Water-soluble supramolecular porphyrin dimer: self-organization of mono(imidazolyl)-substituted Zn porphyrin to a special-pair type dimer in water

Article abstract of DOI:10.1016/j.tetlet.2010.03.115

Tris(4-carboxylphenyl)-mono(N-methylimidazolyl)-substituted Zn porphyrin was synthesized as a precursor for a water-soluble supramolecular porphyrin dimer. The dimer formation was performed in a NaHCO₃ aq solution (pH 8.4) and phosphate buffer solutions (pH 7.4-9.0). The split Soret bands of Zn porphyrin observed in the absorption spectra clearly showed self-organization to a special-pair type slipped cofacial dimer via metal coordination of imidazole even in water.

Full text of DOI:10.1016/j.tetlet.2010.03.115

Tetrahedron Letters 51 (2010) 2979–2982
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Tetrahedron Letters
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Water-soluble supramolecular porphyrin dimer: self-organization
of mono(imidazolyl)-substituted Zn porphyrin to a special-pair
type dimer in water
*
Hidekazu Miyaji , Junko Fujimoto
Department of Biomolecular Science, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Tris(4-carboxylphenyl)-mono(N-methylimidazolyl)-substituted Zn porphyrin was synthesized as a pre-
cursor for a water-soluble supramolecular porphyrin dimer. The dimer formation was performed in a
NaHCO3 aq solution (pH 8.4) and phosphate buffer solutions (pH 7.4–9.0). The split Soret bands of Zn por-
phyrin observed in the absorption spectra clearly showed self-organization to a special-pair type slipped
cofacial dimer via metal coordination of imidazole even in water.
Received 14 January 2010
Revised 5 March 2010
Accepted 30 March 2010
Available online 1 April 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Porphyrin
Imidazole
Self-organization
Water solubility
Mimicking biological functions using synthetic molecules is a
challenging area of biomimetic chemistry.1 In particular, porphy-
rins have been widely used to mimic biological functions.2 For
example, imidazole- and other ligand-substituted porphyrins have
been reported as models for artificial photosynthesis3–7 and artifi-
cial hemoglobins.8,9 However, these biomimetic supramolecules
have been studied in organic solvents, such as chloroform. We
are interested in making water-soluble biomimetic supramolecular
systems via the self-organization of porphyrins. Water-soluble
supramolecular porphyrins are expected to be used not only as a
catalyst10 that works in water but also in medical applications,
such as photodynamic therapy (PDT),11 in the future. As a first
and isolated by column chromatography (Al2O3, activity stage II,
CH2Cl2/EtOAc = 9/1 as an eluent) in 93% yield. The Zn porphyrin
methyl ester 2 was hydrolyzed in a 1 N NaOH/THF solution with
stirring at room temperature, and the Zn porphyrin carboxylic acid
3 was obtained by the addition of 1 N HCl as a precipitate in 80%
yield (see Supplementary data for details).
The 1H NMR spectrum of the Zn porphyrin methyl ester 2 in
CDCl3 elucidated a supramolecular porphyrin dimer formation.
As already reported in similar special-pair type slipped cofacial di-
mers,3,8 signals for protons in the stacking area of two porphyrin
planes were found to shift the up-field. For example, the b-pyrrolic
protons of free base porphyrin 1 that appeared at 8.84 ppm (2H)
and 8.85 ppm (2H, next to N-methylimidazole) shifted the up-field
at 8.26 ppm (2H) and 5.59 ppm (2H, next to N-methylimidazole) in
the case of Zn porphyrin 2, respectively. Significant up-field shifts
were observed at the N-methylimidazole moiety of Zn porphyrin
2. Signals for the imidazole protons of porphyrin 1 that appeared
at 7.47 ppm (1H) and 7.66 ppm (1H) shifted the up-field at 2.05
ppm (1H) and 5.57 ppm (1H) in the case of Zn porphyrin 2, respec-
tively. N-Methyl protons (3H) also shifted the up-field from
3.45 ppm (for 1) to 1.72 ppm (for 2). These results indicated the
shielding effect of porphyrin ring current due to stacking of 2 via
coordination. On the other hand, the b-pyrrolic protons of 2 that
were expected to be in the non-stacking area did not show such
up-field shifts; they appeared at 8.91 ppm (2H) and 8.96 ppm
(2H) (see Supplementary data).
approach, we herein report
a tris(4-carboxyphenyl)-mono(N-
methylimidazolyl)-substituted Zn porphyrin as a precursor for a
water-soluble supramolecular porphyrin dimer and its carboxylate
salt, which forms a special-pair type slipped cofacial arrange-
ment12 via metal coordination of the imidazolyl ligand in water.
Scheme 1 shows a synthetic route of the imidazole-substituted
porphyrin. Three methyl benzoate- and one N-methylimidazole-
substituted porphyrin was synthesized by condensation of methyl
4-formylbenzoate and 1-methylimidazole-2-carboxaldehyde with
pyrrole in acetic anhydride/propionic acid at 110 °C. Mono(N-
methylimidazolyl)-substituted porphyrin 1 was isolated by col-
umn chromatography (Al2O3, activity stage II, CH2Cl2/EtOAc = 15/1
as an eluent) in 22% yield. The Zn porphyrin 2 was synthesized
by stirring with zinc acetate in MeOH/CH2Cl2 at room temperature
Table 1 lists the absorption and fluorescence spectral data of the
free-base porphyrin 1, the Zn porphyrin methyl ester 2, 2 with
* Corresponding author. Tel.: +81 58 293 2464; fax: +81 58 293 2794.
E-mail address: miyaji@gifu-u.ac.jp (H. Miyaji).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.115

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H. Miyaji, J. Fujimoto / Tetrahedron Letters 51 (2010) 2979–2982
O
O
H
N
zinc acetate,
CH2Cl2,MeOH
0.5 hours
O
+
+
CH3CH2COOH, Ac2O,
reflux, 3 hours
HN
N
N
N
N
O
O
O
20%
93%
NH
OHC
CHO
N
N
1
O
O
O
O
R1
N
N
N
N
N
R1
Zn
N
CH2Cl2
N
N
N
N
O
O
Zn
R1
R1
N
N
N
N
O
O
N
N
Zn
R1 R1=
N
N
2
22
R1
O
O
NaOHaq,
THF,
80%
21 hours
HO
O
R2
N
N
N
NaHCO3,
H2O
N
N
R2
Zn
N
Zn
N
N
N
N
N
N
O
R2
HO
R2
O
N
N
N
N
R2=
Zn
R2
N
N
ONa
3
32
R2
O
OH
Scheme 1. Synthetic route of mono(N-methylimidazolyl)-substituted porphyrin dimer.
Table 1
Absorption and fluorescence spectral data of the free-base and the Zn complex of mono(N-methylimidazolyl)porphyrin and ZnTCPP
Porphyrin
Solvent
Absorption
Half-band width (nm)
Soret band (nm)
Q band (nm)
Fluorescence (nm)
1
2
CH2Cl2
CH2Cl2
CH2Cl2
NaHCO3 aq (pH 8.4)
NaHCO3 aq (pH 8.4)
Phosphate buffer (pH 7.4)
Phosphate buffer (pH 7.4)
Phosphate buffer (pH 7.4)
Phosphate buffer (pH 7.4)
420
418, 435
431
413, 430
427
413, 430
427
16
35
12
32
15
32
19
12
12
515, 550, 587, 543
565, 612
565, 605
565, 610
564, 606
564, 609
564, 606
556, 596
562, 603
650, 712
616, 667
614, 666
615, 661
613, 660
611, 656
612, 664
610, 661
617, 667
2(MeIm)
3
3(MeIm)
3
3(MeIm)
ZnTCPP
ZnTCPP (MeIm)
422
427
excess amounts of N-methylimidazole, 2(MeIm) in CH2Cl2,13 the Zn
porphyrin carboxylic acid 3 in a saturated NaHCO3 aq solution (pH
8.4), 3 with excess amounts of N-methylimidazole, 3(MeIm) in a
saturated NaHCO3 aq solution (pH 8.4), 3 in a phosphate buffer
solution (pH 7.4),14 3 with excess amounts of N-methylimidazole,
3(MeIm) in a phosphate buffer solution (pH 7.4) along with Zn tet-
rakis(4-carboxyphenyl)porphyrin, ZnTCPP,15 as the reference com-
pound with or without the axial coordination from MeIm in a
phosphate buffer solution (pH 7.4). The free-base porphyrin 1 gave
a normal Soret band and four Q bands. After Zn was introduced

H. Miyaji, J. Fujimoto / Tetrahedron Letters 51 (2010) 2979–2982
2981
(shown as 2) to the free-base porphyrin, the Soret band was split
by 17 nm, and the Q-bands changed to two peaks. The split Soret
bands are explained by exciton coupling theory,16 i.e., interactions
of two transition dipoles (face to face and head to tail) between
two porphyrin rings arising from the slipped cofacial dimer forma-
tion 22.
Figure 1a shows the absorption spectra of 2 at various concen-
trations (dilution experiments) in CH2Cl2. As we expected, the
slipped cofacial dimer 22 was very stable in CH2Cl2. The split Soret
fer solutions. When 3 was dissolved in basic solutions, such as
phosphate buffer solutions (pH 7.4 and 8.0) and pH 9–12 solutions,
similar split Soret bands were observed.14 At pH values higher than
12, a new peak seemed to be overlapping at 430 nm due to the
coordination of OHÀ. When 3 was dissolved in acidic solutions, such
as a phosphate buffer solution with pH 6.8, a new peak appeared at
421 nm. At pH 6.8, dimer 32 seemed to dissociate (or to exist in
equilibrium between the dimer and monomer) due to the proton-
ation of imidazole (see Supplementary data). In a tris buffer with
pH 8.0 and pH 7.5, the equilibria between the dimer and monomer
seemed to exist, probably due to coordination of the buffer.
To dissolve 3 in a physiologically similar pH condition, the
phosphate buffer with pH 7.4 was chosen as a solvent. Figure 2a
shows the absorption spectra of 3 at various concentrations (dilu-
tion experiments) in a phosphate buffer solution (pH 7.4). Differ-
ently from 2 in Figure 1a, the split Soret bands of 3 appeared at
413 and 430 nm were difficult to discern at concentrations lower
bands remained until the lowest concentration (0.1 lM) of the
absorption spectra.17 After the addition of MeIm (Fig. 1b), the equi-
librium between dimer 22 and monomer 2(MeIm) was observed.
Furthermore, after the addition of excess amounts of MeIm, most
of the dimer 22 dissociated, and a new peak appeared at 427 nm
(2(MeIm), a coordination species of MeIm as an axial ligand).
The Zn porphyrin carboxylic acid 3 was first dissolved in a satu-
rated NaHCO3 aq solution (pH 8.4). The NaHCO3 aq solution was
chosen in order to avoid a protonation of the imidazole moiety
and to form carboxylate salts in water. As we expected, 3 was dis-
solved in the aqueous solution, and the Soret band of the absorption
spectrum was split by 17 nm, as we observed in 22. The split Soret
bands show the formation of a slipped cofacial dimer 32 in the
than 0.5 lM, and a new peak appeared at 421 nm. This result indi-
cated the concentration dependence between the dimer and the
monomer. The monomer seemed to be 3(H2O), a coordination spe-
cies of H2O as an axial ligand in the aqueous solution. At concentra-
tions higher than 0.5 lM, the slipped cofacial dimer 32 predomin-
aqueous solution. We also tested dissolving 3 (2
lM) in various buf-
antly existed in the solutions, but, at lower concentrations,
Figure 1. Absorption spectra of the Zn porphyrin 2 recorded in CH2Cl2, (a) at
various concentrations, (b) before and after the addition of N-methylimidazole
(MeIm).
Figure 2. Absorption spectra of the Zn porphyrin 3 recorded in phosphate buffer
solution (pH 7.4), (a) at various concentrations, (b) before and after the addition of
N-methylimidazole (MeIm).

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H. Miyaji, J. Fujimoto / Tetrahedron Letters 51 (2010) 2979–2982
equilibria between dimer 32 and monomer 3(H2O) seemed to exist
in the solutions. After the addition of MeIm (Fig. 2b), the equilib-
rium between the dimer 32 and monomer 3(H2O) was found to
shift to the monomer, 3(MeIm). Upon the addition of excess
amounts of MeIm, most of the dimer 32 dissociated, and a new
peak appeared at 427 nm (3(MeIm), a coordination species of
MeIm as an axial ligand). The fluorescence emission maxima also
shifted from kem = 611 and 656 nm to 612 and 664 nm upon the
addition of excess amounts of MeIm, and the fluorescence intensity
increased five times.18 These results also supported the dissocia-
tion of dimer 32 (including 3(H2O)) to monomer, 3(MeIm) by the
addition of excess amounts of MeIm.19
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.03.115.
References and notes
1. Breslow, R. J. Biol. Chem. 2009, 284, 1337–1342. and references cited therein.
2. The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: New York, 2000.
3. Kobuke, Y.; Miyaji, H. J. Am. Chem. Soc. 1994, 116, 4111–4112.
4. Kobuke, Y.; Miyaji, H. Bull. Chem. Soc. Jpn. 1996, 69, 3563–3569.
5. Ogawa, K.; Kobuke, Y. Angew. Chem., Int. Ed. 2000, 39, 4070–4073.
6. Takahashi, R.; Kobuke, Y. J. Am. Chem. Soc. 2003, 125, 2372–2373.
7. Satake, A.; Kobuke, Y. Tetrahedron 2005, 61, 13–41.
8. Inaba, Y.; Kobuke, Y. Tetrahedron 2004, 60, 3097–3107.
9. Yang, Q.-Z.; Khvostichenko, D.; Atkinson, J. D.; Boulatov, R. Chem. Commun.
2008, 963–965.
10. Supramolecular Catalysis; van Leeuwen, P. W. N. M., Ed.; Wiley-VCH: Weinheim,
2008.
11. Young, S. W.; Qing, F.; Harriman, A.; Sessler, J. L.; Dow, W. C.; Mody, T. D.;
Hemmi, G. W.; Hao, Y.; Miller, R. A. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 6610–
6615.
12. Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. J. Mol. Biol. 1984, 180,
385–398.
13. Spectroscopic-grade dichloromethane (containing amylene as
purchased from Aldrich was used instead of chloroform because chloroform
contains small amounts of acid and methanol as a stabilizer.
14. Phosphate buffer solutions (pH 7.4 and pH 8.0) were purchased from TCI. These
buffer solutions consisted of KH2PO4–NaOH. Solutions with pH 9.0 and higher
were prepared by the addition of NaOH into the pH 8.0 phosphate buffer
solution.
15. Rochford, J.; Chu, D.; Hagfeldt, A.; Galoppini, E. J. Am. Chem. Soc. 2007, 129,
4655–4665.
16. Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371–392.
17. Further dilution experiments until the detection limit of the fluorescence
(10À9 M) also supported the stability of dimer 22 in CH2Cl2.
18. Control experiments using ZnTCPP also supported the formation of dimer 32 in
a phosphate buffer solution (pH 7.4). Although red shifts of the absorption and
fluorescence maxima (Table 1) were observed by the coordination of MeIm, no
increase in the fluorescence intensity was observed in the case of
The association constant for dimer 32 in a phosphate buffer
solution (pH 7.4) was estimated by the reported method.8 First,
the association constant Ka for the control compound, ZnTCPP with
MeIm in a phosphate buffer solution (pH 7.4), was determined by a
UV–vis. titration experiment (Ka = 1.1 Â 102 MÀ1). Second, the
association constant Kb for 3(MeIm) was determined from the
titration of dimer 32 in a phosphate buffer solution (pH 7.4) with
MeIm (Kb = 1.9 MÀ1). The association constant Kc (=Ka2/Kb) for di-
mer 32 in a phosphate buffer solution (pH 7.4) was obtained as
6.4 Â 103 MÀ1. This value was 58 times larger than Ka.20 Dimer 32
a stabilizer)
seemed to be stabilized by cooperative coordination and p-stack-
ing. The pH dependence of the association constant for dimer 32
was evaluated in phosphate buffer solutions with pH 8.0 and 9.0.
The association constants Kc for dimer 32 in phosphate buffer solu-
tions with pH 8.0 and 9.0 were obtained as 4.0 Â 103 MÀ1 and
3.0 Â 103 MÀ1, respectively. At pH values higher than pH 7.4, the
association constants for dimer 32 were found to decrease due to
the influence of OHÀ.
In conclusion, tris(4-carboxylphenyl)-mono(N-methylimidazo-
lyl)-substituted Zn porphyrin was synthesized. Zn porphyrin
was dissolved in a NaHCO3 aq solution (pH 8.4) and phosphate
buffer solutions (pH 7.4–9.0). Though equilibria between the di-
ZnTCPP(MeIm). The excitation wavelength (kex) was set to the isosbestic
point of the Soret bands of the absorption spectra. Self-quenching of the
fluorescence seemed to occur in the case of dimer 32 in a phosphate buffer
solution (pH 7.4). See Supplementary data S-2 and S-3.
mer and the monomer were observed at the
lM range in such
aqueous solutions,21 the splitting of the Soret bands of Zn por-
phyrin clearly showed self-organization to a special-pair type
slipped cofacial dimer even in water. Further applications using
the Zn porphyrin and other central metals (e.g., Mn, Fe, Co) of
porphyrin will be reported elsewhere.
19. Excess amounts (more than 300,000 equiv) of MeIm are required to dissociate
dimer 32; this result also supported the stability of dimer 32 in water due to
cooperative coordination and p-stacking.
20. The association constant Kc’ for dimer 22 in CH2Cl2 was determined in the same
way (Kc’ = Ka’2/Kb’ = 2.1 Â 1010 MÀ1). Ka’ was obtained as 5.2 Â 105 MÀ1 from the
titration of Zn tetrakis(4-methoxycarbonylphenyl)porphyrin in CH2Cl2 with
MeIm. Kb’ was obtained as 13 MÀ1
.
Acknowledgment
21. A similar equilibrium between dimer 32 and monomer 3 (H2O) was observed in
the case of dilution experiments in
Supplementary data S-4 and S-5.
a NaHCO3 aq solution (pH 8.4). See
We thank united graduate school of drug discovery and medical
information sciences, Gifu University for financial support.