Covalent fixation of the cyclic tetramer of a metallo-porphyrin based on self-complementary quadruple hydrogen bonding

Article abstract of DOI:10.1246/cl.2006.1076

The stability of quadruple hydrogen bonding in the cyclic tetramer of a metallo-porphyrin before and after a covalent fixation was investigated. According to competition and energy-transfer experiments, the exchange reaction between the cyclic tetramers o

Full text of DOI:10.1246/cl.2006.1076

1076
Chemistry Letters Vol.35, No.9 (2006)
Covalent Fixation of the Cyclic Tetramer of a Metallo-porphyrin
Based on Self-complementary Quadruple Hydrogen Bonding
Haruki Ohkawa,1 Akihiro Takayama,2 Satoshi Nakajima,2 and Hiroyuki Nishideꢀ1;2
1Consolidated Research Institute for Advanced Science and Medical Care, Waseda University (ASMeW),
513 Waseda Tsurumakicho, Shinjuku-ku, Tokyo 165-0041
2Department of Applied Chemistry, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555
(Received June 26, 2006; CL-060718; E-mail: nishide@waseda.jp)
The stability of quadruple hydrogen bonding in the cyclic
of the exchange reaction of the quadruple hydrogen bonding.
The porphyrin 1, 2, and 4 were synthesized by the reaction
between the activated isocytosine and amino-substituted tetra-
phenylporphyrin. The covalent fixation of the porphyrin 2 was
performed by olefin metathesis using Grubbs’ catalyst first gen-
eration in a dilute condition (Figure 1).2b,6 By diffusion-ordered
spectroscopy (DOSY), each porphyrin derivative was reveled to
exist as a tetramer in solution.5
tetramer of a metallo-porphyrin before and after a covalent
fixation was investigated. According to competition and ener-
gy-transfer experiments, the exchange reaction between the
cyclic tetramers of the metallo-porphyrin was fully suppressed
by the covalent fixation. This spontaneous cyclization–covalent
fixation strategy based on olefin metathesis can open the way to
the use of cyclic assemblies in a solid phase.
The stability of the cyclic tetramer before and after the co-
valent fixation was examined by detecting the energy transfer
(ET) between the donor and acceptor porphyrins. It has been
shown already that ET is induced when an acceptor porphyrin
exists adjacent to a donor porphyrin via the quadruple hydrogen
bond.5 We were able to monitor the exchange reaction success-
fully by the degree of the ET between the donor and acceptor
porphyrins (Figure 2).5,7 Without the covalent fixation, a slow
exchange between the cyclic tetramer of 1(2H) and 1(Zn) was
observed (case 1, red circles), which required 6 h until equilibra-
tion was reached. Compared to the lifetime of the dimer of the
4(2H), which was determined to be 120–130 ms by exchange
spectroscopy (EXSY),4a,8 this slow exchange is direct evidence
for the cooperative quadruple hydrogen bonds in the tetramer.
Furthermore, upon the addition of the donor porphyrin 4(Zn),
the degree of ET was immediately equilibrated within 5 min
Porphyrin-based supramolecules, which rely on a noncova-
lent strategy, have been extensively studied over the past decade,
motivated by their potential functions, such as light-harvesting
antenna1,2 and hemoprotein-models.3 The noncovalent approach
to build porphyrin assemblies is quite advantageous, because it
can take advantage of the porphyrin’s preorganized structure
to be a molecular junction while avoiding the porphyrin’s
synthetic difficulties. Very recently, we have reported the spon-
taneous formation of the cyclic tetramer from the synthetically
accessible tetraphenylporphyrin derivative substituted with two
strong self-complementary hydrogen-bonding units, 2-ureido-
4[1H]-pyrimidinone (UPy).4,5 However, we could not complete-
ly exclude the possibility of the cyclic tetramer’s ring-opening
upon concentrating because of the reversibility of the quadruple
hydrogen bonding. Therefore, in regard to solid-state use of
the cyclic tetramer, we investigated its stabilization by covalent
fixation, as well as its tolerance against the ring-opening in terms
720 nm
550 nm
ET
shuffle
(case 1)
(case 2)
O
N
+
+
(slow)
ring opening
H
N
RO
RO
N
n
(fast)
H
H
O
N
N
N
N
H
OR
M
N
H
N
O
H N
N
O
599 nm
550 nm
N
N
H
C H O
4
9
M
N
H
N
N
N
(case 3)
(case 4)
“NO”
+
+
C H O OC H
4 9
4
9
O
H N
N
exchange
O
ET
no emission
4(2H)
4(Zn)
M: 2H
M: Zn
RO
RO OR
(exsit as a dimer)
R: -CH2(CH2)10CH3
M: Zn
1(Zn)
2(2H)
0
50
100
150
time / min
200
250
300
350
(CH ) CH=CH
R: -CH2
M: 2H
2 8
2
spontaneous cyclization
covalent fixation
Figure 2. Time-dependency of the fluorescence intensity emit-
ted from the acceptor zinc porphyrin. The equilibration profiles
were examined for the equimolar mixture of the donor porphyrin
(zinc) and the acceptor porphyrin (free base): (red circles) mix-
ture of the porphyrin 1(Zn) and 1(2H), (red squares) 4(Zn) and
1(2H), (blue circles) 1(Zn) and 3(2H), (blue squares) 4(Zn) and
3(2H). Donor porphyrins were illuminated at ꢀexc ¼ 550 nm,
with the constant concentration of at 3 mM in CHCl3.
Grubbs’ cat. 1st Generation
-CH2(CH2)8CH
R:
-CH2(CH2)8CH
3(2H)
M: 2H
Figure 1. Porphyrins used in this study.
Copyright ꢀ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.9 (2006)
1077
100
(a)
“NO”
exchange
+
+
80
60
40
20
0
(b)
(c)
ring opening
end-capping
n
+
immediate
exchange
+
statistical mixture
0
1
2
3
4
5
6
UPy in Porphyrin / C12-UPy molar ratio (-)
Figure 4. The percentage of the ‘‘UPy of the porphyrin’’ contri-
buting to the ‘‘porphyrin–porphyrin linkage’’ upon addition of
aliphatic UPy 5 as a competitor. The ratio was determined by
1H NMR recorded in CDCl3, at 300 K. (blue circles) mixture
of porphyrin 3(2H) and 5, (red circles) mixture of porphyrin
1(2H) and 5, (green square) statistical ratio, anticipated when
there is no selectivity or cooperativity in UPy dimerization.
The concentration of the porphyrins was constant at 10 mM.
14
13
12
11
10
9
8
7
6
chmical shift in ppm
Figure 3. 1H NMR spectra recorded in CDCl3. (a) aliphatic
UPy 5, (b) porphyrin 4(2H), (c) the equimolar mixture of the
porphyrin 4(2H) and UPy 5. The NMR signals indicated with
red circles, red squares, blue circles, and blue squares correspond
to the UPy of porphyrin in 4(2H) 4(2H), UPy of porphyrin in
.
4(2H) 5, UPy of 5 in 5 5, and UPy of 5 in 5 4(2H), respectively.
.
.
.
olefin metathesis, inter-tetramer shuffling of the porphyrin was
strongly suppressed. Our spontaneous cyclization–covalent fixa-
tion strategy can open the way to the use of cyclic assemblies in a
solid phase. Currently, the incorporation of the ‘‘porphyrin tetra-
mer’’ into the membrane is in progress.
(case 2, red squares), which was ascribed to the fast ring-opening
and the end-capping of the tetramer of 1(2H). This exchange-
ability in UPy–UPy linkage, as well as the direct detection of
the ring-opening reaction, indicates that a tetramer without cova-
lent fixation does not necessarily exist as a tetramer in a solid
state. On the contrary, neither the shuffling between tetramers
of 1(Zn) and covalently fixed 3(2H) nor the ring-opening of
3(2H) upon the addition of the dimer of 4(Zn) was observed
(case 3, blue circles and case 4, blue squares, respectively), sug-
gesting that a tetramer with covalent fixation would strongly re-
sist ring-opening upon concentrating to the solid phase.
This work was financially supported by Encouraging Devel-
opment Strategic Research Centers Program and the Special Co-
ordination Funds for Promoting Science and Technology, from
MEXT, Japan.
References and Notes
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To investigate the stability of the tetramer in more severe
conditions, we performed competition experiments, in which
N-dodecyl-2-ureido-6-(1-ethylpentyl)-4-pyrimidinone (UPy, 5)
was added to break up the tetramer. As seen in Figure 3, the
UPy moiety of the porphyrin bound to aliphatic UPy 5 was dis-
tinguishable from the one linking two porphyrins as a result of
2
1
the strong ring current effect in H NMR spectra. By using the
3
4
integral values of the NMR signals of the UPy moiety of the por-
phyrin, we determined the percentage of the UPy contributing
to the porphyrin–porphyrin linkage (Figure 4). The porphyrin
1(2H) underwent the ring-opening and end-capping by 5, despite
the existence of a trace amount of the survived tetramer, which
was indicated by the slight deviation of the plots (red circles)
over statistical values. However, for the covalently fixed tetra-
a) S. H. M. Sontjens, R. P. Sijbesma, M. H. P. van Genderen, E. W.
¨
Meijer, J. Am. Chem. Soc. 2000, 122, 7487. b) F. H. Beijer, R. P.
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H. Ohkawa, A. Takayama, S. Nakajima, H. Nishide, Org. Lett. 2006, 8,
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EXSY spectrum of the mixture of the porphyrin 4(2H) and aliphatic
UPy 5, plots to determine the lifetime, and experimental detail of
competition experiment can be seen in the Supporting Information
which is also available electronically on the CSJ-Journal Web site,
http://www.csj.jp/journals/chem-lett.
5
6
7
8
.
mer, no signals assigned as a 3(2H) 5 linkage were detected,
which supports the finding that no exchange of the quadruple hy-
drogen bonding in the tetramer of 3(2H) occurred. Moreover,
even after evaporating the solution of 3(2H) containing large ex-
cess of the competitor 5, still no exchange was observed in the
redissolved solution.8 All these results demonstrate that the ex-
change reaction of the quadruple hydrogen bond in the covalent-
ly fixed porphyrin tetramer was frozen.
In conclusion, as a result of covalent fixation based upon
Published on the web (Advance View) August 26, 2006; doi:10.1246/cl.2006.1076