N-confused porphyrin possessing glucamine-appendants: Aggregation and acid/base properties in aqueous media

Article abstract of DOI:10.1016/j.bmcl.2008.10.079

A water-soluble derivative of N-confused porphyrin (NCP: 5,10,15,20-tetraaryl-2-aza-21-carbaporphyrin) was synthesized by introducing glucamine groups at the para-position of meso-aryl groups. The tetraglucamine-appended NCP (TG-NCP) exists as monocation in aqueous solution containing 6 mM sodium dodecyl sulfate (SDS) but exists as freebase to form aggregates in pure water. These properties are distinct from those of corresponding regular porphyrin, which exists as freebase in the micellar solution and practically insoluble in water.

Full text of DOI:10.1016/j.bmcl.2008.10.079

Bioorganic & Medicinal Chemistry Letters 18 (2008) 6394–6397
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
N-confused porphyrin possessing glucamine-appendants: Aggregation
and acid/base properties in aqueous media
Yoshiya Ikawa a,b, Hiroaki Ogawa a, Hiroyuki Harada a, Hiroyuki Furuta a,
*
a Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 744 Nishi-Ku Moto-oka Fukuoka 819-0395, Japan
b PRESTO, Precursory Research for Embryonic Science and Technology, Japan Science and Technology Corporation, Saitama 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A water-soluble derivative of N-confused porphyrin (NCP: 5,10,15,20-tetraaryl-2-aza-21-carbaporphy-
rin) was synthesized by introducing glucamine groups at the para-position of meso-aryl groups. The
tetraglucamine-appended NCP (TG-NCP) exists as monocation in aqueous solution containing 6 mM
sodium dodecyl sulfate (SDS) but exists as freebase to form aggregates in pure water. These properties
are distinct from those of corresponding regular porphyrin, which exists as freebase in the micellar solu-
tion and practically insoluble in water.
Received 7 September 2008
Revised 15 October 2008
Accepted 17 October 2008
Available online 22 October 2008
Keywords:
Ó 2008 Elsevier Ltd. All rights reserved.
N-confused porphyrin
Water-soluble porphyrin
Aggregate
Gulcamine
pKa
Water-soluble porphyrins possessing hydrophilic substituents
have been widely studied because of their promising properties
for biological and medicinal application as well as environmental
analyses. Some water-soluble porphyrins have been employed as
photosensitzers for photodynamic therapy (PDT),1 a core compo-
nent in hybrid molecules for boron neutron capture therapy,2 ion
sensors in water3 and so on. More recently, porphyrin analogues
including expanded porphyrins and porphyrin isomers have also
attracted much attention owing to their unique properties unseen
in regular porphyrin.4
at the para-position of meso phenyl groups, were prepared accord-
ing to the literature procedure.9 Tetracarboxylic acid 1b and 2b
were obtained by the KOH catalyzed hydrolysis of 1a and 1b in
H2O/CH3OH. An NCP derivative bearing four glucamine units (TG-
NCP) and corresponding porphyrin (TG-P)10 were synthesized by
a coupling of 1b and 2b with 1-amino-1-deoxy-D-sorbitol (D-gluc-
amine) using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide
hydrochloride (EDC) and 1-hydroxybenzotriazole (HOBT), respec-
tively.10,11 1b and TG-NCP were characterized by spectroscopically,
using 1H NMR and MALDI-TOF mass.12 IR spectrum of TG-NCP (as
KBr pellet) exhibits a characteristic stretch of amide C@O at
N-confused porphyrin (NCP) is one of the porphyrin isomers5
that has been considered as a promising candidate for a PDT sensi-
tizer because some NCP derivatives have shown to generate singlet
oxygen efficiently in organic media.6 NCP has also been expected to
apply to biosensors since it can bind various anions including
biologically important phosphate and chloride.7 To investigate
interactions between NCP and biomolecules in aqueous media,
water-soluble NCP derivatives are necessary but such molecules
have been scarcely known.8 Herein, we report a synthesis of
water-soluble NCP derivative possessing neutral polyol moieties
at the meso-aryl groups. Its aggregation behavior as well as acid/
base properties in aqueous solutions are also described.
1629.6 cmÀ1
UV–vis absorption spectra of 25
.
l
M TG-NCP in ultrapure water
show the Soret band at 429 nm (Fig. 1A, [SDS] = 0 mM), which is
significantly blue-shifted and broadened compared to that of
5,10,15,20-tetraphenyl N-confused porphyrin (NCTPP), in which
the Soret band is observed at 438 or 442 nm in CH2Cl2 or DMF,
respectively.13 Thus, such blue-shift of the Soret band of TG-NCP
in water strongly suggests the formation of self-aggregates in
water.
In contrast to TG-NCP, TG-P was found insoluble in ultrapure
water.10 TG-P became slightly soluble in the water containing
0.2 mM sodium dodecyl sulfate (SDS) (Fig. 1B). The Soret band is
observed at 400 nm, which is also significantly blue-shifted and
broadened compared to that of tetraphenylporphyrin (TPP) in or-
ganic solvent (kmax = 419 nm in benzene).14
A water-soluble NCP and corresponding derivative of regular
porphyrin were synthesized as shown in Scheme 1. NCP 1a and
regular porphyrin 1b, both of which bear methoxycarbonyl groups
To examine whether the aggregates are formed in water with
TG-NCP and TG-P, SDS was added to their aqueous solutions. The
* Corresponding author. Fax: +81 92 802 2865.
E-mail address: hfuruta@cstf.kyushu-u.ac.jp (H. Furuta).
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.10.079

Y. Ikawa et al. / Bioorg. Med. Chem. Lett. 18 (2008) 6394–6397
6395
Scheme 1. Structures and synthesis of glucamine-appended N-confused porphyrin (TG-NCP) and regular porphyrin (TG-P). Reagents and conditions: (i) 20% aq KOH/CH3OH
(v/v = 1/4), ambient temp. (ii) EDC (10 equiv), HOBT (10 equiv), -glucamine (20 equiv) in DMF/H2O (v/v = 6/1), 50 °C.
D
7 mM SDS shows the Soret band at 419 nm, which is identical to
that of TPP in benzene (419 nm).14 These data indicate that TG-P
exists as freebase in neutral pH consistent with the previous re-
ports,8,10 and is more acidic than TG-NCP.
To elucidate the acid/base properties of TG-NCP under the
monomeric and aggregated conditions, quantitatively, pH titra-
tions were carried out in the presence/absence of 6 mM SDS. In
presence of SDS, titration profile reveals four regions which
showed distinct spectral changes (Fig. 2): (i) from pH 12.6 to
10.5, intensity of the Soret band decreases modestly without a shift
of kmax (446 nm) (Fig. S1A); (ii) from pH 10.5 to 7.8, intensity of the
Soret band increases, coinciding with a red-shift of the kmax by a
11 nm (446–457 nm). Q-type bands also show spectral changes
which give an increase of a broad Q-type band at 817 nm
(Fig. 2A); (iii) from pH 7.8 to 4.2, intensity of the Soret band de-
creases slightly without a shift of kmax (458 nm) (Fig. S1B); (iv)
from pH 4.2 to 0.83, intensity of the Soret band increases with a
modest red-shift (458–463 nm) accompanied by an emergence of
two new Q-type bands at 660 and 804 nm (Fig. 2B). Spectral fea-
tures of TG-NCP at pH 11 (Fig. S1A), pH 7.8–4.2 (Fig. S1B), and
pH 0.83 (Fig. 2B) are closely similar to those of the freebase, mon-
ocation, and dication of monomeric NCTPP in organic solvent.5a,13
Thus, the dominant species of TG-NCP in 6 mM SDS solution at pH
11, 7.8–4.2, and 0.83 can be assigned as freebase, monocation, and
dication, respectively (Table 1). The equilibrium constants between
freebase and monocation (pK3) and between monocation and dica-
tion (pK4) were estimated to be 9.3 and 3.0, respectively. These val-
ues are comparable to those of NCTPP measured in the aqueous
micellar SDS solution, in which pK3 and pK4 were determined to
be 8.4 and 3.3.5a
In the absence of SDS, TG-NCP shows a modest spectral change
between pH 13.3 and 7.7 with the Soret band around 430 nm
(Fig. 3A). In the pH range from 6.8 to 2.2, however, a drastic in-
crease in the intensity of the Soret band accompanying red-shift
of the kmax is observed (Fig. 3B). In the pH range from 2.2 to 0.83,
Figure 1. UV–vis absorption spectral changes of TG-NCP (A) and TG-P (B) in
ultrapure water by the addition of various amounts of SDS. The pH of solutions with
0, 1, 3, or 7 mM SDS was 7.3, 6.3, 6.0, or 5.7, respectively.
changes in the absorption spectra of TG-NCP or TG-P were satu-
rated when the concentrations of SDS were reached to 6 or
7 mM, respectively, which values are slightly smaller than the crit-
ical micelar concentration of SDS in water (8.5 mM). Along with
the addition of SDS, both the absorption spectra became sharper
and red-shifted (Fig. 1). This change strongly suggests that both
TG-NCP and TG-P form H-type aggregates in neutral water.15
In the presence of 6 mM SDS, the absorption spectrum of TG-
NCP exhibits the Soret band at 457 nm (Fig. 1A), which value is
close to that of NCTPP monocation showing the kmax at 451 nm
in CH2Cl2.5a Therefore, TG-NCP can exist dominantly as monoca-
tion in neutral water (Table 1). On the other hand, TG-P with
Table 1
Absorption maxima (kmax) of TG-NCP in different forms observed in aqueous solution.
TG-NCP
Soret band kmax (nm)
Q-type bands kmax (nm)
Freebase (monomer)a
Monocation (monomer)b
Dication (monomer)c
Freebase (aggregates)d
446
457
463
431
546, 593, 666, 730
817
660, 804
605, 753
a
From Figure S1A.
From Figure S1B.
From Figure 2B (at pH 0.83).
From Figure 3A (at pH 13.3).
b
c
d

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Y. Ikawa et al. / Bioorg. Med. Chem. Lett. 18 (2008) 6394–6397
Figure 2. pH-dependent spectral changes of TG-NCP in ultrapure water containing
6 mM SDS. Absorption spectra during a titration between pH 10.5 and 7.8 (A), and
between pH 4.2 and 0.8 (B). Titrations were performed with aqueous HCl or NaOH.
Figure 3. pH-dependent spectral changes of TG-NCP in ultrapure water without
SDS. Absorption spectra during a titration between pH 13.3 and 6.8 (A), and
between pH 6.3 and 0.83 (B). Titrations were performed with aqueous HCl or NaOH.
both the Soret and Q-like bands decrease without a shift of kmax
values (Fig. 3B). Comparison with the spectra in the presence of
6 mM SDS indicates that the spectral changes of TG-NCP observed
in acidic pH without SDS would represent a disassembly process of
aggregated freebases to monomeric dications. Increase of the
absorption around pH 2.2 suggests that this process may involve
an intermediate whose nature remains to be elucidated. This
assignment is supported by the fact that the Soret band of TG-
NCP under acidic conditions (pH <4) shows kmax at 462 nm that
is almost identical to that of TG-NCP with 6 mM SDS (463 nm)
and that of NCTPP dication in CH2Cl2 (461 nm). At pH 0.83, the
spectrum of TG-NCP is virtually unchanged by the addition of
SDS (Fig. S2), indicating that TG-NCP dication exists as a mono-
meric form even without SDS. It is also worth to note that the
pH titration experiments without SDS (Fig. 3) afford no strong evi-
dence for accumulation of TG-NCP monocation although it is dom-
inant species between pH 4 and 8 in the solution containing 6 mM
SDS (Fig. 2A and S1B). Spectral changes suggest that pK value for
the transition between aggregated freebases and monomeric dica-
tions (pKagg) is around 4 (Fig. 3B). These data indicate that acid/
base properties of NCP macrocycle are strongly affected by its
aggregation (Scheme 2).
In summary, a water-soluble derivative of N-confused tetra-
arylporphyrin bearing four glucamine units and corresponding
regular porphyrin have been synthesized. At neutral pH, gluc-
amine-appended NCP (TG-NCP) exists as monocation in 6 mM
SDS solution whereas it forms aggregates without protonation in
pure water. These characteristic properties of TG-NCP as well as
its water-solubility would originate from the confused pyrrole in
the porphyrin core, while the corresponding regular porphyrin
(TG-P) is practically insoluble in neutral water without SDS. In liv-
Scheme 2. A plausible dynamics of aggregation/disaggregation and acid/base
behaviors of TG-NCP in aqueous solution. Spectral changes of TG-NCP by the
addition of SDS at pH 13.3 are shown in Figure S3.
ing cells, linear and cyclic polyols are known to play various
roles.16 Based on the findings that mono- and oligo-saccharides of-
ten act as specific markers or targeting-tags for proteins, organelle,
and cells,15 a number of porphyrin–carbohydrate conjugate mole-
cules have been designed and investigated for pharmaceutical and
diagnostic application.17 This work therefore would allow NCP to
register as a new player in these fields.
Acknowledgments
This work was partly supported by Grants-in-Aid for Young Sci-
entists (A) (No.18685020 to Y.I.), Exploratory Research

Y. Ikawa et al. / Bioorg. Med. Chem. Lett. 18 (2008) 6394–6397
6397
11. Sessler, J. L.; Davis, J. M.; Kral, V.; Kimbrough, T.; Lynch, V. Org. Biomol. Chem.
2003, 1, 4113.
(No.19657071 to Y.I.), and the Global COE program, ‘‘Science for
Future Molecular Systems” (to H.F.) from the Ministry of Education,
Culture, Science, Sports and Technology of Japan.
12. Compound 1b: 1a (150 mg, 0.18 mmol) was dissolved in methanol (50 mL) and
cooled to 0 °C and 20% aq KOH (12.5 mL) was added. The reaction mixture was
allowed to warm to ambient temperature. After stirring for 48 h, the reaction
mixture was acidified with trifluoroacetic acid (TFA) until precipitation began
to form. After removal of the solvent, the residue was extracted with ethyl
acetate. The organic layer was dried and evaporated and the residue was
recrystallized with methanol/ethyl acetate to yield 1b as a green solid. Yield;
122 mg (87%). 1H NMR (CD3OD): d À2.57 (s, 1H), 8.21–8.32 (m, 8H), 8.37–8.50
(m, 9H), 8.54–8.61 (m, 4H), 8.65 (d, 1H, J = 4.8 Hz), 8.76 (d, 1H, J = 4.8 Hz); MS
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2008.10.079.
(MALDI, positive) 790.71 (M+). TG-NCP: 1b (28 mg, 72
l
mol) and EDC (70 mg,
mol) were dissolved in 20 mL DMF. After adding HOBT (47 mg,
mol), reaction was stirring for 30 min. solution of -glucamine
360
360
l
l
References and notes
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D
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25 mg (49%). 1H NMR (DMSO-d6 + TFA): d À1.91 (br, 1H), 3.37–3.54 (m, 8H),
3.58 (br s, 8H), 3.64 (br, 4H), 3.68 (br, 4H), 3.78 (br, 4H), 3.95 (br, 4H), 8.28 (br,
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[Note: Due to the aggregation, each proton signals show some broadening. In
addition, the presence of several conformers or magnetically inequivalent
species makes the spectrum complicated to assign the signal explicitly]; MS
(MALDI, positive) 1441.99 (M+).
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