The Helix to Super-Helix Transition in the Self-Assembly of π-Systems: Superseding of Molecular Chirality at Hierarchical Level

Article abstract of DOI:10.1002/anie.201707392

Higher-order super-helical structures derived from biological molecules are known to evolve through opposite coiling of the initial helical fibers, as seen in collagen protein. A similar phenomenon is observed in a π-system self-assembly of chiral oligo(p

Full text of DOI:10.1002/anie.201707392

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Accepted Article
Title: Helix to Super-Helix Transition in π-Systems Self-assembly:
Superseding of Molecular Chirality at Hierarchical Level
Authors: Mohamed Hifsudheen, Rakesh K Mishra, Balaraman
Vedhanarayanan, Vakayil K Praveen, and Ayyappanpillai
Ajayaghosh
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707392
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10.1002/anie.201707392
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Helix to Super-Helix Transition in π-Systems Self-assembly:
Superseding of Molecular Chirality at Hierarchical Level
Mohamed Hifsudheen, Rakesh K. Mishra, Balaraman Vedhanarayanan, Vakayil K. Praveen,* and
Ayyapanpillai Ajayaghosh*
synthetic helical systems.[2-6] Transfer of chirality can takes place
Abstract: Higher-order super-helical structures derived from
from (i) single molecules to one-dimensional (1D) assemblies
biological molecules are known to evolve through opposite coiling of
and then from (ii) 1D helical assemblies to macroscopic
the initial helical fibers as seen in the case of collagen protein.
superstructures. In principle, each step in this propagation
Herein we describe a similar kind of phenomenon in a π-system self-
process can determine the overall supramolecular chirality of the
assembly of chiral oligo(phenyleneethylene) derivatives (S)-1 and
system and therefore, sometimes creates ambiguity in predicting
the handedness of helical assemblies. Recently, Meijer and co-
workers have demonstrated that pathway complexity in the
nucleation-elongation process can determine the helical
handedness of 1D assemblies formed from a synthetic π-
system.[7] While the role of hierarchical processes like helix to
super-helix transformation in defining the supramolecular
chirality of assemblies formed by biopolymers is well
established,[5a,c] this aspect has been overlooked in the case of
synthetic supramolecular polymers.[5d,8] Herein, we report a
mechanistic insight into the transformation of the initial 1D
helical structures to super-helical fibers with opposite
handedness. In this hierarchical assembly process, the overall
chirality supersedes the molecular chirality, resulting in
oppositely coiled helical bundles, which can be systematically
decoiled under appropriate experimental conditions.
(R)-1 which explains the unequal formation of both left- and right-
handed helices from molecule having a specific chiral center. The
concentration and temperature-dependent circular dichroism (CD)
and UV-Vis spectroscopic studies revealed that the initial formation
of helical aggregates is in accordance with the molecular chirality.
However, at the next level of hierarchical self-assembly, coiling of
the fibers occurs with opposite handedness, thereby superseding the
command of the molecular chirality. This has been confirmed with
the help of solvent-dependent decoiling of super-helical structures
and concentration-dependent morphological analysis. While this
phenomenon is common in biological system, this is the first time to
have detailed mechanistic insight for synthetic molecules, which
explains the occurrence of both left- and right-handed helical
assemblies from the same chiral molecule.
Helical architectures constitute one of the most aesthetic and
unique systems evolved during the origin of life on earth. DNA
and proteins are some of the well-known examples of helical
biopolymers. These biopolymers often undergo transition from
lower-order structures to higher-order super-helical structures to
achieve more precision, selectivity and efficiency in their
functions.[1] For instance, coiled-coil super-helical structures of
proteins such as collagen are responsible for providing high
strength to cartilage and bone. On the other hand, in some other
cases formation of higher-order helical structures are
responsible for several neurodegenerative disorders.[1b] It is
interesting to note that in most of the cases the final helicity of
the super structures are not in agreement with the chirality of
molecular building blocks involved. Therefore, mechanistic
insights on their formation are of paramount importance.
We
have
chosen
the
amide
functionalized
phenyleneethynylene (PE) derivatives (S)-1 and (R)-1 having a
single chiral handle[9] at the center of the molecule. (S)-1 and
(R)-1 were synthesized by multistep palladium-catalyzed
Sonogashira–Hagihara cross coupling reactions (Scheme S1)
To mimic the complex biological functions involving helical
structures, chemists have been trying to replicate these
processes functionally and morphologically by developing
1
and characterized by FT-IR, H and 13C NMR spectroscopy as
M. Hifsudheen, Dr. R. K. Mishra, Dr. B. Vedhanarayanan,
Dr. V.K. Praveen, Prof. Dr. A. Ajayaghosh
Photosciences and Photonics Section, Chemical Science and
Technology Division
CSIR-National Institute for Interdisciplinary Science and Technology
(CSIR-NIIST), Thiruvananthapuram 695019 (India)
E-mail: ajayaghosh@niist.res.in
well as by high-resolution mass spectrometry (Figure S1-S3). At
a concentration of 5 × 10-6 M, the UV-Vis absorption spectra of
(S)-1 and (R)-1 in chloroform at 280 K showed a broad π-π*
absorption band at 404 nm (Figure S4a), whereas in n-decane,
the absorption spectrum was more structured with the
appearance of an additional band at 460 nm (Figure S4a). Upon
increasing the temperature from 280 to 330 K, the absorption
spectrum in n-decane lost its features and became broad similar
to that in chloroform (Figure S4b) implying that at 280 K, both
(S)-1 and (R)-1 form aggregates in n-decane, whereas in
chloroform they are dissolved.
vkpraveen@niist.res.in
M. Hifsudheen, Dr. B. Vedhanarayanan, Dr. V.K. Praveen,
Prof. Dr. A. Ajayaghosh
Academy of Scientific and Innovative Research (AcSIR)
CSIR-NIIST Campus, Thiruvananthapuram 695019 (India)
Supporting information for this article is given via a link at the end of
the document
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10.1002/anie.201707392
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Figure 1. a) CD Spectra of (S)-1 (black) and (R)-1 (gray) at concentrations of 5 × 10-6 M (upper panel) and 5 × 10-5 M (lower panel) in n-decane. b) CD spectral
intensity variations of (S)-1 and c) (R)-1 measured at λ = 460 nm upon cooling (1 K min-1) of n-decane solutions at different concentrations.
Aggregates of both (S)-1agg and (R)-1agg prepared by heating
and controlled cooling (1 K min-1) exhibited mirror image CD
spectra with opposite Cotton effects (Figure 1a and S5). For
example, at 280 K, CD spectrum of (S)-1agg (5×10-6 M) showed a
bisignate Cotton effect with a negative signal at 460 nm followed
by a positive signal at 410 nm with a zero crossing at 419 nm.
On the contrary, (R)-1agg (5×10-6 M) exhibited a positive signal at
460 nm followed by a negative signal at 410 nm (Figure 1a
upper panel). Surprisingly, the CD spectra recorded at a higher
concentration of 5 × 10-5 M displayed a complete reversal in the
chiral signature of both (S)-1agg and (R)-1agg with 3-fold
enhancement in CD spectral intensity (Figure 1a lower panel).
The observed chirality reversal at higher concentration was
studied by temperature dependent (cooling rate 1 K min-1) CD
spectral intensity variation of (S)-1agg at λ = 460 nm. Cooling
curves obtained at different concentrations of (S)-1 showed an
independent nature (Figure 1b). At a lower concentration of 5 ×
10-6 M, CD cooling curve shows a complete negative signal,
while upon increasing concentration up to 2 × 10-5 M, cooling
curves initially showed a negative trend, however, revert back to
zero. Above this critical concentration, the CD intensity for all the
cooling curves reaches to a positive value, after
concentration to 5 x 10-5 M, significant changes are observed in
ΔHe (−258.8 kJ mol−1), Te (312.1 K) and Ka (1.1 x 10-5). This
observation implies that (S)-1 self-assembles through an
enthalpically driven cooperative process involving multiple
noncovalent interactions such as π-π stacking and H-bonding
(Figure S8). The concentration dependent increase in Te shows
a linear relationship in the van’t Hoff plot (Figure S9) yielding the
standard enthalpy (ΔH°) and entropy (ΔS°) values of -101.7 kJ
mol-1 and 244.4 J mol-1 K-1, respectively. At lower concentrations,
ΔHe values show close agreement with ΔH°, while at higher
concentrations a significant deviation is noticed. The increased
negative enthalpy release observed at higher concentrations
suggests clustering[5a,10a] and coiling of helical aggregates to
super-helical structures,[5a,5d,8] which can be correlated to
inversion of CD signal with increase in intensity at higher
concentrations. Temperature-dependent heating of (1 K min-1)
aggregates in n-decane exhibited decrease in the CD spectral
intensity of (S)-1agg at λ = 460 nm from the initial positive to
negative value followed by a gradual increase to zero. The
presence of aggregates having negative CD signal in the
temperature-dependent heating and cooling experiments (Figure
passing through a negative dip. The corresponding
CD spectra also exhibited a gradual change in
Cotton effect from negative to positive upon
increasing concentration (Figure S6a). Exactly
opposite trends were observed for CD spectra and
cooling curves for (R)-1 (Figure 1c and S6b).
To probe the self-assembly pathway of (S)-1
in n-decane, the UV-Vis absorption changes at 460
nm were monitored as a function of temperature by
fixing the cooling rate 1 K min-1. The plot of degree
of aggregation (αagg) against temperature reveals
non-sigmoidal nature of cooling curves (Figure S7).
The cooling curves can be fitted with a nucleation-
elongation model characteristic of a cooperative
assembly[10] and the data are summarized in Table
S1. The curve fitting provided the elongation
enthalpy, ΔHe
=
−128.1 kJ mol−1, elongation
Figure 2. AFM with height profiles and TEM images of (S)-1agg in n-decane at a concentration
of a) and d) 5 × 10-6 M, b) and e) 2 × 10-5 M and c) and f) 5 × 10-5 M, respectively. The insets
of a and b show zoomed images. Helicity of the fibers are indicated with arrows.
temperature, Te
cooperativity, Ka
concentration of 5 x 10-6 M. Upon increasing the
=
294.1
2.2
K
and degree of
10-5 at
given
=
x
a
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10.1002/anie.201707392
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COMMUNICATION
1D M-fibers and the subsequent
depolymerization to the respective
monomer. This has been again
confirmed by probing the time-
dependent variation in CD intensity at λ
=
460 nm after injecting different
fractions of molecularly dissolved (S)-1
in chloroform (5 x 10-5 M) to a solution of
(S)-1agg in n-decane (5 x 10-5 M) at
different time scale (Figure 3b).
Dilution of (S)-1agg or (R)-1agg (5 x
10-5 M) in n-decane with the same
solvent did not show any change in CD
response when Δε is plotted against
Figure 3. a) Decoiling of (S)-1Agg (5 × 10-5 M) in n-decane induced by manually mixing different volume
fractions of (S)-1 in chloroform (5 × 10-5 M) while keeping the total concentration of the mixture as 5 × 10-5
M. b) Decoiling kinetics of (S)-1agg (5 × 10-5 M) in n-decane studied by injecting different volume fractions
of (S)-1 in chloroform (5 × 10-5 M).
concentration (Figure 4a). This observation indicates that the
super-helical fibers once formed in n-decane are quite stable
and is resistant to decoiling or disassembly upon dilution with a
nonpolar solvent. The effect of temperature on the CD intensity
variation of M-helix of (S)-1agg (5 x 10-6 M) and P-super-helix (5 ×
10-6 M) prepared by dilution of (S)-1agg (5 x 10-5 M) is shown in
Figure 4b. Heating the M-helix of (S)-1agg (5 x 10-6 M) leads to
depolymerization to monomers (Figure 4b, pink), whereas
heating the P-super helix of (S)-1agg (5 × 10-6 M) leads to
decoiling followed by depolymerization to monomers (Figure 4b,
blue). Based on the above observation, the chiral self-assembly
of (S)-1agg or (R)-1agg can be explained as shown in Figure 5.
Initially, the molecules form 1D helical fibers in agreement with
S10 and S11) indicate that the formation of helical aggregates at
higher concentration follow a thermodynamic pathway and rules
out the existence of off-pathway or metastable aggregates.[7,11]
AFM images of (S)-1agg prepared at lower concentration
(5×10-6 M) showed the formation of left-handed (M-type) helical
fibers of width ranging from 100-150 nm with helical pitch of 70-
80 nm (Figure 2a). The heights of these fibers were in the range
of 4-6 nm. At 2 x 10-5 M, coiling of fibers is observed (Figure 2b
and S12a) and at 5×10-5 M densely coiled helical bundles of
width 500-700 nm and height of 150-170 nm could be seen
(Figure 2c and S12b). The helicity of these super-helical fibers
were found to be right-handed (P-type) along with a few M-type
helical fibers. The TEM images recorded at lower (Figure 2d, 2e
and S13a) and higher concentrations (Figure 2f and S13b) of
(S)-1 also indicate the coiling of individual 1D helical fibers to
super helical fiber bundles of opposite handedness. In the TEM
image, even though we could not clearly see the initial helicity of
fibers, the final helicity of the supercoils is clear, which is in
agreement with the AFM image.
the molecular chirality, following
a
nucleation-elongation
pathway. At higher concentration, the 1D helical fibers coil to
form super-helical fiber bundles with opposite handedness.
These super helices undergo decoiling upon adding polar
solvents as a denaturant or increasing the temperature.
In summary, insights on the complex pathway associated
with helical to super-helical transformation of a synthetic π-
To rationalize the formation of super-helix with opposite
handedness, a solvent assisted[12,13] decoiling study of (S)-1agg
was performed. Careful addition of a solution of (S)-1 in
chloroform (5 x 10-5 M) to a solution of (S)-1agg in n-decane (5 x
10-5 M) at different ratios resulted in
system assembly, which follows
a
nucleation-elongation
mechanism is disclosed. The helical to super-helical
transformation observed in the present system is reminiscent of
the super-helix formation of biopolymers such as proteins. This
changes in CD spectral intensity at λ =
460 nm, which was plotted against
chloroform volume fraction (v/v),
f
(Figure 3a and S14). Even at a very
small volume fraction (f = 0.024), a
substantial decrease in CD intensity
was observed. As the chloroform ratio
was increased, the changes in CD
intensity became negative at a volume
fraction of
f = 0.111, and showed
maximum negative dip at f = 0.149.
Upon further increase in the chloroform
Figure 4. a) The plot of CD intensity in Δε (λ = 460 nm) versus concentration. The data is obtained from the
dilution experiment conducted by adding n-decane to a solution of (S)-1Agg and (R)-1Agg in n-decane (5 x
10-5 M) prepared by controlled cooling (1 K min-1). b) Change in CD spectral intensity at λ = 460 nm upon
heating M-helix of (S)-1Agg (5 × 10-6 M) in n-decane prepared by controlled cooling (¡) and P-super-helix of
(S)-1 (5 × 10-6 M) (✩) prepared by diluting (S)-1Agg (5 × 10-5 M) in n-decane with the same solvent.
fraction
(f
=
0.183),
complete
disappearance of CD signal was
observed. These observations confirm
the decoiling of the P-super helix to the
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Acknowledgements
A.A. is grateful to the DST-SERB, Govt. of India, for a J. C. Bose
National Fellowship (SB/S2/JCB-11/2014). M.H. and B.V. are
thankful to CSIR, Govt. of India, for research fellowships. R.K.M.
acknowledges, DST for an Inspire Faculty Fellowship. V.K.P.
thanks DST-SERB, for a Young Scientist Fellowship. We thank
Mr. Kiran Mohan and Dr. R. Panneerselvam (CSIR-NIIST) for
TEM and data fitting, respectively.
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Keywords: π-system • self-assembly • chirality • super-helix •
CD-inversion
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COMMUNICATION
COMMUNICATION
Insights on the evolution of chiral
super-helix of π-systems through
helicity reversal are reported.
Concentration and temperature
dependent decoiling and
M. Hifsudheen, Dr. R. K. Mishra, Dr. B.
Vedhanarayanan, Dr. V. K. Praveen,*
Prof. Dr. A. Ajayaghosh*
Page No. – Page No.
depolymerization experiments and
morphological analysis reveal that the
initial formation of the chiral
elementary fibers are controlled by
molecular chirality, whereas the
hierarchical assembly of the chiral
fibers to super-helical bundles goes to
opposite direction.
Helix to Super-Helix Transition in π-
Systems Self-assembly: Superseding
of Molecular Chirality at Hierarchical
Level
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