues to rationalise the
difference in the stoichiometry, depending on the guest
chirality. In 1R · (2R)2, two intramolecular COOHZPy
hydrogen bonds force the porphyrin rings to slightly twist
leftward (from a top view). The orientation of two pairs of
Py–Py binding sites thus generated is compatible with the
orientation of two COOH groups in 2R. In other words, the
intramolecular hydrogen bonds used for porphyrin
twisting can operate cooperatively with the intermolecular
hydrogen bonds formed between 1R and 2R. The resultant
most stable complex has the 1:2 stoichiometry. In the
1R · (2S)3 complex, on the other hand, the leftward
hydrogen bond orientation dominant in original 1R is
opposite to the rightward hydrogen bond orientation in 2S.
Thus, the original intramolecular COOHZPy hydrogen
bonds are collapsed to form one pendent amide–amide
Figure 2. CD spectra of 1R (0.10 mmol dm23; light grey line)
and 1R (0.10 mmol dm23) þ triethylamine (100 mmol dm23
;
black line) in a TCE–THF 30:1 (v/v) mixed solvent at 298 K (cell
length ¼ 1.0 mm). The hydrogen bond-induced CD signal
around 310 nm, which was assigned to the chiral pyridine pair,
disappeared upon the addition of triethylamine.
We also analysed this binding isotherm according to
the stepwise binding process and evaluated the association
constants in detail by a nonlinear curve-fitting method
assuming the stepwise association constants K1 and K2.
They were estimated to be log K1 ¼ 1.6, log K2 ¼ 4.1 and
log Ktotal ¼ 5.6 (correlation coefficient, R ¼ 0.997) for 2R
(Kn/M21). The relation K1 , K2 clearly indicates that this
guest-binding process features the positive homotropic
allosterism. It may be a little curious that 1R, being pre-
organised suitably for the 2R binding, affords the sigmoidal
curvature. We consider that 1R can still adopt several
Figure 3. CD spectra of 1R (0.10 mmol dm23) recorded after the addition of (a) 2R and (b) 2S in a TCE–THF 30:1 (v/v) mixed solvent at
298 K (cell length ¼ 1.0 mm). Bold lines in a and b are CD spectra of 1R in the absence of guest. (c) Plots of the CD intensity at 310 nm of
1R (0.10 mmol dm23) vs. concentration of 2 and the fitted theoretical curves (shown in solid lines) for 1:2 and 1:3 complexation with guest
2R and 2S, respectively. Hill plots for the (d) 2R and (e) 2S recognition systems.

62
T. Ikeda et al.
Figure 4. The energy minimised structures of (a) 1R, (b) 1R · (2R)2 and (c) 1R · (2S)3 as generated by MM2 calculations with Insight
II/Discover 3.0. Carbon and oxygen atoms of guest molecules are darkened for clarity. The arrows and symbols of stacked squares denote
the hydrogen bonds and the twisting direction of two porphyrins. In the model for 1:3 1R · (2S)3 complex, the intramolecular hydrogen
bonds are collapsed.
linkage and three 2-(Py)2 linkages, all of which should
have the rightward orientation.
standard. Mass spectral data were obtained using a
Perceptive Voyager RP MALDI-TOF mass spectrometer.
CD spectra were recorded using JASCO J-720WI
spectrophotometers.
To demonstrate the high enantioselectivity towards 2R,
we conducted the titration experiment using racemic 2.
Upon the addition of racemic 2, the CD signal arising from
the 1R · (2R)2 complex ([1R] ¼ 0.10 mmol dm23) was the
same as that of pure 2R (error is within 2%) until [2S] in
racemate reaches 1.0 mmol dm23 and then gradually
decreased because of the competitive formation of the
1R · (2S)3 complex (Figure 3(c) and S4). The critical 2R
selectivity (1.0 mmol dm23) is higher than that observed
for DDPy7R (0.6 mmol dm23) (7), supporting the
improved chiral memory effect by two 2R groups
covalently introduced into 1R. Apparently, 1R exhibits
unconventional enantioselectivity towards 2R in a specific
concentration region, giving rise to the information that an
error molecule (2S) is precisely filtered off by the control of
the nonlinear responses (error filtering). This is affected by
proper incorporation of the structural information of two
enantiomers into the host and utilisation of multiple
equilibrium characteristic of positive allosterism.
Binding isotherm analysis
Cooperative guest-binding process was analysed accord-
ing to the Hill equation: log½y=ð1 2 yÞꢀ ¼ n log½guestꢀþ
log K, where K, y and nH are the association constant, the
extent of complexation and the Hill coefficient, respect-
ively. The slope and the intercept of the linear (Hill) plots
allow K and nH to be estimated; these values are useful
measures of cooperativity. A higher value of nH is related
to a higher degree of cooperativity; the maximum value is
equal to the number of binding sites. In the analysis of the
binding isotherm from the Hill plot, we evaluated the
concentration of the unbound guest – [guest] – by
assuming that the 1:2, (vs. 2R) or 1:3 (vs. 2S) complex of 1R
is formed quantitatively when the CD change is saturated.
The stoichiometry of 1:2 (vs. 2R) and 1:3 (vs. 2S) was
estimated by the continuous variation plot or the Hill plot.
The stepwise binding constants were evaluated by the non-
linear least square curve-fitting based on the assumption of
stoichiometry. The obtained theoretical curves are super-
imposed on Figure 3(c).
In conclusion, we consider that a new imprinting
memory system can be designed by utilising the positive
allosteric effect, and double-decker porphyrins act as
excellent scaffolds for this kind of molecular design.
Experimental
General
CD spectroscopy
All starting materials and solvents were purchased from
Tokyo Kasei Chemicals (Chuo-ku, Tokyo, Japan) or Wako
Chemicals (Osaka-shi, Osaka, Japan) and used as received.
The 1H NMR spectra were recorded on a Brucker DRX 600
(600 MHz) spectrometer. Chemical shifts are reported in
ppm downfield from tetramethylsilane as the internal
A stock solution of 2 prepared in THF was added to a
solution (0.10 mmol dm23) of 1R in 1,1,2,2-TCE.
The solvent ratio of the measurement solution was
TCE:THF ¼ 30:1 (v/v). The CD spectra from 250 to
500 nm were recorded with a JASCO J-720WI spectro-
polarimeter at 10–15 different concentrations of guest

Supramolecular Chemistry
63
molecules. The measurement temperature was 278 K and
the cell length was 1.0 mm.
6.4 (br, 2H, exo-m-Ph), 7.5 (br, 2H, endo-m-Ph), 8.21–
8.31 (m, 12H, pyrrole-CH), 8.45–8.51 (m, 4H, pyrrole-
CH), 8.58 (br, 6H, exo-m-Py), 9.44 (br, 2H, endo-o-Ph) 9.5
(br, 12H, endo-o-Py, endo-m-Py). MALDI-TOF mass
[dithranol] m/z calcd for [M]þ ¼ 1400.36, found 1400.73.
Synthesis
Compound 1R was synthesised according to the procedures
depicted in Scheme 1.
Synthesis of double-decker complex 1R
Double-decker complex 3 (14.0 mg, 10.0 mmol) was
dissolved in dichloromethane (3 ml). (1R,2R)-Cyclohex-
ane-1,2-dicarboxylic anhydride (15.0 mg. 970 mmol) was
added to this solution, which was then stirred at rt for
1 h. After the solvents were removed, the remaining
solid was purified by gel column chromatography (Bio-
Beads S-X3, chloroform). Precipitation treatment using
dichloromethane ethyl acetate gave a brown solid
Synthesis of porphyrin 3
4-Acetamidobenzaldehyde (4.0 g, 24.5 mmol) and pyri-
dine-4-carbaldehyde (2.5 g, 23.3 mmol) were dissolved in
propionic acid (200 ml). After the mixture was stirred at
808C for 30 min, pyrrole (3.4 ml, 50.0 mmol) was added.
The solution was refluxed for 5 h. After cooling to room
temperature (rt), ethylacetate (300 ml) was added to the
flask and insoluble was removed by paper filtration. Half
amount of solvent was removed in vacuo and n-hexene
(500 ml) was added to the solution. A purple solid
precipitate was collected by filtration. The purple solid was
dissolved in mixed solvent of trifluoroacetic acid (30 ml)
distilled water (30 ml) and the solution refluxed for 2 h.
After cooling to rt, half amount of solvent was removed in
vacuo, which was then neutralised with aqueous
potassium carbonate solution until the colour of the
solution changed from green to red. A crude product was
extracted with dichloromethane from the aqueous phase.
The organic extract was dried over anhydrous sodium
sulphate and the solvents were removed. The resulting
solid was purified by column chromatography (silica gel,
dichloromethane/ethyl acetate/methanol ¼ 5/5/1 (v/v/v))
to afford a purple solid (236 mg, 15% yield).
1
(17 mg, quant.). H NMR (pyridine-d5, 600 MHz, J/Hz,
2208C); d 1.18 (m, 2H, cyc-hex), 1.37 (m, 2H, cyc-
hex), 1.65 (m, 2H, cyc-hex), 1.74 (m, 4H, cyc-hex), 1.96
(m, 2H, cyc-hex), 2.48 (m, 2H, cyc-hex), 2.57 (m, 2H,
cyc-hex), 3.42 (m, 2H, cyc-hex), 3.62 (m, 2H, cyc-hex),
6.78-91 (m, 8H, exo-o-Ph and exo-o-Py), 8.15–8.35 (m,
2H, exo-m-Ph), 8.65–8.73 (m, 4H, pyrrole-CH), 8.77–
8.82 (m, 16H, pyrrole-CH), 8.90–8.97 (m, 18H, exo-m-
Py and CONH), 9.15–9.31 (m, 2H, endo-m-Ph), 9.61–
9.67 (m, 6H, endo-m-Py), 9.93–10.0 (br, 8H, endo-o-Ph
and endo-o-Py), 12.10 (m, 2H, COOH). MALDI-TOF
mass [dithranol] m/z calcd for [M]þ ¼ 1708.49, found
1709.74.
Acknowledgements
T.I. thanks the JSPS Research Fellowship for Young Scientists
for financial support. This study was supported partially by
KAKENHI to M.T. from the Ministry of Education, Culture,
Science, Sports and Technology, Japan.
1H NMR (CDCl3, 600 MHz, J/Hz, rt); d 22.84 (s, 2H,
pyrrole-NH), 4.08 (br, 2H, NH2), 7.09 (d, J ¼ 7.9, 2H, Ph),
7.98 (d, J ¼ 7.9, 2H, Ph), 8.16 (d, J ¼ 4.7, 6H, Py), 8.81
(d, J ¼ 4.0, 2H, pyrrole-CH), 8.84 (br-s, 4H, pyrrole-CH),
9.02 (d, J ¼ 4.1, 2H, pyrrole-CH), 9.05 (d, J ¼ 5.0, 6H,
Py). MALDI-TOF mass [dithranol] m/z calc. for
[M]þ ¼ 632.24, found 633.47.
Notes
1. Analysis of the stoichiometry for complex with 2S by
continuous-variation plots did not indicate a significant
change in the CD intensity because the concentration,
[1R] þ [2S] was too low to form the complex as shown in
Figure 3(c). Moreover, this concentration cannot be raised
because of the precipitate formation
Synthesis of double-decker complex 4
Mixture of porphyrin
3
(100 mg, 0.16 mmol),
2. For the 1:3 1R · (2S)3 complex, the COOH–COOH interaction
between the remaining pendent substituents (see also
Graphical abstract) would be possible; the theoretical
estimation shown in Figure 4 indicates that the CONH–
CONH interaction is more likely because of steric
orientation of the hydrogen bonding sites. In other words,
in the molecular modelling for 1R · (2S)3, formation of the
amide dimer is sterically more favourable than the formation
of carboxylic acid dimer
Ce(acac)3 · 3H2O (233 mg, 0.47 mmol) and p-t-butyl ani-
line (0.3 ml) in dry 1,2,4-trichlorobenzene (31 ml) was
refluxed for 6 h under Ar. It was noted that the colour of the
solution turned from red to green. After cooling to rt, the
mixture was stirred for 15 min under air. Upon air
oxidation of cerium, the colour of the solution turned to
brown. The mixture was directly purified by column
chromatography (silica gel, chloroform/methanol ¼ 1/0–
3/1 (v/v)) and gel column chromatography (Bio-Beads S-
X3, chloroform) to afford a brown solid (26 mg, 24%
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¨
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