Allosteric regulation of the ligand-binding ability of Zn-porphyrin by metal complexation

Article abstract of DOI:10.1016/j.tetlet.2013.04.130

Zn-porphyrin with bipyridyl units at the ends of a conjugated chain, in addition to two alkyl side chains, was prepared as an artificial allosteric system. The axial ligand-binding ability of the compound was considerably reduced by the formation of a Fe(bpy)₃-type complex. The degree of the allosteric suppression strongly depended on both alkyl chain length and the steric demand of the pyridyl ligand.

Full text of DOI:10.1016/j.tetlet.2013.04.130

Accepted Manuscript
Allosteric Regulation of the Ligand-binding Ability of Zn-porphyrin by Metal
Complexation
Masatoshi Kozaki, Yoshikazu Ninomiya, Shuichi Suzuki, Keiji Okada
PII:
S0040-4039(13)00746-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.04.130
TETL 42898
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
27 March 2013
24 April 2013
30 April 2013
Please cite this article as: Kozaki, M., Ninomiya, Y., Suzuki, S., Okada, K., Allosteric Regulation of the Ligand-
binding Ability of Zn-porphyrin by Metal Complexation, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/
j.tetlet.2013.04.130
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Allosteric Regulation of the Ligand-binding
Ability of Zn-porphyrin by Metal
Complexation
Leave this area blank for abstract info.
Masatoshi Kozaki,* Yoshikazu Ninomiya, Shuichi Suzuki, and Keiji Okada*

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Allosteric Regulation of the Ligand-binding Ability of Zn-porphyrin by Metal
Complexation
Masatoshi Kozaki,* Yoshikazu Ninomiya, Shuichi Suzuki, and Keiji Okada
Graduate School of Science, Osaka City University 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Zn-porphyrin with bipyridyl units at the ends of a conjugated chain, in addition to two alkyl side
chains, was prepared as an artificial allosteric system. The axial ligand-binding ability of the
compound was considerably reduced by the formation of a Fe(bpy)3-type complex. The degree
of the allosteric suppression strongly depended on both alkyl chain length and the steric demand
of the pyridyl ligand.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Allosteric system
Porphyrin
Bipyridine
Axial ligand
Sonogashira coupling
The design and construction of artificial allosteric systems
have recently been attracting considerable interest as intelligent
catalysts, drug delivery reagents, and molecular machines.1 As
metalloporphyrins are extremely valuable functional units for the
construction of artificial enzymes,2 a designed allosteric system
with a metalloporphyrin unit as an active site would find a wide
range of applications in catalysis and receptors with switchable
activity.3 One of the most effective methods for controlling
substrate-accessibility to an active site is steric shielding, which
has already been applied to the allosteric regulation of catalytic
activity of salen complexes.4 However, this shielding method has
not been investigated in detail for use in the allosteric regulation
of ligand-binding, or for controlling the catalytic ability of
metalloporphyrins. We recently reported a unique method for
inducing a large conformational change in alkyl chains by metal
complexation.5 We also applied this conformational change to
alter the binding constant between the Zn-porphyrin unit and
axial ligands. Unfortunately, the conformational change had only
a limited effect on the binding constant (1.2 times enhancement
by Fe(II)), which was probably due to the large separation
between the Zn-porphyrin unit and the alkyl chains. Here, we
reconsider the configuration of the functional units of the newly
designed allosteric system 1, in which Zn-porphyrin was
embedded in the middle of the conjugated backbone (Fig. 1). The
conjugated chain had a bipyridyl unit at one end and a 1,1’:3’,1”-
terphenyl-4,2’,4”-triyl unit at the other, to which two additional
bipyridyl units were attached as terminal groups through flexible
alkyl chain bridges. Thus, compound 1 had a rigid bipyridyl
opposite side. When MX2 (M = metal cation, X = counter anion)
was added to a solution of 1, the two flexible bipyridyl groups
would come across above and below the Zn-porphyrin surface,
producing an [M(bpy)3]-type complex, where the alkyl chain
bridges are expected to shield the Zn-porphyrin and restrain its
axial ligand-binding ability. In this report, we present the
preparation of 1 and the allosteric regulation of the axial ligand-
binding activity.
Figure 1. Chemical Structure of allosteric system 1.
g—ro—up —on one side and two flexible bipyridyl groups on the
 Corresponding authors. Tel.: +81-6-6605-2564 (M.K.), 81-6-6605-2568 (K.O.); fax: +81-6-6605-2522; e-mail: kozaki@sci.osaka-cu.ac.jp
(M.K.), okadak@sci.osaka-cu.ac.jp (K.O.)

2
Tetrahedron
The length of the alkyl chain is significant in this system, as
and Q-band absorption (550−630 nm) of the Zn-porphyrin
skeleton. Addition of the Fe(II) solution resulted in the
appearance of the characteristic metal-ligand charge transfer
(MLCT) absorption for Fe(II)(bpy)3-type complexes at around
500 nm.11 The intensity of this absorption increased linearly till it
reached a plateau, which was brought about by increasing the
concentration of Fe(II) to 1 equiv in the solution (Fig. 2 inset).
These results indicate quantitative formation (K1a•Fe > 106 M-1) of
Fe(II)(bpy)3-type complex 1a•Fe. The production of the 1:1
complex 1a•Fe is further supported by an MS ion peak observed
it affects both the stability of the [M(bpy)3]-type complexes and
the degree of shielding at the axial ligand-binding site in the Zn-
porphyrin unit (see below). Molecular mechanics (MMFF)
calculations were performed for 1 with alkyl chains of different
lengths, and the results suggested that alkyl chains of n = 7–11
were suitable for the formation of the stable metal complex as
well as effective shielding of the porphyrin unit.6
+
-
at m/z = 2537 ([1a•Fe]2 •BF4 , calcd for C160H170BF4FeN10O6Zn:
2534).
1.0
0.5

0
3.0
1.0 2.0
[Fe2+] / [1a]
400
500
600
700
wavelength / nm
Figure 2. UV-vis spectra resulting from the titration of 1a (2 × 10-5
M) with Fe(BF4)2•6H2O in a toluene/acetonitrile (1/1 (v/v)) solution
(3 × 10-4 M). The inset shows the change in the absorbance of MLCT
band at 500 nm as a function of Fe(II):1a concentration.
The degree of shielding at the axial ligand-binding Zn site in
1•Fe was investigated by adding an axial ligand. 4-
Phenylpyridine (L1) was first selected for this purpose (Fig. 3),
and the ligand-binding constants for Zn coordination were
compared before and after the Fe(bpy)3 complexation (Table 1).
The axial ligand-binding ability of 1a was investigated by means
of UV-vis titration. The selective formation of 1:1 complexes of
Zn-porphyrins and pyridine derivatives is a well-established
phenomenon.12 The titration was carried out by the addition of a
solution of ligand L1 to a solution of 1a (5 × 10-5 M) in
toluene/acetonitrile (1/1 (v/v)) (Fig. 4). On adding the solution,
the porphyrin Q-band of 1a was red-shifted, which was attributed
to the axial coordination of L1 to the Zn center in 1. The binding
constant K1a•L1 was determined as 0.867 ± 0.014 × 104 M-1 from
the curve fitting, using the change in the absorbance at 608 nm
(Fig. 4(b)).
Scheme 1. Synthesis of 1.
Allosteric system 1 was synthesized according to Scheme 1.
The three-component Sonogashira coupling reaction7 of Zn-
porphyrin 28 with terminal alkynes 39 and 45 afford Zn-porphyrin
5 in acceptable yield (41%). Side chain unit 6 was prepared from
hydroquinone by stepwise condensation with bipyridyl- and
pinacole borate-terminated alkyl bromides (Scheme S1). The
Suzuki-Miyaura coupling reaction between 5 and 6 afforded 1 as
a purple solid in moderate yields (67-86%).10 The structures of 1
were unambiguously characterized by NMR and FAB MS
measurements.
An aliquot of Fe(BF4)2 (3 × 10-4 M) in toluene/acetonitrile (1/1
(v/v)) was added to a solution of 1a (2 × 10-5 M) in the same
solvent. The complexation of 1a (n = 11) and Fe(II) in the
resulting solution was monitored by means of UV-vis spectral
measurement (Fig. 2). Before the addition of Fe(II), the solution
of 1a showed the characteristic Soret band absorption (440 nm)
Figure 3. Chemical structures of pyridyl ligands.
Similar UV-vis titration of 1a•Fe (1a: 2 × 10-5 M in the
presence of 1 equiv of Fe(BF4)2) in toluene/acetonitrile (1:1 v/v)
with the addition of L1 gave the binding constant K(1a•Fe)•L1
=
0.528 ± 0.005 × 104 M-1 (Fig. S2). These results suggest that the
formation of Fe(II)(bpy)3-type complexes reduces the stability of
the axial coordinated complex, (1a•Fe)•L1. The degree of the
allosteric suppression for the binding of axial ligands Ln with

3
1a•Fe compared to that with 1a can be defined by
K1a•Ln/K(1a•Fe)•Ln with the higher values indicating more
allosteric
suppression
was
dramatically
enhanced
,
(K1•L3/K(1•Fe)•L3 = 4.12 for 1b (n = 9) and 4.53 for 1c (n = 7)).
suppression (Table 1). Sterically more demanding ligands L2 and
L3 were prepared to enhance the degree of the allosteric
suppression (Fig. 3), with the value for K1a•L3 /K(1a•Fe)•L3
calculated to be 1.93, which was higher than that for L1 (Table 1,
entries 1 and 3). This suggests that the steric demand of a ligand
is an important factor in controlling the ligand-binding activity.
Table 1. The binding constants and the degrees of the allosteric
suppressiona
K1•Ln
K(1•Fe)•Ln
entry
1
Ln
K1•Ln /K(1•Fe)•Ln
(× 10-4 M-1)
(× 10-4 M-1)
0.528 ± 0.005
0.465 ± 0.006
1.73 ± 0.02
1
2
3
4
5
1a L1 0.867 ± 0.014
1a L2 0.707 ± 0.005
1.62 ± 0.03
1.52 ± 0.02
1.93 ± 0.05
4.12 ± 0.09
4.53 ± 0.11
(a)
1a L3
1b L3
1c L3
3.34 ± 0.08
3.68 ± 0.07
2.94 ± 0.05
2.5
0.882 ± 0.008
0.649 ± 0.012
2.0
1.5
a Procedures and conditions are described in Supplementary Material.
In conclusion, we developed a novel methodology for the
allosteric regulation of the axial ligand-binding ability of a Zn-
porphyrin unit. Ligand accessibility to the Zn atom was
considerably restrained when a metal ion (Fe(II)) was added as
an external stimulus. This induced the formation of Fe(bpy)3-type
complex, 1•Fe, in which two alkyl chain bridges came across
above and below the Zn center in the porphyrin unit. The degree
of the allosteric suppression was strongly enhanced by utilizing
the sterically demanding pyridyl ligand and the allosteric system
with shorter alkyl chains. The method presented in this paper is
applicable for the preparation of metalloporphyrin catalysts and
receptors with allosterically switchable activity.
1.0

0.5
520 540 560 580 600 620 640
wavelength / nm
Acknowledgments
(b)
1.5
This work was partially supported by a Grant-in-Aid for
Science Research (No. 23350022) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
1.4
1.3
1.2
References and notes
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1.1

1.0
0.9
0.8
0
0.001 0.002 0.003 0.004 0.005
[L1] / M
Figure 4. (a) UV-vis spectra resulting from the titration of 1a (5 ×
10-5 M) with L1 in a toluene/acetonitrile (1/1 (v/v)) solution. (b) The
change in the molar extinction coefficient of Q-band at 608 nm as a
function of concentration of L1. The solid line is a theoretical
binding curve obtained by a curve fitting.
Interestingly, the absolute values of binding constants of L3
were found to be approximately four times higher than those of
L1 and L2 due to stronger Lewis basicity of the nitrogen atom in
L3.13 A slightly smaller value of K1a•L2/K(1a•Fe)•L2 suggest that the
suppression effect is orientation-dependent on the phenyl
substituents on the axial pyridine ring.14
The degree of suppression of the accessibility of the ligand
could be further increased by utilizing Zn-porphyrin 1 with a
shorter alkyl chain. UV-vis titration using 1b (n = 9) or 1c (n = 7)
were performed, and the binding constants with the bulkiest
ligand (L3) in the presence and absence of Fe(II) were
determined. The axial ligand-binding constant of 1 was relatively
insensitive to the alkyl chain length; however, the degree of the
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4
Tetrahedron
11. (a) Nabeshima, T.; Yoshihira, Y.; Saiki, T.; Akine S.; Horn, E. J.
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3541–3544.
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Hawley, J. K.; Kim, H.-J.; Mak C. C.; Webb, S. J. In Porphyrin
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13. UV-vis titration of 5 (2 × 10-5 M) in toluene/acetonitrile (1:1 v/v)
with the addition of L3 gave the binding constant K5•L3 = 3.43 ±
0.06 × 104 M-1 (Fig. S11). This value is essentially identical to
those obtained for 1 in Table 1. These results suggest that the
superior metal-binding ability of L3. Attractive interaction
between the alkyl chain and L3 is ruled out.
14. Uemori, Y.; Takinami, S.; Takahashi, A.; Munakata, H.; Imai, H.;
Nakagawa S.; Kyuno, E. Inorg. Chim. Acta. 1994, 224, 157-161.
Supplementary Material
Synthetic procedures, characterization data, UV-vis titration
data, and MMFF optimized structures. This material is available
free of charge via the Internet.