Helicity induction in hydrogen-bonding-driven zinc porphyrin foldamers by chiral C60-incorporating histidines

Article abstract of DOI:10.1002/anie.200502465

(Chemical Equation Presented) High chiral amplification is exhibited by a new series of zinc porphyrin appended, hydrogen-bonded foldamers on binding with C₆₀-incorporating chiral histidines as a result of the cooperation of two discrete noncovalent interactions: zinc porphyrin/ imidazole coordination and zinc porphyrin/C₆ π-π stacking (see picture).

Full text of DOI:10.1002/anie.200502465

Communications
Foldamers
DOI: 10.1002/anie.200502465
Helicity Induction in Hydrogen-Bonding-Driven
Zinc Porphyrin Foldamers by Chiral
C60-Incorporating Histidines**
Jun-Li Hou, Hui-Ping Yi, Xue-Bin Shao, Chuang Li,
Zong-Quan Wu, Xi-Kui Jiang, Li-Zhu Wu, Chen-
Ho Tung, and Zhan-Ting Li*
Helical biomacromolecules such as DNA and polypeptides or
proteins play important roles in living systems. Extensive
efforts have been devoted to helicity induction in unnatural
oligomers, polymers, and dendrimers to explore new proper-
ties and functions.[1–3] In many cases, chiral amplification has
been observed, which has found applications in molecular
recognition,[4] light harvesting,[5] and catalysis.[6] We are
motivated to explore new applications of hydrogen-bond-
ing-driven aromatic oligoamide foldamers in chiral recogni-
tion.[7–9] Herein we report a new efficient strategy for helicity
induction based on porphyrin-appended hydrogen-bonded
foldamers and chiral C60-incorporating histidines.
It is well established that zinc porphyrin is coordinated by
N ligands such as pyridine and imidazole,[10,11] and also
spontaneously attracted to C60 by p–p interaction.[12] Since
zinc porphyrin is five-coordinate, we envisioned that ration-
ally designed zinc porphyrin-appended foldamers might
complex N ligand–C60 adducts through cooperation of the
two discrete noncovalent interactions. Therefore, receptors 1–
4 were synthesized and characterized. Previously,[13] we
demonstrated that the repeating oligoamide skeletons in 2–
4 adopt rigid, planar folding conformations due to intra-
molecular three-centered hydrogen bonding.[14] Indeed, the
1H NMR spectra of 2, 3, and 4 in CDCl3 revealed that all the
signals of their NH protons appear in the downfield region,
indicating the existence of intramolecular hydrogen binding
and folding conformation for these molecules. The appended
[*] J.-L. Hou, H.-P. Yi, Dr. X.-B. Shao, C. Li, Z.-Q. Wu, Prof. X.-K. Jiang,
Prof. Z.-T. Li
State Key Laboratory of Bio-Organic and Natural Products
Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Lu, Shanghai 200032 (China)
Fax: (+86)21-6416-6128
E-mail: ztli@mail.sioc.ac.cn
Prof. L.-Z. Wu, Prof. C.-H. Tung
Technical Institute of Physics and Chemistry
Chinese Academy of Science
Beijing 100101 (China)
[**] We thank the Ministry of Science and Technology (G2000078101),
the National Natural Science Foundation (20321202, 90206005,
20372080, 20332040, 20425208) of China, and the Chinese Acad-
emy of Sciences for financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
zinc porphyrin units should adopt a conformation that is on
average perpendicular to the folding skeleton.[4a] Such an
arrangement would facilitate cooperative, “domino” interac-
tion with structurally matching C60–ligand adducts. Receptors
1–4 have good solubility in CHCl3, and their absorption and
1H NMR spectra in this solvent showed no evidence of
intermolecular aggregation of the oligoamide skeletons or
inter- or intramolecular aggregation of the porphyrin units.
With 1–4 in hand, guests 5–7 were synthesized. Upon
addition of 5–7, the UV/Vis spectra of 1–4 in CHCl3 changed
substantially, indicative of strong complexation. The UV/Vis
titration results of 2, 3, and 4 with (R)-7 are presented in
Figure 1. The lmax of the Soret and Q bands of 4 were red-
shifted from 427.6 and 552.4 nm to 436.8 and 556.8 nm,
respectively, with tight isosbestic points at 433.1and 560.0 nm.
In contrast, the shorter oligomers 2 and 3 exhibited smaller
red shifts. Interestingly, a clear isosbestic point is observed for
the Soret band in the UV/Vis spectra of all complexes. This
result may reflect that the zinc porphyrin units in the
foldamers have very similar binding affinities due to their
Figure 1. Absorption spectral changes of a) 2 (3.6”10À6 m), b) 3
(2.4”10À6 m), and c) 4 (1.2”10À6 m) on titration with (R)-7 (0–
7.2”10À4 m) in CHCl3 at 258C.
ordered arrangement. The apparent association constants Ka
between the zinc porphyrin unit of the receptors and 5–9 in
CHCl3 were determined on the basis of the titration experi-
ments (Table 1).[15,16]
For a given guest, the Ka value increases pronouncedly
with elongation of the foldamer. This increased binding
affinity is clearly the result of increased p–p stacking between
zinc porphyrin and C60.[12,17] This increasing p–p interaction in
the longer oligomers may be attributed to the enhanced
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Table 1: Apparent association constants Ka [mÀ1] for the complexes of the zinc porphyrin unit of
receptors 1–4 with guests 5–9 in CHCl3 at 258C, measured by UV/Vis titration.[a]
ciation constants of the complexes of 4 with
5–7 were also determined (Table 1), which
are quite comparable with those evaluated
by UV/Vis titration. As an example, the
titration spectra of 4 with (R)-7 are shown in
Figure 2.
Chirality amplification was then inves-
tigated for the new porphyrin-appended
foldamers. When (R)-7 or (S)-7 was added
to solutions of 2–4 in CHCl3, mirror-image
Cotton effects resulted (Figure 3, the CD
1:5
3.8”104
1.4”105
2.8”105
2.0”105
2.1”105
1:6
3.7”104
1.6”105
1.6”105
1.5”106
1.7”106
1:(R)-7
2:(R)-7
3:(R)-7
4:(R)-7
4:(R)-7[b]
5.9”103
1.1”104
2.1”104
3.5”104
2.3”104
2:5
3:5
4:5
4:5[b]
2:6
3:6
4:6
4:6[b]
2:8
3:8
4:8
8.7”103
8.5”103
1.1”104
2:(R)-9
3:(R)-9
4:(R)-9
4:(S)-7
9.7”102
3.4”103
5.4”103
3.6”104
[a] With error of ꢁ15%. [b] Determined by fluorescence titration method (excitation wavelength
559 nm, emission wavelength 600 nm).
spatial matching between the C60 and porphyrin units due to
warping of the porphyrin units in one direction. The larger 6
and 7 exhibit even greater binding ability than their C60-free
counterparts 8 and 9, and this clearly shows the important
contribution of p–p stacking to complexation. Although
imidazole is a stronger ligand to zinc porphyrin than
pyridine,[11] 7 has lower binding capability than both 5 and
6, which may be rationalized by considering the greater
structural flexibility of 7.
spectrum of the guest has been subtracted). In contrast, 1 is
completely silent under identical conditions. The second
Cotton effect is noticeably stronger than the first and
increased substantially with elongation of the oligomers.
The maximum CD amplitude De is 162, 1098, and
The stoichiometry of the complexes was also estimated.
For example, the 1H NMR spectra for [2]/[5] ([5] = 11 mm) in
CDCl3 revealed a clear inflexion point for the change in the
chemical shift of Ha of 5 at [2]/[5] = 0.5. This value indicates
that these two compounds form a stable complex with 1:2
stoichiometry. Due to signal overlap, association constants
1
could not be obtained from the H NMR titration experi-
ments. The Job plot for the 4:7 system, based on the UV/Vis
experiments, revealed maximum change in absorbance at [4]/
([4] + [(R)-7]) ꢀ 0.17, which corresponds to 1:5 stoichiometry
and is smaller than the value of 0.14 expected for 1:6
stoichiometry. This result may indicate that with both com-
pounds being at low concentration for the UV/Vis measure-
ment, the maximum stoichiometry was not achieved. Never-
theless, Job plots revealed a 1:6 stoichiometry for the
complexes of 4 with 5 and 6 due to increased binding affinity
(see Supporting Information).
Fluorescence studies revealed that the emissions of the
zinc porphyrin units of 1–4 could be quenched significantly by
the guests.[18] The quenching data for their fluorescent
emission at 600 nm (excitation wavelength: 559 nm at the
isosbestic point of the Q band) is presented in Table 2.
Quenching efficiency increases remarkably with elongation of
the oligomers. This result is consistent with the above UV/Vis
study and also supports the existence of cooperativity for the
complexation of the oligomers with the guests. Based on the
fluorescence quenching experiments,[17a,19] the apparent asso-
Figure 2. Fluorescence spectral changes of 4 (1.2”10À6 m) on addition
of (R)-7 (0–1.3”10À4 m) in CHCl3 at 258C. Inset: plot of emission
intensity of 4 at 600 nm versus [(R)-7].
2346mÀ1 cmÀ1 for 2, 3, and 4, respectively, and the ability of
4 for chiral amplification is about 14.5 and 2.1 times as large as
those of 2 and 3. Because the total concentration (20 mm) of
porphyrin units is identical for all oligomers, the results
clearly indicate a remarkably large cooperativity in 4 for
transferring the chirality of the histidine unit to the whole
supramolecular architecture. In contrast, the chiral induction
of C60-free 9 is much smaller (Figure 4). Compared to those of
2 and 3, the CD signal of 4 in the region of the porphyrin Soret
band is also shifted remarkably on complexation with 7. The
CD band between 500 and 580 nm in Figure 3 should be
produced from the chiral C60 unit of 7, since similar signals
were not observed when 7 was replaced by
C60-free 9 (Figure 4). Interestingly, another
strong Cotton effect was also displayed
Table 2: Fluorescent quenching data of receptors 1–4 by adducts 5–7 in CHCl3 at 258C.
I0
I[b] [(I0ÀI)/I0][c]
[5] [m][d]
[6] [m][d]
[(R)-7] [m][d]
[a]
between 580 and 640 nm in the assembly of
4 and 7,[20] which is not observed in the
5
6
(R)-7
assembly of 2 or 3. The chiral amplification,
[e]
[e]
[e]
1
2
3
4
179
213
214
210
173 (0.04)
129 (0.39)
117 (0.45)
51 (0.76)
161 (0.10)
107 (0.50)
90 (0.58)
21 (0.90)
163 (0.09)
195 (0.08)
188 (0.12)
174 (0.17)
–
–
–
1.2”10À5
8.2”10À6
3.6”10À6
8.6”10À6
5.7”10À6
2.9”10À6
–
[e]
signal shifting, and new additional Cotton
effect in the region of long wavelength
displayed by the longer foldamers on com-
plexation with the chiral C60 derivatives may
be explained by considering that the
increased binding affinity of the chiral
3.7”10À5
3.0”10À5
[a] Values of pure receptors at [porphyrin]=7.2”10À6 m. [b] Values when 1 equiv of guest (7.2”10À6 m)
was added. [c] Quenching efficiency. [d] Guest concentration at [I0ÀI]/I0 =0.5. [e] [I0ÀI]/I0 >0.5 in the
concentration range investigated.
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guest to the longer receptors would lead to more ordered
chiral assembling architecture in which all the porphyrin and
60 units are warped in one direction. Based on these spectral
C
investigations, we propose that for the complexation of the
porphyrin-appended foldamer receptors, a clockwise or
anticlockwise one-direction binding mode should be of
lowest energy. Figure 5 presents a plausible binding mode
for the system of 4 with (R)-7.
Chiral induction or amplification of nonnatural oligomers
and polymers has been the subject of intensive research in
recent years.[4,20] In this work, we demonstrated that folda-
mers are suitable skeletons for achieving this target. In
principle, the chiral-recognition subunits can also be incorpo-
rated into one folding skeleton, which would lead to increased
chiral amplification, while chiral induction based on rigid,
unfolding architectures might also exhibit new interesting
features.
Figure 3. CD spectra of 4 (3.3”10À6 m) in the presence of (R)-7 (a)
and (S)-7 (f), 3 (6.7”10À6 m) in the presence of (R)-7 (b) and (S)-7 (e),
and 2 (1.0”10À5 m) in the presence of (R)-7 (c) and (S)-7 (d) in CHCl3
at 258C ([(R)-7]=[(S)-7]=5.0”10À3 m).
Received: July 14, 2005
Revised: September 7, 2005
Published online: December 15, 2005
Keywords: asymmetric amplification · foldamers · fullerenes ·
.
hydrogen bonds · porphyrins
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