Sequential and spatial organization of metal complexes inside a peptide duplex

Article abstract of DOI:10.1021/ja502898t

To generate integrated organized molecular properties, multiple molecular components are required to be assembled into the molecular system with sequential and spatial accuracy in accordance with the design of the molecular assembly. Herein, we present a novel programmable synthesis of a cofacially stacked porphyrin array via repetitive construction of a peptide duplex. We designed and synthesized a novel porphyrin having two artificial amino acid moieties at the trans meso-positions. The amino acid moieties can be connected with another porphyrin unit by repetitive doubly coupling reactions to afford the peptide duplex bridged by the porphyrins. In the duplex, the porphyrin units are stacked cofacially, and the efficient electronic communication among the arrayed porphyrin units was characterized by split redox waves in the cyclic voltammograms. We also demonstrated the three different square-planar metal ions, namely Cu²⁺, Ni²⁺, and Pd²⁺, were arranged inside the ladder-type porphyrin array in a programmable fashion.

Full text of DOI:10.1021/ja502898t

Article
pubs.acs.org/JACS
Sequential and Spatial Organization of Metal Complexes Inside a
Peptide Duplex
Yasuyuki Yamada,†,‡ Takayuki Kubota,† Motoki Nishio,† and Kentaro Tanaka*,†,§
‡
†Department of Chemistry, Graduate School of Science and Research Center for Materials Science, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8602, Japan
§CREST, Japan Science and Technology Agency, Honcho 4-1-8, Kawaguchi-shi, Saitama 332-0012, Japan
S
* Supporting Information
ABSTRACT: To generate integrated organized molecular
properties, multiple molecular components are required to be
assembled into the molecular system with sequential and
spatial accuracy in accordance with the design of the molecular
assembly. Herein, we present a novel programmable synthesis
of a cofacially stacked porphyrin array via repetitive
construction of a peptide duplex. We designed and synthesized
a novel porphyrin having two artificial amino acid moieties at
the trans meso-positions. The amino acid moieties can be
connected with another porphyrin unit by repetitive doubly
coupling reactions to afford the peptide duplex bridged by the porphyrins. In the duplex, the porphyrin units are stacked
cofacially, and the efficient electronic communication among the arrayed porphyrin units was characterized by split redox waves
in the cyclic voltammograms. We also demonstrated the three different square-planar metal ions, namely Cu2+, Ni2+, and Pd2+,
were arranged inside the ladder-type porphyrin array in a programmable fashion.
INTRODUCTION
■
Organized assemblies of functional molecules occasionally
demonstrate chemical and physical properties that are
unpredictable from the sum of their individual molecular
components.1 In elaborate molecular systems, the properties of
individual molecules translate to the integrated properties of
organized molecules via the generation of efficient intermo-
lecular communications, which bring synergy effects.2 To
Figure 1. Representations of (a) an artificial metallo-DNA, (b) a
generate such molecular properties, multiple functional
discrete one-dimensional halogen-bridged platinum nanowire in a
molecular components are required to assemble the molecular
peptide duplex, and (c) a metalloporphyrin stacked array in a peptide
system with sequential and spatial accuracy in accordance with
duplex.
the design of the molecular assembly. Although it is generally
difficult to synthesize a precise molecular assembly composed
metallo-DNAs, the arrayed metal complexes exhibited charac-
teristic spin−spin interactions defined by the spatial arrange-
ments of metallo-base pairs.7 We have also reported the
formation of discrete nanowires of halogen-bridged platinum
complexes via duplex formation between a PtII-pendant peptide
and a PtIV-pendant peptide (Figure 1b).8 However, as related to
the heterogeneous arraying of metals inside a single scaffold,
the assembly of only limited kinds of metal ions has been
achieved, because the bioinspired strategies are based on self-
assembly between metal ions and presynthesized arrays of
metal ligands as the template strands. Hence, the development
of a novel synthetic strategy composed of the coupling of
premetalated building blocks is required to manipulate multiple
of many different molecular components, one practical
approach toward the programmed arrangement of building
units is to utilize the well-defined nanostructures of
biopolymers such as DNA and polypeptides as organizing
scaffolds.3−5 This strategy allows for the use of polymer chains
as a chemical elongation method, and predictable folding
structures such as helices, sheets, duplexes, triplexes, and higher
structures allow structural diversity. Incorporation of such
moieties ensures that the building components are precisely
arrayed in a monodisperse primary structure that can be folded
into a secondary or a tertiary structure to maintain a relative
positional, angular, or configurational relationship between the
components. For instance, both we and others have developed
artificial metallo-DNAs in which metal complexes are aligned
along the DNA helix axes inside the duplexes, via the formation
of artificial metallo-base pairs (Figure 1a).6,7 In the artificial
Received: March 22, 2014
Published: April 15, 2014
© 2014 American Chemical Society
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metal ions at a time and assemble them into a preplanned
fashion.
Scheme 1. Synthesis of the Porphyrin Trimer in the Peptide
Duplex 5, through Two-Fold Peptide Elongation Reactions
a
Porphyrins satisfy such criteria as premetalated units, and
their metal complexes are key building blocks within a wide
range of molecular materials due to their unique photophysical,
electrochemical, magnetic, and catalytic properties.9 Organized
porphyrin assemblies are also demonstrating promise in the
construction of higher functionalized systems,10 while the
arraying of multiple porphyrins has been investigated by
employing bioinspired templates and regulated stacking
interactions.11−15
We have recently reported supramolecular stacked arrays of
porphyrins and phthalocyanines connected by two- or four-fold
rotaxanes,16 demonstrating that connection of the structural
units by two or four columns is an effective way to control their
relative spatial arrangements via π−π stacking interactions. In
this paper, we describe the organized arraying of metal-
loporphyrins in a peptide duplex (Figure 1c), achieved via the
layering of various metalloporphyrins possessing two amino
acid side chains through repetitive peptide bond formation.
Accordingly, the parallel peptide duplexes are bridged by
stacked arrays of metalloporphyrins, and the cofacially stacked
porphyrins are aligned along and in the middle of the peptide
duplex.
a
Reagents and conditions: (i) DMT-MM, DBU in DMA, 17%, (ii)
TFA in CH2Cl2, 80%, (iii) monomer 1, DMT-MM, DBU in DMA,
27%.
RESULTS AND DISCUSSION
carbodiimide (DCC) was not practical, and therefore the
sterically hindered structure of 3 was expected. After
deprotection of the Boc groups by treatment with trifluoro-
acetic acid, structural elongation was carried out via a second
round of two-fold peptide formation, between [4·6H]6+·6TFA−
and 1 analogous to the preparation of 3, to afford the trimer 5
in 27% isolated yield. During the course of the repetitive
coupling reactions, the structures of the formed porphyrin
■
Molecular Design. Before the synthesis of the porphyrin
monomer containing two amino acid linker moieties, we
assessed the structure of the peptide duplex by using computer-
assisted molecular modeling. The sequential length of natural
α-amino acids was judged to be too short to allow efficient
stacking of the arrayed porphyrins. To circumvent this problem,
we designed the monomer 1 with longer artificial amino acids
in the trans meso-positions to increase the number of atoms of
the repeating unit; from the amino (N) to carboxy (C) termini
is same distance as found in a repeating DNA or peptide
nucleic acid (PNA) units, in which planar aromatic moieties are
stacked in the middle of the helical double strands (Figure 2).
1
arrays were fully characterized by H NMR spectroscopy and
ESI-TOF MS, the results of which clearly demonstrated the
stepwise elongation of the peptide duplexes (Figure S11).
NMR Study of Ladder-Type Stacked Arrays. The
stacking interactions among the porphyrin units inside the
duplex were revealed by the NMR signals of the pyrrolic NH
protons; the 1H NMR N−H resonances of the dimer 3
(observed at −3.24 and −3.25 ppm) and the trimer 4
(observed at −3.34, −3.47, and −4.22 ppm) are shifted to
high field as compared to those in the monomer 1 (observed at
−2.74 ppm) (Figure 3). Similar phenomena have been
observed for supramolecular porphyrins stacked with a
phthalocyanine unit as a result of the shielding effects of the
adjacent phthalocyanine.15 In addition, although the C2-
1
Figure 2. Structural comparison of the peptide duplexes and DNA.
symmetric structure of the dimer 3 was revealed by H NMR
spectroscopy at 140 °C in 1,1,2,2-tetrachloroethane-d2 as
shown in Figure S8, significant broadening of the signals in
the aromatic region was observed at room temperature. This is
considered to be a result of a temperature-dependent folding
via efficient π−π stacking interactions between the porphyrin
rings, and the dimer seems to form a single folding structure. In
fact, the pyrrolic NH protons showed a couple of single and
sharp signals as mentioned above. For the trimer 5, the
aromatic signals were still broadened even at elevated
temperature in 1,1,2,2-tetrachloroethane-d2 as shown in Figure
S10, indicating a more tightly folded elongated duplex
structure. Hence, the porphyrins are located in specific spatial
arrangements within the duplexes.
Stepwise Chemical Elongation of Ladder-Type Pep-
tide Arrays. The peripheral artificial amino acid was
subsequently synthesized from L-serine in a good yield and
introduced to the porphyrin center by Stille coupling to afford a
monomer porphyrin 1, as described in Scheme S1.
The synthetic procedure for the porphyrin array in a peptide
duplex is summarized in Scheme 1. Two-fold peptide bond
formation between 1 and the terminal porphyrin 2 was
successfully achieved by using 4-(4,6-dimethoxy-1,3,5-triazin-2-
yl)-4-methylmorpholinium chloride (DMT-MM) as a coupling
reagent at comparatively low concentrations of each
component in N,N-dimethylacetamide (DMA) (<0.5 mM) to
give the dimer 3 in 17% yield. In the coupling reaction, the use
of more bulky coupling reagents such as N,N′-dicyclohexyl
UV−vis and Electrochemical Studies of Ladder-Type
Stacked Arrays. The distinct stacking interactions between
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Figure 3. Partial 1H NMR spectra (600 MHz, CDCl3/TMS): (a)
porphyrin monomer 1; (b) porphyrin dimer 3; (c) porphyrin trimer 5
at 20 °C.
Figure 5. Cyclic voltammograms of (a) porphyrin monomer 1, (b)
porphyrin dimer 3, and (c) porphyrin trimer 5 in CH2Cl2 including 0.1
M nBu4NPF6 at 20 °C at a scan rate of 100 mV s−1. [Substrate] = 400
μM.
number of porphyrin units increased. These phenomena reflect
the efficient π−π electronic interaction in the closely stacked
system and the Coulombic repulsion between the oxidized
porphyrin cations.
The synthetic strategy also allows the incorporation of
heterogeneous metal arrays into the duplex. To demonstrate
the repetitive synthesis, we attempted to arrange three different
metalloporphyrins inside the stacked trimer. Figure 6 shows the
reaction scheme of the stepwise layering from the terminal PdII-
porphyrin 6 to a CuII−NiII−PdII trimer 8 along with the ESI-
TOF MS spectra snapshotted in each elongation step. Although
structural information on the trinuclear complex was not
obtained from NMR measurements because of the para-
magnetic CuII center, it was confirmed that precise assembly of
CuII-, NiII-, and PdII-porphyrins were successfully achieved
without any metal migration from the porphyrin centers during
the peptide elongation reactions, as demonstrated by the
elemental analyses and the ESI-TOF MS spectra (Figure S13).
UV−vis spectra of the NiII−PdII dimer 7 and the CuII−NiII-PdII
trimer 8 showed a slight blue shift with broadening compared
to that of their monomers similar to that when the metal-free
porphyrins were measured, indicating the stacking interactions
between the porphyrins (Figure S15).
Figure 4. UV−vis absorption spectra of the dimer 3 (red) and the
trimer 5 (blue) in comparison with S9 (a precursor of 1: the detailed
structure is shown in the Supporting Information) (black) in CH2Cl2
at 20 °C. [3] = [5] = [S9] = 1.0 μM. Inset: magnified Q-bands.
the porphyrins in the dimer 3 and the trimer 5 were also
observed in UV−vis absorption spectra (Figure 4). In the
absorption spectrum of 3, the Soret band showed a slight blue
shift with broadening compared to that of the monomer 1,
whereas four Q-bands red-shifted significantly. Similar behavior
was observed for 5. Such observations are characteristic of the
H aggregates of stacked porphyrin arrays.17
In cyclic voltammetric experiments, only one quasi-reversible
redox wave was observed at 0.46 V vs Fc/Fc+ for the monomer
1. Conversely, the redox peaks were split into two peaks for 3
(appearing at 0.39 and 0.53 V) and into three different peaks
for 5 (appearing at 0.27, 0.42, and 0.52 V) (Figures 5 and S14).
The first oxidation of a porphyrin array became easier as the
CONCLUSION
■
In summary, we have developed a repetitive stepwise synthesis
of programmable cofacially stacked porphyrin arrays in a
peptide duplex. The duplex, number, sequence, and spatial
arrangement of the porphyrins were precisely controlled and
significant electronic communications were observed between
the stacked porphyrins. Two peptide backbones connected to
the trans meso-positions of each porphyrin effectively
minimized the conformational and configurational diversity.
Since most metalloporphyrins are stable under Boc/peptide
conditions, the synthesis of diverse arrays of metalloporphyrins
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Figure 6. Synthetic procedure for the CuII−NiII−PdII trimer 8 and ESI-TOF mass spectra of (a) PdII monomer 6, (b) NiII−PdII dimer 7, and (c)
CuII−NiII−PdII trimer 8.
+ 6Et2O): C, 72.69; H, 6.81; N, 7.44. Found. C, 72.87; H, 6.44; N,
7.23 (0.37% error).
with designer sequences becomes possible. This concept,
therefore, would allow the potential development of program-
mable molecular wires, spin devices, and supramolecular
catalysts.
Synthesis of a Deprotected Porphyrin Dimer 4. Porphyrin
dimer 3 (74 mg, 36 μmol) was suspended in CH2Cl2 (2 mL). To the
mixture was added TFA (0.2 mL) at 0 °C. After stirring for 9 h, an
additional amount of TFA (0.4 mL) was added, followed by stirring
for 7 h at room temperature. The reaction mixture was added
dropwise to Et2O (400 mL) at 0 °C. The resulting precipitate was
collected by centrifugation. The resulting purple solid was purified by
reprecipitation (MeOH/Et2O) to afford the title compound 4 was
obtained as a purple solid (60 mg, 66%). 1H NMR (400 MHz,
CD3OD/TMS): δ = 8.53 (br, 8H), 8.14 (m, 10H) 7.78 (br, 8H), 7.58
(m, 12H) 7.15 (m, 8H), 7.44 (d, J = 8.2 Hz, 4H), 5.59 (br, 4H), 5.23
(br, 2H), 4.65 (m, 4H), 4.47 (m, 4H), 4.32 (m, 4H), 4.15 (m, 6H),
4.04 (m, 2H) 3.86 (m, 2H) 1.20 (m, 36H). ESI-TOF MS (positive);
m/z calcd for [M + H]+, 1831; found, 1831.
Synthesis of a Porphyrin Trimer 5. Monomer porphyrin 1 (14
mg, 11 μmol) and DMT-MM (11 mg, 39 μmol) were dissolved in
DMA (14 mL) to make solution A. The solution A was stirred at room
temperature for 1 h. Porphyrin dimer 4 (23 mg, 9.1 μmol) and DBU
(4.0 μL, 27 μmol) were dissolved in DMA (14 mL) to make solution
B. Using dropping funnels, the solutions A and B were then added
simultaneously over 50 min to a solution of DMT-MM (7.1 mg, 26
μmol) in DMA (10 mL) at 0 °C. The mixture was stirred for 16 h. The
mixture was poured into brine (300 mL), and the resulting purple
precipitate was dissolved in CH2Cl2 (200 mL). The organic layer was
washed with H2O (100 mL, twice), dried over Na2SO4, filtered, and
concentrated under reduced pressure. The crude purple solid was
purified by silica gel column chromatography (2.5 cm ϕ × 11 cm,
CH2Cl2:MeOH = 20:1, including Et3N 0.5%) and GPC (JAIGEL
2.5H-2.5H, CHCl3) and recrystallization (vapor diffusion, CHCl3/
Et2O) to afford the title compound (9.0 mg, 33%) as a reddish purple
solid. 1H NMR (400 MHz, 120 °C, 1,2-tetrachloroethane-d2/TMS): δ
= 8.21−6.21 (br, 74H), 5.63 (br, 2H), 6.59 (br, 6H), 5.13 (d, J = 5.6
Hz, 2H), 4.82 (m, 2H), 4.65 (m, 2H), 4.24−4.51 (m, 20H), 4.05 (m,
10H), 3.86 (m, 2H), 1.62 (s, 18H) 1.38 (m, 54H). ESI-TOF MS
(positive); m/z calcd for [M + H]+, 3014; found, 3014. Anal. calcd for
C410H458Cl6N36O42 (2M + 3CHCl3 + 4Et2O): C, 71.68; H, 6.44; N,
7.54. Found. C, 71.34; H, 6.10; N, 7.35 (0.34% error).
EXPERIMENTAL SECTION
■
General Procedure. Synthetic procedures were carried out under
dry nitrogen atmosphere, unless otherwise specified. All reagents and
solvents were purchased at the highest commercial quality available
and used without further purification, unless otherwise stated. A
monomer porphyrins 1, a Cu2+ complex of 1 (Cu-1), a Ni2+ complex
of 1 (Ni-1), a terminal porphyrin 2, a Pd2+ complex of 2 ([6·
2H]2+(CF3COO−)2) were synthesized according to the procedure
shown in the Supporting Information. 1H and 13C spectra were
recorded on a JEOL JNM-A600 (600 MHz for 1H; 150 MHz for 13C)
1
spectrometer or a JEOL JNM-ECS400 (400 MHz for H; 100 MHz
for 13C) spectrometer at a constant temperature of 298 K.
1
Tetramethylsilane (TMS) was used as an internal reference for H
and 13C NMR measurements in CDCl3. Elemental analyses were
performed on a Yanaco MT-6 analyzer. Silica gel column
chromatographies and thin-layer (TLC) chromatography were
performed using Merck silica gel 60 and Merck silica gel 60 (F254)
TLC plates, respectively. GPC was performed using a JAI LC-9204
equipped with JAIGEL 1H-40/2H-40 columns. ESI mass spectrometry
was performed with a Waters LCT-Premier XE Spectrometer
controlled using Masslynx software.
Synthesis of a Porphyrin Dimer 2. Monomer porphyrin 1 (317
mg, 0.26 mmol) and DMT-MM (224 mg, 0.81 mmol) were dissolved
in DMA (260 mL), and the mixture was stirred at room temperature
for 2 h (solution A). On the other hand, porphyrin 2 (283 mg, 0.22
mmol) and DBU (0.090 mL, 0.60 mmol) were dissolved in DMA (260
mL) (solution B). Using two different dropping funnels, the solutions
A and B were added simultaneously to a solution of DMT-MM (143
mg, 0.52 mmol) in DMA (200 mL) over 8 h at 0 °C. The mixture was
stirred for 10 h. The solvent was removed under reduced pressure.
The residual purple precipitate was dissolved in CH2Cl2 (300 mL).
The organic layer was washed with H2O (200 mL, twice), dried over
Na2SO4, filtered, and concentrated under reduced pressure. The crude
purple solid was purified by silicagel column chromatography (4 cm ϕ
× 10 cm, CH2Cl2:MeOH = 20:1, including Et3N 0.5%) and GPC
(JAIGEL 2.5H-2.5H, CHCl3) and crystallization (vapor diffusion,
CHCl3/Et2O) successively to afford the title compound 3 as a reddish
purple solid (89 mg, 20%). 1H NMR (400 MHz, 140 °C,
tetrachloroethane-d2/TMS): δ = 8.26 (m, 8H), 7.80 (m, 10H) 7.16
(br, 8H), 6.99 (br, 3H) 6.79 (br, 7H), 6.59 (br, 6H), 4.98 (d, J = 4.0
Hz, 2H), 4.36 (m, 10H), 4.13 (s, 4H), 3.88 (m, 8H), 1.49 (s, 18H)
1.32 (m, 36H). ESI-TOF MS (positive); m/z calcd for [M + H]+,
2030; found, 2030. Anal. calcd for C410H458Cl6N36O42 (3M + 2CHCl3
Synthesis of a Deprotected NiII−PdII Porphyrin Dimer [7·
2H]2+(CF3COO−)2. A NiII complex of Ni-1 (93 mg, 73 μmol) and
DMT-MM (60 mg, 220 μmol) was dissolved in DMA (80 mL), and
the mixture was stirred at room temperature for 2 h to make solution
A. On the other hand, a PdII porphyrin [6·2H]2+(CF3COO−)2 (86 mg,
73 μmol) and DBU (27 μL, 113 μmol) were dissolved in DMA (80
mL) to make solution B. Using two different dropping funnels, the
solutions A and B were added simultaneously to a mixture of DMT-
MM (40 mg, 140 μmol) in DMA (60 mL) over 5 h at 0 °C. The
reaction mixture was stirred for 19 h. After the solvent was removed
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under reduced pressure. The residual purple solid was purified by silica
gel column chromatography (4.5 cm ϕ × 12 cm, CH2Cl2:MeOH =
20:1 including 0.5% Et3N), GPC (JAIGEL 2.5H-3H, CHCl3, twice),
and reprecipitation (CHCl3/Et2O, CH3CN) successively to afford a
Boc-protected form of a NiII−PdII dimer as a reddish purple solid (32
ACKNOWLEDGMENTS
■
We thank Dr. Kinichi Oyama of the Chemical Instrumentation
Facility, Research Center for Materials Science, Nagoya
University for elemental analysis. This work was financially
supported by Grant-in-Aids for Scientific Research on
Innovative Areas “Coordination Programming” (area 2107,
no. 21108012) to K.T. from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan. This paper is dedicated
to Professor Renji Okazaki on occasion of his 77th birthday.
1
mg, 20%). H NMR (400 MHz, 120 °C, tetrachloroethane-d2/TMS):
δ = 8.43 (d, J = 4.8 Hz, 4H), 8.18 (d, J = 4.2 Hz, 4H), 8.11 (d, J = 4.2
Hz, 4H), 7.91−7.85 (m, 8H), 7.70 (m, 4H), 7.27−6.96 (m, 20H), 6.78
(d, J = 7.8 Hz, 4H), 5.11 (d, J = 7.8 Hz, 2H), 4.49−4.36 (m, 10H),
4.23 (s, 4H), 4.18−3.94 (m, 8H), 1.59 (s, 18H) 1.45−1.43 (m, 36H).
ESI-TOF MS (positive); m/z calcd for [M]+, 2188; found, 2188. Anal.
calcd for C257H257Cl3N24Ni2O24Pd2 (2M + CHCl3): C, 68.56; H, 5.75;
N, 7.47. Found. C, 68.45; H, 5.81; N, 7.43 (0.11% error).
REFERENCES
■
A NiII−PdII dimer (12 mg, 5.3 μmol) was suspended in CH2Cl2 (2
mL). To the mixture was added TFA (0.2 mL) at 0 °C. After stirring
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stirring for 1.5 h at room temperature. The reaction mixture was added
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centrifugation. The resulting purple solid was purified by reprecipi-
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2H]2+(CF3COO−)2 was obtained as a purple solid (9.7 mg, 83%). ESI-
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ASSOCIATED CONTENT
* Supporting Information
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(17) Asanuma, H.; Fujii, T.; Kato, T.; Kashida, H. J. Photochem.
Photobiol. C 2012, 13, 124−135.
S
Detailed synthesis for the porphyrin monomer 1 and the
terminal porphyrin 2 as well as additional experimental results.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
kentaro@chem.nagoya-u.ac.jp
■
Notes
The authors declare no competing financial interest.
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dx.doi.org/10.1021/ja502898t | J. Am. Chem. Soc. 2014, 136, 6505−6509