An indolocarbazole-bridged macrocyclic porphyrin dimer having homotropic allosterism with inhibitory control

Article abstract of DOI:10.1039/c1cc00112d

A macrocyclic host, a pair of zinc porphyrins bridged by two anion-acceptable indolocarbazole moieties, has shown strong positive homotropic allosterism upon anionic guest bindings with an inhibitory control mechanism by DABCO addition.

Full text of DOI:10.1039/c1cc00112d

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An indolocarbazole-bridged macrocyclic porphyrin dimer having
homotropic allosterism with inhibitory controlw
Chi-Hwa Lee, Hongsik Yoon, Pyosang Kim, Sung Cho, Dongho Kim* and Woo-Dong Jang*
Received 6th January 2011, Accepted 9th February 2011
DOI: 10.1039/c1cc00112d
A macrocyclic host, a pair of zinc porphyrins bridged by two
anion-acceptable indolocarbazole moieties, has shown strong
positive homotropic allosterism upon anionic guest bindings with
an inhibitory control mechanism by DABCO addition.
Designing synthetic anion-binding receptors is very important
because many anions play critical roles in chemical and
biological processes.1 Recently, several indole-based anion
receptors have been reported, in which the NH protons in
the indole moieties bind strongly to several anions via hydrogen
bonds.2 On the other hand, allosterism is a common but a very
important regulation mechanism in biological events.3,4 Many
proteins possess multiple guest-binding sites that show allosteric
guest binding phenomena, in which the first guest binding
causes a structural alteration of the host protein and influences
additional guest-binding affinity. Designing such artificial
allosteric systems is of great interest because not only they
play crucial roles in biosystems but also it would be helpful to
design them for the functional nano-devices. Many scientists
have tried to synthesize artificial receptors with multiple guest
binding sites to mimic biosystems, but only a limited number
of synthetic receptors have shown positive allosterism.5 In this
regard, we have recently reported a new type of molecular
tweezers composed of biindole moieties and porphyrins,
demonstrating strong positive allosteric bindings between
1,4-diazabicyclo[2,2,2]-octane (DABCO) and chloride.6 As a
continuation of this research, we report herein a new type of
macrocyclic host, and wish to highlight positive homotropic
allosterism of anionic guests and the control of allostericity. A
macrocyclic host compound (1) (Scheme 1), a pair of zinc
porphyrins bridged by two anion-acceptable indolocarbazole
Scheme 1 Structure of 1 and expression of binding constants for each
guest binding processes.
indicating that DABCO and anionic guests have different
binding modes. With the addition of DABCO, a major change
in the 1H NMR spectrum occurs in the phenyl protons of
porphyrin, whereas the pyrrolic protons as well as protons in
indolocarbazole units are shifted by acetate addition, indicating
that the structural alternation of 1 is induced by acetate
bindings. To determine the binding affinity of 1, several
anionic guests and DABCO were titrated with 1 in THF at
25 1C, and spectroscopic changes in Soret and Q bands
were monitored. The absorption spectrum of 1 was changed
by the addition of anionic guests with clear isosbestic points
(see ESIw). The continuous variation method (Job’s plot)7a
together with the spectroscopic titration demonstrated that 1
and all anionic guests form a 1 : 2 host–guest complex. Inter-
estingly, based on the spectroscopic titration of various anionic
guests, it has been found that the anionic guest bindings occur
in a positive allosteric manner. As shown in Fig. 2, the
1
moieties, was synthesized and characterized by H, 13C NMR
and MALDI-TOF-MS analyses. In the 1H NMR study,
addition of either DABCO or acetate to 1 resulted in the
chemical shift changes. As shown in Fig. 1, the signal of NH
protons in indolocarbazole moieties is strongly downfield-shifted
by the addition of acetate ions, indicating that the NH protons
participate in hydrogen bonding. In contrast, the signal of NH
protons is slightly upfield-shifted by the addition of DABCO,
Department of Chemistry, Yonsei University, Seoul 120-749, Korea.
E-mail: wdjang@yonsei.ac.kr, dongho@yonsei.ac.kr
w Electronic supplementary information (ESI) available: Synthetic pro-
cedures and spectroscopic titration data. See DOI: 10.1039/c1cc00112d
Fig. 1 Partial 1H NMR spectra (250 MHz, THF-d8, 25 1C) of (c) 1
(1 mM); containing (a–b) DABCO and (d–g) acetate.
c
4246 Chem. Commun., 2011, 47, 4246–4248
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maintained as 1 : 2. Interestingly, however, the Hill coefficient
and log K of anion bindings to 1*DABCO were greatly
decreased, suggesting that DABCO binding to 1 decreases
the allostericity of anionic guest binding. To know how the
DABCO binding influences the allosterism of acetate binding
to 1, acetate was titrated to 1 by changing the equivalence of
DABCO addition. As shown in Fig. 3, the slope of the Hill
plot was decreased by an increment of DABCO addition,
illustrating that the allostericity of acetate binding to 1 can
be controlled by DABCO addition. The Hill coefficient n and
log K are also summarized in Fig. 3. Using the nonlinear curve
fitting method, the association constants K1* and K2* of
anions to 1*DABCO were again estimated (Table 1).
Compared to the case of 1 without DABCO, 1*DABCO
showed increased association constant K1*, representing
heterotopic allosterism between DABCO and anions. The
binding of DABCO to 1 may create an excellent cavity for
anionic guest binding, thereby the free energy change (DG)
needed for the anionic guest-induced conformational alter-
nation of 1 might be reduced. On the other hand, the second-
ary association constant K2* for anion binding was decreased
by the accommodation of DABCO, possibly because of the
steric repulsion between DABCO and anionic guests. The first
anion binding process to 1*DABCO may include two
different effects, one is conformational optimization of 1
induced by DABCO binding and the other is steric repulsion
between DABCO and the anionic guest. On the other hand,
the second anion binding process includes only steric repulsion
effect. As a result of DABCO binding to 1, the secondary
association constant K2* became smaller than the primary
association constant K1*, and 1 lost positive allosterism of
anionic guest binding. In other words, DABCO works as an
excellent modulator for the homotropic allostericbinding of
anionic guests, because the accommodation of DABCO can fix
the conformational flexibility of 1. In this regard, we have
calculated and compared DG values for each process (Table 2).
The loss of DG by the steric repulsion can be calculated from
the difference between DG2 and DG2*. The smallest fluoride
Fig. 2 Titration binding isotherms of acetate, azide, and fluoride ions to 1.
titration binding isotherms of acetate, azide, and fluoride are
obviously sigmoidal shape curves. These guest-binding profiles
were analysed with the nonlinear curve fitting method and the
Hill equation: log(y/(1 À y)) = n log[guest] + log K, where
y, K and n are the fractional saturation of the host, the
association constant, and the Hill coefficient, respectively.7
The maximum n value is equal to the number of binding sites
of a host molecule. If the Hill coefficient is 2.0, it implies that a
host–guest complex shows perfectly cooperative two-site binding.
From the slope and the intercept of the linear plot, we
obtained log K and n values for various anionic guests. As
shown in Table 1, the values of the Hill coefficients for acetate,
azide, and fluoride bindings are 1.83, 1.88, and 1.72, respectively.
From the spectroscopic titration data, the primary and
secondary association constants, K1 and K2, respectively, were
also determined by the nonlinear curve fitting method using
the commercially available Hypspec software. The ratios of
K2/K1 are 24.5, 35.3, and 3.4 for acetate, azide, and fluoride
bindings to 1, respectively. Compared with K2/K1 = 0.25 for
the non-cooperative system, these K2/K1 values are very large,
reinforcing the positive allosterism for anion bindings to 1.
Considering the structural rigidity of porphyrin and indolo-
carbazole moieties in 1, the only rotatable bond is ethynyl
linkages between porphyrin and indolocarbazole units. When
the first anionic guest binds to 1, the angle between porphyrin
and indolocarbazole units would be changed. In other words,
the first anionic guest binding onto the indolocarbazole unit
induces conformational alternation of 1, which facilitates
additional guest binding. When the DABCO was added to 1,
both Soret and Q bands were also drastically changed with
clear isosbestic points. Job’s plot demonstrated that DABCO
and 1 had a 1 : 1 stoichiometry when forming a host–guest
complex, where the association constant (KDABCO) was
determined to be 1.06 Â 106
M
À1. The absorption spectra
of 1*DABCO were changed with clear isosbestic points
by the addition of anions, indicating that 1*DABCO can
simultaneously accommodate anions. The stoichiometry of
1*DABCO to anions, determined by Job’s plot, was still
Fig. 3 Hill plots for acetate binding to 1 in the presence of DABCO.
Hill coefficient n and log K are summarized in Table 1.
Table 1 Binding constants for various anionic guest bindings to 1
Hill analysis
Nonlinear curve fitting analysisb
na
Log K
K1
K2
K2/K1
K1*
K2*
K2*/K1*
Ions
FÀ
1.72
1.83
1.88
10.93
12.19
10.46
1.60 Â 106
1.06 Â 106
7.11 Â 104
5.42 Â 106
2.60 Â 107
2.50 Â 106
3.39
24.5
35.2
1.18 Â 108
1.50 Â 107
6.44 Â 105
4.09 Â 106
4.86 Â 106
9.69 Â 104
3.47 Â 10À2
AcÀetate
0.324
0.150
N3
a
b
Hill coefficient, Estimated errors o10%.
c
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Table 2 DG changes for each processa
Ions
DG1
DG2
DG1*
DG2*
DG1 À DG1*
À10.7
À6.5
À5.5
DG2 À DG2
0.68
4.2
(DG1 À DG1*) À (DG2 À DG2*)
11.4
10.7
13.5
FÀ
À35.4b
À34.5
À27.7
À38.4
À42.3
À36.5
À46.1
À41.0
À33.1
À37.7
À38.1
À28.5
AcÀetate
N3
8.0
a
b
Estimated errors o10%, kJ molÀ1
.
This research was supported by the CBMH at Yonsei
University and the Star Faculty project funded by the Korea
Research Foundation. C.-H. Lee, H. Yoon and P. Kim
acknowledge fellowships from the BK21 program from the
Ministry of Education, Science and Technology, Korea. The
quantum calculations were performed by using the super
computing resource of the Korea Institute of Science and
Technology Information (KISTI).
Fig. 4 The optimized structures for (a) 1, (b) 1*2acetate, and
(c) 1*DABCO.
anion exhibited only 0.68 kJ molÀ1 of energy loss by the
accommodation of DABCO, whereas acetate and azide gave
4.2 and 8.0 kJ molÀ1 of energy loss, respectively. From the
difference between DG1 and DG1*, DABCO-induced energy
gains in the first anion binding also can be calculated. The
smallest fluoride anion exhibited the largest energy gain
(10.7 kJ molÀ1) by the accommodation of DABCO, which
can be explained by the smallest steric repulsion effect.
DABCO-induced energy gains for acetate and azide were
determined as 6.5 and 5.5 kJ molÀ1, respectively. To com-
pensate the steric repulsion effect on the energy gain by the
DABCO accommodation, (DG1 À DG1*) À (DG2 À DG2*)
were again calculated. As a result, similar values were obtained
for all three different anionic guest bindings, indicating that
the energy consumed for conformational optimization of 1 by
DABCO was 11.8 Æ 1.6 kJ molÀ1. The structures of 1,
1*DABCO, and 1*2acetate have been optimized by using
DFT at the B3LYP/6-31G level. As shown in Fig. 4, 1 exhibits
a parallel alignment of two porphyrin moieties with dihedral
angle about 301 against indolocarbazole moieties. The cavity
between two porphyrin moieties is too small to fit guest
molecules. Therefore, space opening between the two porphyrin
moieties should be needed for anionic guest bindings. The
optimized structure of 1*2acetate clearly shows structural
alternation of host molecules. Nevertheless, when the DABCO
binds to 1, two porphyrin moieties should become perfectly
parallel, and indolocarbazole moieties should have a perpendi-
cular alignment against porphyrin moieties. Considering the
optimized structure of 1*DABCO, additional anionic guest
binding may not induce structural alternation of host molecule.
Therefore, after DABCO binding, 1 eventually loses homotropic
allosterism upon anionic guest bindings. In conclusion, we
have designed a new type macrocyclic host for multiple guest
bindings which can simultaneously accommodate anionic
guest as well as DABCO. Upon anionic guest binding, the
host molecule exhibited strong positive homotropic allosterism.
Interestingly, DABCO successfully worked as a heterotopic
modulator for the allosteric anion binding to the host. By the
accommodation of DABCO to the host molecule, the allosteri-
city of anion bindings was greatly decreased. The present
system is therefore an excellent biomimetic model having
homotropic allosterism with an inhibitory control mechanism.
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4248 Chem. Commun., 2011, 47, 4246–4248
This journal is The Royal Society of Chemistry 2011