Hierarchy of Asymmetry at Work: Chain-Dependent Helix-to-Helix Interactions in Supramolecular Polymers

Article abstract of DOI:10.1002/chem.201706070

A detailed investigation of the hierarchy of asymmetry operating in the self-assembly of achiral (1) and chiral ((S)-2 and (R)-3) 1,3,5-triphenylbenzenetricarboxamides (TPBAs) is reported. The aggregation of these TPBAs is conditioned by the point chirality at the peripheral side chains for (S)-2 and (R)-3. An efficient helix-to-helix interaction that goes further in the organization of fibrillar bundles is experimentally detected and theoretically supported only for the achiral TPBA 1. The effective interdigitation of the achiral aliphatic side chains produces a social self-sorting to form preferentially heterochiral macromolecular aggregates.

Full text of DOI:10.1002/chem.201706070

A Journal of
Accepted Article
Title: Hierarchy of Asymmetry at Work: Chain-Dependent Helix-to-Helix
Interactions in Supramolecular Polymers
Authors: Sandra Díaz-Cabrera, Yeray Dorca, Joaquín Calbo, Juan
Aragó, Rafael Gómez, Enrique Ortí, and Luis - Sanchez
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To be cited as: Chem. Eur. J. 10.1002/chem.201706070
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Hierarchy of Asymmetry at Work: Chain-Dependent Helix-to-Helix
Interactions in Supramolecular Polymers
Sandra Díaz-Cabrera,[a] Yeray Dorca,[a] Joaquín Calbo,[b] Juan Aragó,[b] Rafael Gómez,[a] Enrique
Ortí,*[b] and Luis Sánchez*[a]
Abstract: A detailed investigation of the hierarchy of asymmetry
operating in the self-assembly of achiral (1) and chiral ((S)-2 and
(R)-3) 1,3,5-triphenylbenzenetricarboxamides (TPBAs) is reported.
The aggregation of these TPBAs is conditioned by the point chirality
at the peripheral side chains for (S)-2 and (R)-3. An efficient helix-to-
helix interaction that goes further in the organization of fibrillar
bundles has been experimentally detected and theoretically
supported only for the achiral TPBA 1. The effective interdigitation of
the achiral aliphatic side chains produces a social self-sorting to form
preferentially heterochiral macromolecular aggregates.
of 1–3 produces helical columnar stacks in which the molecular
atropisomerism is cancelled (Figure 1c). In this first level of
hierarchy, the handedness of the helical supramolecular
structures is biased by the peripheral stereogenic centers.
Importantly, the achiral or chiral nature of the peripheral side
chains of 1–3 plays a crucial role in the consecution of a
superior level of hierarchical organization. Columnar stacks of
TPBA 1 experience an efficient interdigitation through the achiral
side chains forming bundles of fibers (Figure 1d). These bundles
generate a spontaneous anisotropy spectroscopically visualized
by circular (CD) and linear dichroism (LD). In contrast, the
branched nature of chiral (S)-2 and (R)-3 impedes the
interdigitation and the tertiary level is not achieved. The results
presented highlight the hierarchy of different structural elements
(point chirality and branched nature of the peripheral side
chains) to bias the handedness of the supramolecular helical
arrangements, which further determine a tertiary organization
level through helix-to-helix interdigitation.
The structure and function of most biopolymers stem from the
interaction of their constitutive units at different levels of
hierarchy. Initially, the covalent bonding of the building blocks
produces a first level of organized backbones. These secondary,
chiral structures further interact to yield tertiary and quaternary
assemblies. To emulate the construction of such complex
structures, supramolecular polymers are being elegantly
exploited.[1,2] A number of helical supramolecular polymers in
which the self-assembling units are endowed with stereogenic
centers at peripheral side chains have been reported.[3,4] More
scarce are the examples in which different elements of
asymmetry (stereogenic centers and/or axial chirality) compete
to dictate the helicity of the polymer.[5] In all them, axial chirality
prevails with no influence of the point chirality.[6,7] At the same
time, and despite the examples describing the generation of
superhelical fibers from supramolecular polymers,[8] no clear
rules are yet established to go further into the level of hierarchy
of the different elements of asymmetry to build up helical
supramolecular polymers.
Herein, we report on the self-assembling features of a series
of 1,3,5-triphenylbenzenetricarboxamides (TPBAs) endowed
with achiral (1) or chiral ((S)-2 and (R)-3) paraffinic side chains
(Figure 1a). At the molecular level, the restricted rotation of the
benzamide units would afford a propeller-like configuration and,
consequently, the generation of the M and P atropisomers
(Figure 1b).[9] Experimental and theoretical evidences
demonstrate that the cooperative supramolecular polymerization
Figure 1. a) Chemical structure of TPBAs 1, (S)-2 and (R)-3. b) Minimum-
energy structure calculated for the two possible atropisomers of TPBA. c) Left-
handed helical columnar arrangement of a decamer of the TPBA model
system. d) Illustration of a heterochiral bundle of two helices of 1 formed by
the interdigitation of the achiral side chains.
[a]
[b]
S. Díaz-Cabrera, Y. Dorca, Dr R. Gómez, Prof. L. Sánchez
Departamento de Química Orgánica I
Facultad de Ciencias Químicas, UCM
Ciudad Universitaria, s/n, 28040, Madrid (Spain)
E-mail: lusamar@quim.ucm.es
Dr. J. Calbo, Dr. J. Aragó, Prof. E. Ortí
Instituto de Ciencia Molecular
Universidad de Valencia, 46980 Paterna, Spain.
E-mail: enrique.orti@uv.es
The preparation of TPBAs 1–3 requires a simple multistep
protocol, in which
a Suzuki cross-coupling between the
corresponding 4-(alkylcarbamoyl)phenyl boronic acid and 1,3,5-
tribromobenzene is the key step (Scheme S1). All the newly
reported compounds were fully characterized by NMR and FTIR
Supporting information for this article is given via a link at the end of
the document
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spectroscopy as well as by high-resolution mass spectrometry
(see the Supporting Information).
To derive the thermodynamic parameters associated to the
supramolecular polymerization of 1–3, variable-temperature UV-
Vis (VT-UV-Vis) experiments were performed. A 10 µM solution
maximize the H-bonding interactions, the amide groups are
twisted 36º giving rise to N−H···O=C distances of 2.198 Å, the
adjacent molecules in the stack being separated by 3.190 Å.
This arrangement favours the planarization of the
triphenylbenzene moiety (dihedral angles < 15º).
of 1 in
a
mixture of methylcyclohexane/1,2-dichloroethane
Theoretical calculations substantiate and give further insight
into the cooperative supramolecular polymerization mechanism
(MCH/DCE) 95/5 at 20 ºC shows a broad absorption centered at
270 nm. This band increases in intensity and slightly shifts
bathochromically (272 nm) upon heating the solution to 80ºC
(Figure S1). The V-shaped cooling curve (Figure S2a), that
of 1. The calculated interaction energy per monomer unit (Eint,n
)
in a columnar stack of the TPBA model system corroborates the
cooperative character of the self-assembly of 1. Plotting Eint,n
values against the number of monomeric units results in a
hyperbolic curve in which the Eint,n grows rapidly until reaching
the asymptotic limit (–211 kJ/mol) at around n = 10 units (Figure
S6a). The asymptotic behaviour of Eint,n is diagnostic of an initial
nucleation process, in which the stability of the aggregate rapidly
increases with n, followed by an elongation process, in which the
incorporation of new monomeric units has no additional
effect.[4c,15] The cooperative self-assembly of 1 is also supported
by the increase of the dipole moment per monomer unit (DMn)
calculated for (TPBA)n aggregates (Figure S6b). The exponential
growth of DMn along the stacking direction implies the
enhancement of the polarization of the H-bonding network
during the nucleation.[16]
could be indicative of
a
hierarchical supramolecular
polymerization recently described for referable systems,[10,11]
cannot be fitted neither to a cooperative nor to an isodesmic
model.[2,12] Only by decreasing the concentration to a very low
value (5 µM) it is possible to achieve a non-sigmoidal curve
characteristic of a cooperative supramolecular polymerization
(Figure S2b).
Similarly to related tricarboxamides,[4d,4e,13,14] the self-
assembly of 1 proceeds through the formation of a triple array of
hydrogen bonds between the amide groups together with the π-
stacking of the aromatic as confirmed from concentration-
dependent 1H NMR and FTIR experiments. The former show
that the aromatic protons shield and the amide protons shift
downfield as concentration increases (Figure S3). In the latter,
the N−H and Amide I (C=O) stretching bands as well as the
Amide II (C−N) bending band recorded at 3287, 1623, and 1532
VT-UV-Vis experiments performed with (S)-2 and (R)-3
using the same experimental conditions (MCH/DCE, 95/5)
allows deriving all the thermodynamic parameters associated to
the supramolecular polymerization of these two chiral TPBAs.
cm−1, respectively, indicate the formation of
a three-fold
intermolecular hydrogen-bonding array (Figure S4).[4d,4e,13,14]
Density functional theory (DFT) calculations at the B3LYP-D3/6-
31G** level for a TPBA model, in which the long alkyl chains are
replaced by methyl groups to reduce the computational cost
Plotting the variation of the molar fraction of aggregates, aaggr,
against temperature results in non-sigmoidal curves, diagnostic
of a cooperative mechanism (Figure S7). A complete set of
thermodynamic parameters can be derived by fitting these
curves to the equilibrium (EQ) model (Table S1).[12] All the
thermodynamic parameters are very similar to those reported
previously for referable tricarboxamides showing a high degree
of cooperativity (s ~ 10-6).[14]
(see
the
Supporting
Information),
corroborate
this
supramolecular organization. The TPBA molecules form
a
helical columnar stack in which the monomeric units are one on
top of each other rotated by 31º (Figure 1c and S5). To
Figure 2. Theoretical simulation of the CD spectra calculated for the P and M monomers (a) and the left- and right-handed dimers (b) of model TPBA; c)
Experimental CD spectra of (S)-2 and (R)-3 (MCH/DCE, 95/5, 10 µM, 298 K).
To disentangle the chiroptical features of the supramolecular
assemblies formed by the reported TPBAs, a computational
study was performed under the DFT approach (see the
Supporting Information). The isolated monomer of model TPBA
benzamide units are rotated 37º clockwise to yield a propeller-
like geometry (Figure 1b and S5a). The rotation in
counterclockwise manner (–37º) results in an isoenergetic
mirror-image analogue. The theoretical CD spectra calculated
for these two atropisomers show a complex bisignate pattern
a
displays
a
minimum-energy C3 geometry in which the
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with positive and negative peaks at 304, 275, 249, 207 and 193
nm (Figure 2a). However, the CD spectra computed for M- and
P-type dimers of model TPBA are simpler with bisignate signals
peaking at 309 and 268 nm and a zero-crossing point at 290 nm
(Figure 2b). These bisignate CD bands originate from high-
energy electronic excitations involving the aromatic TPBA core
(Figure S8), and their sign is dictated by the handedness of the
dimer.
The experimental CD spectra of (S)-2 and (R)-3 show a
clear bisignate Cotton effect with maxima at 281 and 258 nm
and a zero-crossing point at 270 nm (Figure 2c and S9a). By
comparison with the simulated CD spectra of the dimeric
species, the self-assembly of (S)-2 and (R)-3 can be
unequivocally assigned to right- and left-handed helical
structures, respectively, in good agreement with previous results
for related tricarboxamides.[4c,4d,13] Note that the dichroic pattern
of (S)-2 and (R)-3 is much simpler than that predicted for the
TPBA monomer (Figure 2a) due to the planarization
experienced by the benzenes around the central ring upon self-
assembly (Figure 1c and S5). This planarization cancels the
molecular atropisomerism switching the hierarchy of asymmetry.
In this case, and unlike previous examples in which
atropisomerism prevails over point chirality,[5-7] the chiral
information embedded in the stereogenic centers of the side
concentration (cT). Surprisingly, the addition of the sergeant (S)-
2 results in dichroism spectra of different shape and intensity for
the different mixtures (Figure S10). These findings could be
indicative of a strong trend of 1 to form fibrillar structures that
bundle into thick filaments with a concomitant macroscopic
alignment of such filaments.[17] To document this macroscopic
alignment and the subsequent anisotropy generated in the
cuvette, the CD spectra of 1 was registered in MCH/DCE 95/5 at
cT = 10 µM. The CD spectrum, that resembles to that obtained
for (R)-3, slightly increases upon rotary stirring (Figure 3a). In
addition, heating this solution at 80 ºC followed by a rapid
cooling on an ice bath results in a mirror-like CD spectrum of
higher intensity (Figure S11). This inversion of the helicity could
be indicative of a pathway complexity in the self-assembly of the
sample.[18] However, this is not the case because the CD
spectrum is strongly contaminated by a LD response that
decreases upon rotary stirring (Figure 3a, inset). This
unexpected dichroic behaviour of 1 is comparable to that
observed previously for some other achiral self-assembling units
and is attributed to the strong trend to form fibillar structures and
also to the chiral alignment of the fibers in solution as a
consequence of convective flows in the measuring cuvette.[17,19]
Interestingly, this is not the case of chiral (S)-2 and (R)-3 that do
not present any LD contamination of the CD spectra (Figure
chains biases the handedness of the helical stacks in our TPBAs. S9b). The fibrillar nature of the aggregates formed from 1 was
The thermodynamic characterization of the supramolecular
polymerization of TPBAs 1–3 was undertaken by performing
experiments of amplification of chirality.[3] Sergeants-and-
soldiers experiments were first carried out by adding increasing
amounts of chiral (S)-2 into achiral 1 keeping constant the total
visualized by atomic force microscopy (AFM) on mica as surface.
The AFM images show a dense network of thick filaments (tens
of nanometers) forming rope-like structures constituted by the
aggregation of thinner fibrils (Figure 3b and S12).
Figure 3. a) CD spectra of TBPA 1 (10 µM; MCH/DCE, 95/5, 20 ºC). The inset shows the corresponding LD spectra; b) Phase AFM image of the fibrillar
structures of 1 onto mica (z scale = 65 nm); c) Changes in CD intensity as a function of e.e. upon adding (S)-2 to a solution of (R)-3 (MCH, 293 K, c = 25 µM); e.e.
= 1 and e.e. = –1 correspond to pure (S)-2 and (R)-3, respectively. The inset shows the changes in the corresponding CD spectra.
Majority rules experiments, performed by adding dissimilar
system upon incorporating a monomer into an aggregate of its
amounts of the (S)-2 and (R)-3 enantiomers keeping cT constant, unpreferred helicity. The DHmm value is smaller to that derived
were
carried
out
to
complete
the
thermodynamic
for related C3-symmetric tricarboxamides,[12.13] and gives rise to
an efficient amplification of chirality: with only 22% of
enantiomeric excess (e.e.), the chirality of the mixture is the
same as that observed for pristine (S)-2 or (R)-3.
Finally, we attempted to rationalize the helix-to-helix
interactions present in the fibrillar bundles observed
characterization of the supramolecular polymerization of (S)-2
and (R)-3. The non-linear variation of the dichroic response was
fitted to the two-components EQ model to extract the mismatch
enthalpy (DHmm).[12] This energetic term, calculated to be −1.00
kJ/mol (Table S1), indicates the penalization undergone by the
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experimentally for 1 by using theoretical simulations. The
geometry of TPBAs 1 and (S)-2 was obtained from DFT
calculations (Figure S13). From these monomeric units, helical
icosamers were constructed and optimized at the molecular
mechanics level for both 1 and (S)-2 (Figure S14). The rotation
contacts, and a pleasant correlation between the intercolumnar
contact area (ICA) and the Eint,col values is obtained (Table S2
and Figure S19). The complementary supramolecular pockets
calculated for the social self-sorting of 1 result in efficient helix-
to-helix interactions that justify the macroscopic alignment of
supramolecular columns in fibers. This is not the case of chiral
(S)-2 in which the helix-to-helix interaction is much weaker.
To summarize, we report on the self-assembling features of
angle (ca. 30º) between adjacent monomers results in
a
supramolecular cavity suitable to induce efficient van der Waals
interactions between the aliphatic side chains of vicinal
aggregates. Particularly interesting is the case of 1 in which two
different helix-to-helix interactions can be postulated considering
that this achiral monomeric unit yields a racemic mixture of both
right- and left-handed helical structures. The first of these two
possibilities implies a self-recognition or narcissistic self-sorting
phenomenon in which the two enantiomeric structures interact
with those of the same species to form homochiral
superstructures (Figure 4a). The second possibility implies a
self-discrimination or social self-sorting in which P- and M-type
helices interact to form a heterochiral aggregate (Figure 4b).[20]
For chiral (S)-2, the transference of the point chirality embedded
in the side chains provokes the formation of P-type helices, and
only a narcissistic self-sorting is possible through homochiral
helix-to-helix interactions (Figure 4c).
a
new series of C3-symmetric TPBAs 1–3. A detailed
experimental and theoretical study of the hierarchy of
asymmetry elements operating in the formation of right- or left-
handed supramolecular polymers is provided. The restricted
rotation of the lateral benzamide units yields a mixture of P- and
M-type atropisomers in the molecularly dissolved state. The
supramolecular polymerization of these units by H-bonding and
π-stacking interactions cancels this atropisomerism, and the
next level of hierarchy is biased by the point chirality of the
peripheral side chains. An efficient helix-to-helix contact is for
the achiral TPBA 1 to go further in the macromolecular
organization of tertiary superstructures. Theoretical calculations
demonstrate that the efficient interdigitation of the achiral
aliphatic chains affords a social self-sorting effect to form
preferentially heterochiral bundles. The results presented herein
contribute to shed light on the rules governing the transformation
of secondary into tertiary supra- and macromolecular structures
and take a step forward in understanding the hierarchy of the
elements of asymmetry to bias the handedness of the resulting
helical supramolecular polymer.
Acknowledgements
Financial support by the MINECO of Spain (CTQ2014-53046-P,
CTQ2015-71154-P, and Unidad de Excelencia María de Maeztu
MDM-2015-0538),
the
Generalitat
Comunidad
Valenciana
de Madrid
(PROMETEO/2016/135),
the
(NanoBIOSOMA, S2013/MIT-2807), and European FEDER
funds (CTQ2015-71154-P) is acknowledged. J.C. is grateful to
the Generalitat Valenciana for
a post-doctoral fellowship
Figure 4. Top and side views of density maps of homochiral (a) and
heterochiral (b) helix-to-helix interactions in 1 and homochiral helix-to-helix
interaction in (S)-2 (c). The distance between the two column centers is
indicated in the top view.
(APOSTD/2017/081). J.A. is acknowledged to the MINECO for a
“JdC-Incorporación” Fellowship.
Keywords: supramolecular polymers • amplification of chirality •
helicity • self-sorting • theoretical calculations
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The self-assembly of 1,3,5-
triphenylbenzenetricarboxamides
(TPBAs) is reported. The helical
outcome is conditioned by the
point chirality with an efficient
helix-to-helix interaction that
goes further in the organization
of fibrillar bundles. The effective
interdigitation of the achiral
aliphatic side chains produces a
social self-sorting to form
S. Díaz-Cabrera Y. Dorca, J. Calbo J. Aragó, R. Gómez, E.
Ortí,* and L. Sánchez*
Page No. – Page No.
Hierarchy of Asymmetry at Work: Chain-Dependent Helix-
to-Helix Interactions in Supramolecular Polymers
preferentially heterochiral
macromolecular aggregates
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