Chiral transcription and retentive helical memory: Probing peptide auxiliaries appended with naphthalenediimides for their one-dimensional molecular organization

Article abstract of DOI:10.1002/chem.201200197

Right or left paradox: Homochiral, heterochiral and achiral peptide auxiliaries appended with naphthalenediimide (NDI, see figure) were employed to demonstrate chiral transcription. We report an interesting phenomenon coined as retentive helical memory. R

Full text of DOI:10.1002/chem.201200197

DOI: 10.1002/chem.201200197
Chiral Transcription and Retentive Helical Memory: Probing Peptide
Auxiliaries Appended with Naphthalenediimides for Their One-Dimensional
Molecular Organization
M. Pandeeswar, M. B. Avinash, and T. Govindaraju*[a]
The single-handedness of biomolecular building blocks of
life as in l-amino acids and d-sugars has been one of the
greatest mysteries of the biotic world.[1] The biological mac-
romolecules evolved from these chiral building blocks, such
as DNA double helix and secondary a-helix of proteins, are
exclusively homochiral. The origin of this homochirality has
evoked enormous interest in the scientific community for
several decades.[2,3] In this regard, pioneering work on syn-
thetic covalent polymers by Green and co-workers led to
the phenomenon of amplification of chirality.[4] The nonco-
valent interactions that drive supramolecular polymers are
of interest as they offer modularity, tunability and reversibil-
ity. The Meijer and Reinhoudt groups independently devel-
oped various noncovalent systems to construct elegant chiral
supramolecular assemblies.[5,6] Since then, supramolecular
chirality has gained unprecedented importance from the
fundamental science perspective and for practical applica-
tions in areas such as asymmetric catalysis, determination of
absolute configuration, sensors, optoelectronics, materials
science and biomedicine.[7,2f] Benefits of introducing chirali-
ty into synthetic covalent polymeric structures and the ef-
fects on function have been well documented.[8] However,
supramolecular chirality is still in its infancy and will have
a profound role in the development of fast emerging dynam-
ic supramolecular polymers.
In particular, dedicated efforts towards chirogenesis and
chiral enhancement have significantly influenced the struc-
tural and functional evolution of nano- and macroscale
supramolecular polymeric materials through weak noncova-
lent-interaction-driven molecular self-assembly. However,
the use of modular biomolecules as chiral auxiliaries, either
freely or in conjugation with chromophores, is largely un-
known and an unexplored area.[9] Thus, the study of chiral
transcription and enhancement in the design of chiral supra-
molecular polymers could enable the production of novel
hybrid materials with interesting functions as alternative to
covalent polymers. These observations inspired us to design
homochiral, heterochiral and achiral peptide auxiliaries ap-
pended with naphthalenediimides (NDIs) as shown in
Scheme 1 (see the Supporting Information for synthesis and
Scheme 1. Molecular structures of homochiral (ll and dd), heterochiral
(ld and dl) and achiral (AA) peptide conjugates of NDI.
characterization details). Homochiral peptides appended
with NDIs (ll and dd) comprised of identical l or d stereo-
chemistry for the two a-phenylalanine units. Heterochiral
peptides with appended NDIs (ld and dl) comprised of al-
ternative l and d or d and l stereochemistry for the two a-
phenylalanine units. The dipeptide auxiliary with two gly-
cine units formulates the achiral peptide appended NDI
(AA). The choice of phenylalanine was due to its favorable
steric characteristics, which is expected to play a major role
in the stereospecific orientation of supramolecular building
units and to least sterically hinder conformation, and was
partly inspired by the interesting results reported earlier.[10]
Organic chromophores were employed as an additional
optical probe to study the process of chiral supramolecular
assemblies. Herein we have employed NDI (n-type semicon-
ductor) due to its versatile properties and potential applica-
tions ranging from electronics to biomedicine.[11–13] NDI
[a] M. Pandeeswar, M. B. Avinash, Dr. T. Govindaraju
Bioorganic Chemistry Laboratory, New Chemistry Unit
Jawaharlal Nehru Center for Advanced Scientific Research
Jakkur, Bangalore 560064 (India)
Fax : (+91)80-2208-2627
E-mail: tgraju@jncasr.ac.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200197.
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Chem. Eur. J. 2012, 18, 4818 – 4822

COMMUNICATION
chromophore not only serves as an optical probe but also fa-
cilitates a planar central aromatic core structure, and com-
pliments the self-assembly characteristics of the peptide con-
jugates. Here, we report the induction of supramolecular
chirality to achiral NDI with phenylalanine peptide auxilia-
ries. Chiral enhancement is demonstrated by using ser-
geants-and-soldiers and majority-rules experiments. Further-
more, we show that in case of the heterochiral peptide con-
jugates (ld and dl) the chirality of the first stereocenter (ir-
respective of the stereochemistry of second stereocenter)
adjacent to the NDI core determines the supramolecular
helicity; this is termed as retentive helical memory. Remark-
ably, morphological studies reveal that the homochiral ll
and dd self-organize into 1D hierarchical supramolecular
polymers with opposite helicity. Interestingly, the heterochi-
ral peptide conjugates ld and dl form microspheres. The
chiral enhancement experiments and the propensity of ll
and dd conjugates to form 1D architectures, in principle can
support the mechanism of both spontaneous deracemization
and chiral enhancement pathways for biological homochira-
lity.[10b]
The molecular aggregation behavior of peptide conjugates
ll, dd, ld and dl were explored through absorption and
fluorescence spectroscopic studies. All the NDI–peptide
conjugates in dimethylsulfoxide (DMSO) exhibited strong
absorption bands in the region of 300–400 nm (band I),
which is attributed to the characteristic p–p* transitions of
NDI chromophore polarized along the z axis (see the Sup-
porting Information). In aqueous DMSO (water/DMSO,
85:15) slight bathochromic shift of band I was observed.
However, a weak fluorescent emission was observed for
NDI chromophores in DMSO (Figure 1a). Interestingly, in
aqueous DMSO (water/DMSO, 85:15) a very broad band
centered at 475 nm with a shoulder at 530 nm appeared.
This red-shifted fluorescence is attributed to excimer-like
emission due to the formation of ground state aggregates as
reported earlier.[14] The inset in Figure 1a shows the exci-
mer-like emission of ll upon UV irradiation.
Figure 1. a) Fluorescence emission spectra of ll. Inset: photographs of ll
in DMSO (A) and in water/DMSO (85:15) under UV light (B). b) CD
spectra of NDI–peptide conjugates. c) Schematic representation of the
transition of the angles and distance between the z polarized transition
moments of stacked NDI chromophores.
In order to probe the induction of molecular chirality on
achiral NDI chromophores, circular dichroism (CD) spectro-
scopic measurements were performed. The CD spectrum of
ll shows negative excitonic Cotton effects between 300–
400 nm, corresponding to the p–p* transitions of NDI chro-
mophore (Figure 1b). Thus, l-phenylalanyl-l-phenylalanine
methylester based peptide auxiliaries transcript their stereo-
chemical information to achiral NDI chromophore. The neg-
ative Cotton effect indicates M-type supramolecular helical
assembly for ll. On the other hand, dd exhibits positive
Cotton effect suggesting P-type helicity (Figure 1b). Surpris-
ingly, in aqueous DMSO (water/DMSO, 85:15) both ll and
dd undergo a bathochromic shift with an enhanced Cotton
effect. The observed bathochromic shift in CD was attribut-
ed to the p–p stacking as revealed in absorption studies.
Previous reports have shown that the amplitude of CD
signal depends on molar extinction coefficient, length of the
helical polymer, angle and the intermolecular distance be-
tween the chromophores.[15] Therefore, the enhanced CD
signal in aqueous DMSO can arguably be attributed, pre-
dominantly due to modifications in the angle and intermo-
lecular distance between the NDI chromophores by hydro-
phobic effect.
We further investigated sergeants-and-soldiers effect in
the case of homochiral ll and dd. In a conventional ser-
geants-and-soldiers experiment the chiral ll was mixed with
achiral AA in different proportions by keeping the overall
concentration (500 mm) constant. In order to induce self-or-
ganization, the mixture of ll and AA in DMSO was soni-
cated for 15 min (for homogeneity) and was followed by the
addition of deionized water (44%) prior to recording the
CD spectra (see the Supporting Information). The plot of
fraction chiral with net helicity reveals a complex chiral be-
havior (Figure 2a). For a clear visualization of chiral en-
hancement, we performed another experiment in which an
increasing amount of achiral AA (0, 100, 200 and 300 mm)
was added to a fixed concentration of chiral ll or dd
(100 mm). The obtained CD spectra after sonication and ad-
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T. Govindaraju et al.
Figure 2. CD spectroscopy. a) The plot (line+symbol) of fraction chiral with net helicity in a conventional sergeants-and-soldiers experiment expressed
as absorptions at 385, 397 nm and absorption maxima. The chiral ll was mixed with achiral AA in different proportions by keeping the overall concen-
tration constant (500 mm). The mixture was sonicated for 15 min (for homogeneity) followed by the addition of deionized water (44%, water/DMSO
44:56) prior to CD measurements. b) Achiral AA (0, 100, 200 and 300 mm) was added to a fixed concentration of chiral ll or dd (100 mm) in DMSO fol-
lowed by the addition of deionized water (44%). c) The plot (line+symbol) of fraction chiral with net helicity for (b). d) Majority-rule experiment.
e) Spectra of ld, dl and monophenylalanine methylester (l or d isomer) conjugated NDI. f) Summary of temperature-dependant studies of ll, dd, ld
and dl.
dition of water are shown in Figure 2b. The chiral enhance-
ment clearly shows that the achiral “soldier”, AA, under-
goes chiral supramolecular assembly as guided by the helici-
ty of the “sergeant”, ll or dd. The plot of fraction chiral
with net helicity confirms the nonlinear behavior with a re-
markable chiral enhancement (Figure 2c). To further eluci-
date the effect of incorporating homochiral NDI–peptide
conjugates of mismatched chirality into a single column,
a majority-rules experiment was performed. Subsequently,
homochiral NDI–peptide conjugates were mixed in different
ratios by keeping the overall concentration constant
(500 mm; Figure 2d). The net helicity was determined as
a function of enantiomeric excess (ee) at 383 nm (see the
Supporting Information). The marginal nonlinear depend-
ence of net helicity on the enantiomeric excess suggests min-
imal chiral enhancement in the majority-rules experiments.
We further investigated the effect of heterochirality of
peptide auxiliary on supramolecular chirality. The CD spec-
trum of ld showed negative Cotton effect, indicating M-hel-
icity similar to that of ll (Figure 2e). On the contrary, dl
showed P-helicity similar to that of dd (Figure 2e). Conju-
gates ld and dl exhibited relatively low induced chirality
compared to ll and dd, respectively. Furthermore, the am-
plitude of the CD signal observed in ld and dl were almost
similar to monophenylalanine methylester appended NDI
(Figure 2e). These results clarified that the supramolecular
helicity is determined by the first stereocenter adjacent to
NDI core whereas the second regulates chiral enhancement.
This ability of the stereocenter to retain the memorized ste-
reochemical information in supramolecular chirogenesis
even in the presence of neighbors with opposite chirality is
termed as retentive helical memory. The retentive helical
memory of the stereocenter is attributed to its direct con-
nection with the achiral NDI chromophore. A similar helical
retention behavior has very recently been observed with
dendritic dipeptides,[10b] whereas a reverse helical retention
was observed in case of benzenetricarboxamides.[9a] The re-
verse retentive helical memory observed in the latter was at-
tributed to peripheral mesogen-driven aggregation. Unlike
covalent polymers, chiral supramolecular assemblies are
strongly dependant on temperature, as it determines the
strength of intermolecular, noncovalent interactions. Tem-
perature-dependent CD spectra for both homochiral and
heterochiral NDI–peptide conjugates were carried out from
10 to 1008C (see the Supporting Information). The summary
of the temperature-dependant CD data is shown in Fig-
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Chiral Transcription and Retentive Helical Memory
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ure 2 f. Even though noncovalent interactions are weak in
nature, approximately 55% of the CD intensity was retained
at 1008C; this strongly emphasizes the strength of the pep-
tide auxiliaries-driven chiral supramolecular assembly.
The homochiral peptide auxiliaries-driven supramolecular
helical polymerization led to 1D assembly of NDI chromo-
phores (Figure 3a–c) in aqueous DMSO (water/DMSO,
85:15). Both peptide conjugates ll (Figure 3a and b) and
Figure 4. Proposed models. a) Schematic representation of left-handed
(ll and ld) and right-handed (dd and dl) chiral supramolecular assem-
blies. b) Schematic illustration of sergeants-and-soldiers effect in which
the achiral soldier AA follows the chiral sergeant, ll or dd.
the zero dimensional organization of heterochiral ld and
dl. These observations in principle could support the mech-
anism of both spontaneous deracemization and enhance-
ment pathways for biological homochirality. This work is sig-
nificant from the fundamental science perspective as well as
envisioned potential applications of NDI–peptide conjugates
in chiral optoelectronics, biomaterials and chiral technology.
Figure 3. Field emission scanning electron microscopy (FESEM) images
of self-assembled structures from water/DMSO (85:15); a), b) ll, c) dd,
d) ld and e) dl.
dd (Figure 3c) formed 1D belt-like nanostructures of over
50 mm in length. These 1D belts pile up to form hierarchical
structures with helical signatures. Such morphological helici-
ty in a hierarchical structure is rarely observed, to the best
of our knowledge.[16] However, ld (Figure 3d) and dl (Fig-
ure 3e) formed mesospheres in aqueous DMSO (water/
DMSO, 85:15). These morphological studies strongly em-
phasize that the formation of 1D single-handed supramolec-
ular helical polymers are more favorable in homochiral
NDI–peptide conjugates (ll and dd) than in heterochiral
NDI–peptide conjugates (ld and dl). A model is proposed
to explain the supramolecular chirality observed in the solu-
tion and self-assembled structures of ll, dd, ld and dl (Fig-
ure 4a). Figure 4b depicts the induction of supramolecular
chirality to achiral soldiers, AA, by either of the chiral ser-
geants ll or dd, as seen in the sergeants-and-soldiers experi-
ment.
In conclusion, we have successfully demonstrated chirality
transcription and enhancement by employing the homochi-
ral, heterochiral and achiral peptide conjugates of NDI. An
unprecedented enhancement of chirality was observed in
sergeants-and-soldiers experiments. Temperature-dependent
CD data showed high thermal stability of the peptide auxil-
iaries-driven chiral supramolecular assembly of NDI conju-
gates. Moreover, an interesting phenomenon, coined as re-
tentive helical memory, is reported. Remarkably, the homo-
chiral ll and dd showed 1D molecular organization, unlike
Acknowledgements
Authors thank Prof. C. N. R. Rao, FRS, for constant support and encour-
agement; JNCASR, Department of Biotechnology (DBT) and Innovative
Young Biotechnologist Award (IYBA), Department of Science & Tech-
nology (DST), Government of India, New Delhi, for financial support;
Prof. G. U. Kulkarni for FESEM facility; Selvi and Usha for their help in
various measurements.
Keywords: chirality
·
enhancement
·
memory
·
naphthalenediimide · peptides
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Received: January 18, 2012
Published online: March 20, 2012
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