Dibenzodiaza-30-crown-10-appended bis(zinc porphyrin) tweezers: Synthesis and crown-assisted chiroptical behaviour

Article abstract of DOI:10.1039/b701317e

In our program for developing chirality manipulation systems, we synthesized bis(zinc porphyrin) 1, with a dibenzodiaza-30-crown-10 as a linker unit. Two structural features were examined. The aza-crown segment exhibited an intermolecular interaction with

Full text of DOI:10.1039/b701317e

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Dibenzodiaza-30-crown-10-appended bis(zinc porphyrin) tweezers: synthesis
and crown-assisted chiroptical behaviour†
Yusuke Ishii, Toshiharu Yoshizawa and Yuji Kubo*
Received 29th January 2007, Accepted 15th February 2007
First published as an Advance Article on the web 13th March 2007
DOI: 10.1039/b701317e
In our program for developing chirality manipulation systems, we synthesized bis(zinc porphyrin) 1,
with a dibenzodiaza-30-crown-10 as a linker unit. Two structural features were examined. The
aza-crown segment exhibited an intermolecular interaction with the zinc(II) of the porphyrin, capable of
causing aggregation to form spherical nanostructures, as inferred by concentration-dependency of 1H
NMR as well as scanning electron microscopy (SEM) observation. We also consider the crown-based
conformation flexibility, in which accommodated K+ tunes the porphyrin orientation into the tweezers
conformation, assisting chirality induction upon complexation with chiral diamine 2. The circular
dichroism (CD) intensity change essentially reached a plateau at a [(1R,2R)-2] : [1] ratio of 2 : 1 for
which a 45% enhancement in the amplitude of CD spectra was observed compared to the K+-free
conditions. Use of the crown linker of 1 is not limited to promoting chirality induction with diamines in
the presence of K+; chiroptical probing of unprotected amino acids (Lys, His, Trp, and Phe) using 1 was
attained through liquid (1 in CH2Cl2)–liquid (the amino acids in 1 N KOH) two-phase extraction. The
amphiphilic properties of the crown segment, as well as the K+-assisted tweezers conformation, make it
possible to explore a potent way to develop chirality sensors for amino acids in water.
is a helical molecule which undergoes racemization between
right-handed (P) and left-handed (M) conformers in solution.
Since coordination of a chiral guest (e.g., L-Asp(OMe)–OMe)
to the zinc can induce helical chirality due to the shift in P–
Introduction
The design of systems which can manipulate chirality information
has become important in the interdisciplinary area between
supramolecular chemistry and chiral chemistry. As motivation,
M equilibrium, chirality is amplified allosterically in the system.
the chiroptical outcome is predictable and immediately applicable
Recently, by taking advantage of homotropic allostery based
in chirotechnology.1 Applications include sensors,2 asymmetric
on a cerium double decker porphyrin, highly enantioselective
catalysis,3 actuators such as molecular motors,4 photochromic
materials,5 nonlinear optical materials,6 liquid crystalline systems,7
and other valuable products.8 A common methodology of chirality
induction (chirogenesis)9 at the molecular level would be of use
in the approach in which a chiral-orientated conformation is
created by the transfer of chiral information from an external
species by noncovalent interaction. Bisporphyrins are known to
be chiral receptors and circular dichroism (CD) reporters.10 If
a porphyrin bischromophore in the system, with known electric
transitions, can be arranged in a clockwise or anticlockwise
sense upon complexation with chiral guests, its behaviour will
allow us to determine the absolute configuration by means of
CD spectroscopy.9,11 In such molecular architectures, synthetic
exploration of systems capable of controlling chirality induction
is still in its infancy. Use of allostery12 in inducing chirality
is an interesting approach in which shape control of a system
that responds to an external stimulus may affect its function
controllably. In 2000, Mizutani et al. reported allosteric chirality
amplification using a zinc bilinone dimer.13 The zinc bilinone
recognition has successfully taken place.14 Our approach is to
employ dynamic action based on heterotropic allostery which
should be easier to handle than homotropic allostery in molecular
manipulation.15 It is well known that crown ethers are powerful
tools for constructing target systems because of their structural
topology and superior synthetic susceptibility.16 Conformational
flexibility increases with ring size,17 such that the crown ligands
(e.g., 30-crown-10) tend to wrap cations of small size. Accordingly,
we have focused on a highly flexible dibenzo-30-crown-10 scaffold,
in which K+ is significantly wrapped to induce topological change
into a tweezers-like structure,18 and which generates a chiral
screw conformation.19 The insight that chirality can be induced
in the highly flexible crown ether congener led us to synthesize a
bisporphyrin system possessing this unit as the linker.
Another intriguing issue is whether the crowned bisporphyrin
can be applied to chirality sensing of unprotected amino acids.20
Lipophilic zinc porphyrins are not capable of binding with free
amino acids. Thus, to attain significant interactions with amino
acids, it is crucial to insert water soluble groups into the porphyrin
scaffold to provide an electrostatic interaction.21 CD probing
using the porphyrin systems has been therefore applicable mainly
to protected amino acids.22 However, Tamiaki et al. proposed
lanthanide porphyrin systems serving as excellent CD probes of
amino acids, which successfully extracted the amino acids from
neutral water.23 The key factor is the strong binding property of
the lanthanide porphyrin with the COO− site of the guest. As an
Department of Applied Chemistry, Graduate School of Science and Technol-
ogy, Saitama University, 255 Shimo-ohkubo, Sakura-ku, Saitama 338-8570,
Japan. E-mail: yuji@apc.saitama-u.ac.jp; Fax: +81-48-858-3514; Tel: +81-
48-858-3514
† Electronic supplementary information (ESI) available: Cooperative
association model, several spectroscopic data (Fig. S1–S5). See DOI:
10.1039/b701317e
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alternative approach, the crown-assisted chiroptical properties of
1 should allow it to act as a CD probe of free amino acids in water.
Here we describe the synthesis of title compound 1, as well
as its spectroscopic properties.24 In the latter section we begin
by addressing self-association of 1 through intermolecular Zn–
N interaction. Allosteric behaviour for chirality induction in 1 is
then discussed, in which the CD amplitude upon complexation
with chiral diamine 2 is enhanced by K+ accommodated in the
crowned cavity. Further, taking advantage of the crown segment,
the chirality of several unprotected amino acids could be read out
by 1 when we employed liquid–liquid two-phase extraction using
1 N KOH solution. CD activity was not observed when crown
ether-free bisporphyrin tweezers were used, in place of 1.
the terminal benzene rings of the dibenzo-30-crown-10 skeleton.
Regioselective modification is usually not easy.25 However, by tar-
geting a diaza-congener it is possible to regioselectively synthesize
dinitro-substituted 3 from a commercially available material, 5-
nitroguaiacol, in only three steps.18 Compound 3 was reduced to
the corresponding diamino derivative 4 with H2, and condensing
with tetraphenylporphyrin (TPP) acid chloride derivative 526 with
a small amount of NEt3 in dry CH2Cl2 then gave 6 in 48% yield.
Metallation using Zn(OAc)2·2H2O then gave the target 1 in 98%
yield.
Self-association
The structure of 1 was assigned according to various spectroscopic
data; Fig. 1 shows the 1H NMR data at room temperature using
CD2Cl2. The chemical shifts of the crown segment broadened
more than those due to the porphyrins even in low concentration
(0.1 mM) as a result of restricted mobility of the large-ring
crown, which increases the relaxation time in NMR. The spectra
Results and discussion
Synthesis
The synthesis is shown in Scheme 1. The key step in reaching
the target is to regioselectively insert porphyrin segments on
Scheme 1 Synthesis of 1. Reaction conditions: (a) 10% Pd/C, H2 (2.5 atm), EtOH, r.t.; (b) dry CH2Cl2 with a few drops dry Et3N, 0 ◦C, under Ar, 48%;
(c) sat. Zn(OAc)2·2H2O in MeOH, CH2Cl2, r.t., 98%.
Fig. 1 1H NMR spectra of 1 at various concentrations in CD2Cl2 at 23 ◦C.
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also show changes in the shift of protons in 1 with varying
concentration from 8 mM to 0.1 mM; a significant down-field
shift, due to protons of the dibenzodiaza-30-crown-10 as well as
the amido-linked benzene units, was observed in which a large
shift difference of 0.526 ppm for N–CH3 was clearly detected.
Self-association of 1 through intermolecular Zn–N coordination
between the porphyrins and the dibenzodiaza-30-crown-10 most
likely accounts for the observation at relatively high concentration
at room temperature. The change in chemical shift of protons
near to aza-segments of the crown ring can be therefore ascribed
to a ring current effect of porphyrin. Although this behaviour is
partly consistent with the observation in the case of dimerization
of diaza-18-crown-6-bridged zinc-free base bisporphyrin,27 the
N–CH3 resonances simply shifted, meaning that both nitrogens
interact with the central zinc (II) in the porphyrins to form extended
aggregates (vide infra). The 1H NMR dilution experiments in
CD2Cl2 on 1 enable us to estimate an association constant (Ka),
by monitoring the shift of the sensitive singlet (N–CH3) upon
changing the concentration (Ka = 440 32 M−1).28 This parameter
indicates that 1 exists mainly as a monomer (98%) at 0.1 mM,
so that at a UV–vis or CD detectable concentration 1 does not
undergo self-association.
in both asymmetric synthesis29 and as a building block of chiral
receptors.30 A UV–vis titration of 1 upon adding incremental
amounts of (1R,2R)-2 in CH2Cl2–MeCN (4 : 1 v/v) at 25 ◦C
was first carried out, the result is shown in Fig. 3. The Soret band
at 423 nm did not show a clear spectral change with increasing
amounts of (1R,2R)-2, possibly due to an intramolecular exciton
coupling interaction31 of the porphyrin-chromophores accompa-
nying the conformation switch which affects the Soret band (vide
infra). Thus, the complexation phenomenon can be quantified
based on the shift of the Q(1,0)-band, from 555 nm to 562 nm;
use of a Job plot32 suggests a 1 : 1 complex formation (Fig. S1),
and the association constant (log Ka) was estimated to be 6.04
0.09 using a nonlinear curve fitting method. We thus investigated
CD properties of the system upon adding incremental amounts
of (1R,2R)-2 under similar conditions. While 1 is inherently CD
inactive, addition of the chiral guest gave bisignated Cotton effects
at 433 and 422 nm (Fig. 4). The kCD value is in good agreement with
the k value of the Soret band of the porphyrin, leading to a clock-
wise twist between the chromophores. The presence of 6 equiv. of
(1R,2R)-2 allows us to detect positive exciton coupled CD spectra
[De +416 M−1 cm−1 (433 nm), −354 M−1 cm−1 (422 nm)], the total
To obtain insight into the morphology of the association, a field
emission-scanning electron microscopy (FE-SEM) experiment
was carried out; spherical structures of 1 were formed after direct
casting of the toluene solution on an aluminium plate (Fig. 2). This
formation process was found to involve aggregation of 1, followed
by stacking of the aggregates to minimize the interfacial free energy
between the particles and solvent. No such nanostructure was
obtained in the case of zinc tetraphenylporphyrin (ZnTPP) as a
control. Intermolecular Zn–N coordination plays a key role in the
extended aggregation.
Fig. 3 UV–vis spectra of 1 upon adding (1R,2R)-2 in CH2Cl2–MeCN (4 :
1 v/v) at 25 ◦C, [1] = 2.0 lM, [(1R,2R)-2] = 0, 2.0, 4.0, 6.0, 8.0, 10, 12, 16,
20, 30 lM.
Fig. 2 FE-SEM images of 1 (1.2 mM). The scale bar corresponds to 6 lm.
Cation-assisted chirality induction
The conformational flexibility of the crowned spacer in 1 permits
switching of the porphyrin orientation into the tweezers-like
conformation upon complexation with a bidentate ligand. We
selected optically active N,Nꢀ-dimethylcyclohexane-1,2-diamines
2 as target guests, because these are simple C2 symmetric molecules
suitable for investigation of crown-assisted chirality induction
using 1. Moreover, 1,2-diaminocyclohexane derivatives are useful
Fig. 4 CD spectral changes of 1 upon addition of (1R,2R)-2 (blue line)
or (1S,2S)-2 (red line) in CH2Cl2–MeCN (4 : 1 v/v) at 25 ◦C, [1] = 2.0 lM;
[2] = 0, 2.0, 4.0, 6.0, 8.0, 10, 12 lM.
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amplitude being 770 M−1 cm−1. This finding can be explained on
the basis of a steric repulsion mechanism between the coordinated
diamine 2 and the neighboring porphyrin rings, which generates a
chiral screw structure of 1.33 The complexation was verified by 1H
NMR spectra in which the proton resonances due to (1R,2R)-2
were entirely upfield shifted; the methyl resonances of 2 shifted
from 2.38 ppm to −3.71 ppm upon adding 1 (vide infra). On
the other hand, addition of (1S,2S)-2 into a solution of 1 gave a
negative exciton coupled CD spectrum (Fig. 4), indicating that the
chirality induced in 1 reflects the geometry of the chiral diamine.
Dynamic conformational change of the dibenzodiaza-30-
crown-10 skeleton by cation complexation leads us to investigate
heterotropic allostery in the chirality induction, it is expected
that K+ can be wrapped by the crown segment to induce a
tweezers-like conformation. Indeed, UV–vis titration, monitoring
the Soret band of 1 upon adding KClO4 in CH2Cl2–MeCN
(4 : 1 v/v), showed a hypsochromic shift by 2 nm along with
significantly decreasing absorption intensity (Fig. 5a). The change
in absorption intensity at 423 nm is almost saturated upon adding
a stoichiometric amount of K+, indicating that cofacialization of
hyperconjugation which was diminished by K+–N interactions.
Fig. 5b shows how porphyrin aromatic protons reflect the K+-
induced conformation change: upon interaction with K+, the
spectrum changed from broad signals to split patterns. The amide
protons shifted downfield by 0.75 ppm, whereas the changes
of pyrrole-resonances were greater and upfield. A 2D COSY
experiment (Fig. S3) led us to assign the signals as three pairs of
doublets in a 3 : 3 : 2 integral intensity ratio: 8.76 (J = 4.4 Hz) and
8.58 (J = 4.8 Hz), 8.67 (J = 4.4 Hz) and 8.32 (J = 4.8 Hz) as well
as 8.53 (J = 7.6 Hz) and 8.34 (J = 8.0 Hz).34 These observations
support the hypothesis that the dynamic conformational change
of 1 is restricted by encapsulated K+ to the tweezers conformation,
where the terminal porphyrin units are close to each other. It is
interesting to see how the encapsulated K+ could assist in chirality
induction. Fig. 6 shows CD amplitude changes as a function of
the incremental amounts of chiral 2 in the absence or presence
of 5 equiv. of metal ion (K+ or Li+); presence of K+ gave a
steeper ascending behaviour in the CD amplitude compared to
metal ion free conditions. It essentially reached a plateau at a
[(1R,2R)-2] : [1] ratio of 2 : 1, where a 45% enhancement in the
amplitude of CD spectra was observed. Similar enhancement was
also obtained in the case of (1S,2S)-2; the CD amplitude increased
by 43% upon adding 2 equiv. of the diamine. These results
suggest that the allosteric effect in which the K+-binding tunes
the conformation to bind chiral 2, can assist chirality induction
in 1. As a control experiment, when we measured CD spectra
under similar conditions using Li+ instead of K+, there was no
enhancement in the CD spectra. Also use of Cs+ induced some
CD enhancement but to a lesser degree than K+. We postulate
that the enhanced CD intensity in the presence of K+ is due to
a cooperative association of K+ and the chiral diamine toward
1. To verify this, we estimated the apparent association constants
with 2 in the presence or absence of K+ (5 equiv.) by monitoring
1
the porphyrin units takes place. This is supported by H NMR
experiments obtained after solid[KClO4]–liquid[1.0 mM of 1 in
CD2Cl2–CD3CN (4 : 1 v/v)] two-phase extraction, in view of
the low solubility of KClO4 in NMR detectable conditions. K+
coordination in the crown ring was inferred from the upfield-
shifted N–CH3 and N–CH2 resonances, as well as the downfield-
shifted CH2–O resonances (Fig. S2). The upfield-shift of protons
adjacent to the aza-segments could be explained on the basis of
Fig. 6 Changes in CD amplitude [A (= De1 − De2)] of 1 (2 lM) upon
complexation with chiral 2 in the presence or absence of K+, Li+ or
Cs+(5 equiv.) in CH2Cl2–MeCN (4 : 1 v/v) at 25 ◦C: (᭺) (1R,2R)-2 with
K+; (᭛) (1R,2R)-2 with Li+; (ꢀ) (1R,2R)-2 with Cs+; (ꢁ) (1R,2R)-2; (᭹)
(1S,2S)-2 with K+; (᭜) (1S,2S)-2 with Li+; (ꢁ) (1R,2R)-2 with Cs+; (ꢂ)
(1S,2S)-2.
Fig. 5 (a) UV–vis spectral change of 1 upon adding KClO4 in
◦
CH2Cl2–MeCN (4 : 1 v/v) at 25 C. [1] = 2.0 lM; [K+] = 0, 0.5, 1.0,
1.5, 2.0, 2.5, 3.0, 4.0 lM. (b) 1H NMR spectra of 1 and K+–1 complex in
CH2Cl2–CD3CN (4 : 1 v/v) at 23 ◦C. [1] = 1.0 mM.
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the absorption intensity at 602 nm due to the perturbation of
the Q-band in the porphyrin-chromophore. As shown in Fig. 7,
a nonlinear curve-fitting procedure, assuming (K+–1–(1R,2R)-2)
complex formation, reproduces the experimental titration data
with a residual square sum of 2.81 × 10−7, in which the estimated
association constant (log Ka) is 6.27, being twice the value as
without K+ (log Ka = 5.98).
at a rate observable within the NMR time scale. However, solid
[KClO4]–liquid [1 (10 mM) and (1R,2R)-2 (5 mM) in CDCl3–
CD3CN (4 : 1 v/v) solution] two-phase solvent extraction led to a
further upfield shift, sharpening the signals (Fig. 8c). In particular,
the resonances for NHCH3 and NHMe shifted further upfield, by
0.40 and 0.49 ppm, respectively, than under K+-free conditions.35
This means that equilibrium was more shifted to the complex,
supporting the CD enhancement in the presence of K+.
Chirality sensing of unprotected amino acids
We have found that the crown-assisted chirality induction de-
scribed here is applicable to chirality sensing of unprotected amino
acids. This result is highlighted by the fact that hydrophobic
bis(zinc porphyrin) tweezers cannot sense the chirality of free
amino acids using CD spectroscopy.9 Insertion of the amphiphilic
crown segment in 1 allowed us to investigate an assay of several
enantiomer pairs of common amino acids by liquid–liquid two-
phase extraction and subsequent CD measurements. We first
selected basic L-Lys as a putative guest, based on our supposition
that ditopic binding with bis(zinc porphyrin) tweezers takes place.
There are no CD responses from the extraction of the amino acid
from neutral aqueous solution to a CH2Cl2 solution of 1 (39 lM),
but use of 1 N KOH solution as an aqueous phase induced an
exciton-coupled bisignate CD curve [first Cotton, 434 nm (De
−101 M−1 cm−1; second Cotton, 426 nm (De +123 M−1 cm−1)],
Fig. 9, line (b). The negative exciton coupling CD spectrum
suggests an anticlockwise screw sense of the porphyrin units,
consistent with that in the case of 1 with L-Lys–OMe in CH2Cl2
(Fig. 9, line (e)). Of particular interest is that upon replacement
of KOH with LiOH or NaOH, the CD spectra remained silent
(Fig. 9, lines (c) and (d)), consistent with Fig. 6 in which the K+-
induced CD enhancement is more efficient than in the presence of
Li+. This strongly suggests that the K+-coordinated dibenzodiaza-
30-crown-10 linker assists the extractability from the aqueous
solution to CH2Cl2, accompanying K+-induced conformational
rearrangement in 1. Indeed, no CD sign was detected in a similar
Fig. 7 The change in absorption intensity at 602 nm as a function of
incremental amounts of (1R,2R)-2 in the presence (᭺) and absence (ꢁ) of
5 equiv. of K+ in CH2Cl2–MeCN (4 : 1 v/v) at 25 ◦C.
Direct evidence for the K+-assisted interaction between 1 and 2
1
was found by H NMR measurements; a CDCl3–CD3CN (4 :
1 v/v) solution of (1R,2R)-2 (5 mM) was titrated with 1 in
the absence or the presence of K+ (Fig. 8). The methylamino
group of 2 should become a useful probe for monitoring the
complexation motif. The spectrum in Fig. 8a shows protons due to
the guest diamine, where the singlets for NHCH3 and NHMe were
detectable at 2.38 and 3.37 ppm, respectively. When adding 2 equiv.
of 1 (Fig. 8b), the proton resonances due to 2 were observed
wholly in higher magnetic field than 0 ppm with signal broadening,
indicating that the guest was located in the concave cavity between
porphyrin units. The observed broad signals can be ascribed to
the exchange between free and complexed species, which occurs
Fig. 9 CD spectra of 1 in CH2Cl2 after shaking (30 min) with an aqueous
solution of L-Lys (a), with a 1 N KOH solution of L-Lys (b), with a 1 N
LiOH solution of L-Lys (c), with a 1 N NaOH solution of L-Lys (d). CD
spectrum of 1 with L-Lys–OMe in CH2Cl2 (e). CD spectrum of 1 in CH2Cl2
after shaking (30 min) with a 0.1 N KOH solution of D-Lys (f). Conditions:
[1] = 13 lM, [L-Lys] = [D-Lys] = 0.1 M, [L-Lys–OMe] = 260 lM at 25 ◦C
and optical length of employed cell is 0.2 cm.
Fig. 8 1H NMR spectra of: (a) (1R,2R)-2; (b) (1R,2R)-2 with 2 equiv. of
1; (c) (1R,2R)-2 with 2 equiv. of 1 in the presence of K+, in CDCl3–CD3CN
(4 : 1 v/v) at 23 ◦C. [1] = 10 mM, [(1R,2R)-2] = 5 mM.
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liquid–liquid two phase extraction using crown-free bis(zinc por-
phyrin) tweezers, such as ethane-bridged bis(zinc porphyrin).36 We
tried to determine the amount of L-Lys extracted by 1 from the 1 N
KOH phase using ninhydrin colorimetry.37 Defining the extraction
(%) as the ratio of extracted amino acid concentration versus the
concentration of 1, the value is 61%.38 Also investigated was a sim-
ilar liquid–liquid two-phase extraction with the D-Lys enantiomer
to give a mirror image in the CD spectrum [De +124 M−1 cm−1
(434 nm), −117 M−1 cm−1 (426 nm)], see line (f) in Fig. 9.
His, possessing a basic imidazole sidechain, was also checked in
a similar extraction study, giving significant bisignate CD curves
[De +178 M−1 cm−1 (435 nm), −114 M−1 cm−1 (424 nm) for L-
His; De −160 M−1 cm−1 (435 nm), +119 M−1 cm−1 (424 nm) for
D-His] in which the CD intensity is somewhat larger than with
Lys (Fig. 10). The chirality signs induced by His are opposite
to those for Lys. The sign of the induced CD depends on the
steric bulkiness of the substituents at the chiral center, which play
the major role in the stereospecific orientation of the interacting
electric transitions in the complex. This forces the complex motif
to adopt the least sterically hindered conformations.22c We suggest
that the Lys sidechain does not affect the porphyrin arrangement
in the complex, whereas the more rigid imidazole residue in His
mainly determines the chirality screw sense of the porphyrins.
Fig. 11 CD spectra of 1 (13 lM) in CH2Cl2 after shaking (30 min) with
a 1 N KOH solution of Trp and Phe (0.1 M) at 25 ◦C. Optical length of
employed cell is 0.2 cm.
addition of incremental amounts of L-Trp–OMe (0 → 20 equiv.);
there were no bisignate spectra based on the exciton coupling of
porphyrins (Fig. S5). Overall, the extractability of the amino acids
(Trp and Phe) as aminocarboxylate potassium salts is ascribed
to both the amphiphilic properties of the crown segment and the
K+-guided hydrophobic interactions between the guest and the
concave cavity of 1, allowing chirality sensing of the amino acids
in water.
Conclusion
The present results show a novel capability potential of bis-
porphyrin systems possessing a large ring-sized crown ether as
the spacer. Their highly flexible conformation property led us
to explore their utility as a chiral probe because of the ease of
the achiral-to-chiral transformation, along with complexation of
chiral diamines in the terminal porphyrin segments. In particular,
the combination of the crown and porphyrin units in the system
provides cooperative chiroptical properties in the presence of K+,
where K+ accommodated in the crown ring acts as an allosteric
factor to tune the conformation to bind the diamines, resulting
in a CD enhancement. We further found that 1 provides an
extraction and chirality probing of unprotected amino acids
(aminocarboxylates) in 1 N KOH as a result of the amphiphilic
property of the crowned spacers, where encapsulated K+ can
guide carboxylate into the tweezers. We believe that the success
of chirality sensing for amino acids using 1 would provide a new
way to develop porphyrin-based CD probes. Our results further
explore how to manipulate chirality at the molecular level, which
remains an intriguing challenge in supramolecular chemistry.
Fig. 10 CD spectra of 1 (13 lM) in CH2Cl2 after shaking (30 min) with
a 1 N KOH solution of His (0.1 M) at 25 ◦C. Optical length of employed
cell is 0.2 cm.
The assay of various common amino acids was further inves-
tigated; it is interesting to note that chirality sensing for Trp and
Phe was implemented through an extraction study using 1 N
KOH solution, as shown in Fig. 11. The bisignate CD curves
are evidence for dipole–dipole electric interaction between the
two porphyrins. Despite 13C NMR and IR measurements to gain
insight into the interaction between coordinated Zn(II)–COO−,
we have not obtained any evidence as to whether two-point
complexation between 1 and the aminocarboxylate takes place
through the extraction process. However, we postulate that K+ in
the crown moiety guides the carboxylate into the concave cavity
in 1; significant hydrophobic interactions are believed to occur
between the hydrophobic side chains of Trp and Phe39 and the
bisporphyrin tweezers. This effect is not expected for amino acids
with no hydrophobic side chain, such as L-Asp. As a control
experiment, we measured CD spectra of 1 (13 lM) in CH2Cl2 upon
Experimental
General
NMR spectra were taken on Bruker DPX 400 or DRX 400
(400 MHz) spectrometers. Chemical shifts (d) are reported down-
field from the initial standard Me4Si. Matrix-assisted laser desorp-
tion/ionization time-of-flight (MALDI-TOF) mass spectroscopy
was performed with a CHCl3 solution of dithranol (20 mg mL−1) as
a matrix on a Shimadzu AXIMA-CFRTM. Electronic absorption
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spectra were recorded on a Shimadzu UV-3100 spectrophotome-
ter. Circular dichroism (CD) spectra were recorded on a JASCO
J-720 spectropolarimeter. Elemental analyses were obtained using
an EISON EA1108.
18H), 7.59 (d, J = 1.2 Hz, 2H), 7.47 (dd, J = 8.5 and 1.2 Hz,
2H), 6.92 (d, J = 8.7 Hz, 2H), 4.15 (br s, 4H), 3.84 (br s, 4H),
3.67–3.63 (m, 12H), 3.54 (s, 8H), 3.23 (t, J = 5.8 Hz, 4H), 2.80 (s,
6H); 13C NMR (100.7 MHz, DMSO-d6, 23 ◦C) d 165.11, 150.44,
149.39, 149.32, 148.97, 145.83, 142.68, 138.02, 134.14, 133.29,
131.83, 131.66, 131.39, 127.50, 126.60, 125.81, 120.60, 120.47,
119.23, 117.69, 112.76, 106.08, 70.00, 69.93, 69.91, 69.79, 69.20,
69.11, 67.36, 54.27; MALDI-TOF, m/z 1996 (M+), 1997 ([M +
H]+); elemental analysis, anal. calcd for C120H100N12O10Zn2·2H2O:
C, 70.76, H, 5.15, N, 8.25; found: C, 70.67, H, 4.94, N, 8.16%.
Materials
The starting materials were of available commercial grade and
were used without further purification. Dry solvents were pre-
pared according to standard procedures. Optically active N,Nꢀ-
dimethylcyclohexane-1,2-diamines as well as amino acids were
purchased from Tokyo Chemical Industry Co., Ltd, of which
L-Trp–OMe was purchased as a hydrochloride salt and treated
with NaOH to convert the salt to the corresponding free amine.
The CH2Cl2 and MeCN for UV–vis and CD measurements were
purchased as analytical grade and used as received.
FE-SEM measurements
The sample (10 mg) was dissolved in hot toluene (4 mL) and dried
at room temperature on an aluminum plate. It was shielded by Pt–
Pd and examined with a HITACHI S-4100 field emission-scanning
electron microscope.
17,37-Bis[4-(10,15,20-triphenyl-21H,23H-porphine-5-yl)phenyl-
carbonylamino]-21,33-dimethyl-2,5,8,11,14,24,27,30-octaoxa-21,
33-diazatricyclo[32.4.0.015,20 ]octatriaconta-1(38),15(20),16,18,34,
36-hexene (6). Regioselectively dinitro-inserted dibenzodiaza-
30-crown-10 3 (103.5 mg, 0.16 mmol) and 10% Pd/C (52.6 mg)
were dispersed in EtOH (80 mL). The mixture was stirred
for ca. 6 hrs at room temperature under a H2 atmosphere
(2.5 atm). After filtration using Celite, evaporation to dryness
gave the corresponding diamino derivative 4. Subsequently, 4
and a tetraphenylporphyrin (TPP) acid chloride derivative 5
(183.1 mg, 0.27 mmol) were dissolved in dry CH2Cl2 (15 mL).
After adding NEt3 (0.3 mL) the mixture was stirred for 2.5 hrs
at 0 ◦C. The resulting solution was treated with water (50 mL)
and extracted with CH2Cl2 (50 mL × 3). The organic phase was
evaporated, chromatographed on silica gel (Wakogel C-300) using
CH2Cl2–AcOEt–MeOH (10 : 2 : 1 v/v) as eluent, and further
purified by GPC using CHCl3 as eluent. In this way, 122 mg of
Liquid–liquid two-phase extraction
The chirality sensing of amino acids by 1 was carried out by
adding a CH2Cl2 solution of 1 (3.9 × 10−5 M, 0.8 mL) to an
aqueous solution of amino acids (0.1 M, 0.8 mL) in the absence or
presence of 1 N MOH (M = Li, Na, and K). After the mixture was
stirred for 30 min, the extracted organic phase (200 lL) was diluted
with 400 lL of CH2Cl2, and then measured by CD spectroscopy
where a 0.2 cm length cell was used. On another front, the
concentration of L-Lys in the organic phase was determined by
ninhydrin colorimetery;37 the organic phase (20 mL) obtained by
stirring a mixture of a CH2Cl2 solution of 1 (3.9 × 10−5 M) and a
1 N KOH solution of L-Lys (0.1 M) for 30 min was extracted by
water (10 mL × 3). The extracted water phases were collected and
evaporated in vacuo. The residue was dissolved in a pH 5 buffer
solution (AcOH and AcONa in 5 mL) containing ninhydrin (1.0 ×
10−2 M) under an Ar atmosphere, and was then heated to 110 ◦C for
20 min. The resulting solution was cooled to 25 ◦C and measured
by UV–vis spectroscopy.
1
6 were obtained (48% yield). H NMR (400 MHz, DMSO-d6,
10 mM, 23 ◦C) d 10.44 (s, 2H), 8.82 (s, 8H), 8.81 (s, 8H), 8.34 (d,
J = 8.0 Hz, 4H), 8.29 (d, J = 8.0 Hz, 4H), 8.18 (d, J = 6.4 Hz,
12H), 7.83–7.76 (m, 18H), 7.58 (s, 2H), 7.47 (d, J = 8.0 Hz,
2H), 6.91 (d, J = 8.0 Hz, 2H), 4.14 (br s, 4H), 3.83 (br s, 4H),
3.66–3.60 (m, 12H), 3.52 (s, 8H), 3.23–3.20 (m, 4H),◦2.79 (s, 6H),
−2.91 (s, 4H); 13C NMR (100.7 MHz, DMSO-d6, 23 C) d 164.91,
150.41, 144.13, 141.12, 138.02, 134.61, 134.17, 133.21, 131.55,
131.34, 131.16, 128.07, 126.96, 126.13, 120.24, 120.12, 118.92,
117.64, 112.75, 106.05, 69.98, 69.89, 69.76, 69.17, 69.07, 67.33,
54.24; MALDI-TOF, m/z 1872 (M+), 1873 ([M + H]+); elemental
analysis, anal. calcd for C120H104N12O10·2H2O: C, 75.45, H, 5.70,
N, 8.80; found: C, 75.54, H, 5.47, N, 8.70%.
Acknowledgements
This research has been supported by a Grant-in-Aid for Scientific
Research (C) from Education, Science, Sport and Culture of
Japan (No. 16550119). We thank Shimadzu Corporation for
the measurement of MALDI-TOF spectroscopy using Shimadzu
AXIMA-CFRTM.
References
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