A supramolecular helix that disregards chirality

Article abstract of DOI:10.1038/nchem.2397

The functions of complex crystalline systems derived from supramolecular biological and non-biological assemblies typically emerge from homochiral programmed primary structures via first principles involving secondary, tertiary and quaternary structures. In contrast, heterochiral and racemic compounds yield disordered crystals, amorphous solids or liquids. Here, we report the self-assembly of perylene bisimide derivatives in a supramolecular helix that in turn self-organizes in columnar hexagonal crystalline domains regardless of the enantiomeric purity of the perylene bisimide. We show that both homochiral and racemic perylene bisimide compounds, including a mixture of 21 diastereomers that cannot be deracemized at the molecular level, self-organize to form single-handed helical assemblies with identical single-crystal-like order. We propose that this high crystalline order is generated via a cogwheel mechanism that disregards the chirality of the self-assembling building blocks. We anticipate that this mechanism will facilitate access to previously inaccessible complex crystalline systems from racemic and homochiral building blocks.

Full text of DOI:10.1038/nchem.2397

ARTICLES
PUBLISHED ONLINE: 16 NOVEMBER 2015 | DOI: 10.1038/NCHEM.2397
A supramolecular helix that disregards chirality
1
1,2
1
3
1
Cécile Roche , Hao-Jan Sun , Pawaret Leowanawat , Fumito Araoka , Benjamin E. Partridge ,
1
1
1
2
4
Mihai Peterca , Daniela A. Wilson , Margaret E. Prendergast , Paul A. Heiney , Robert Graf ,
Hans W. Spiess , Xiangbing Zeng , Goran Ungar and Virgil Percec
4
5
5,6
1
*
The functions of complex crystalline systems derived from supramolecular biological and non-biological assemblies
typically emerge from homochiral programmed primary structures via first principles involving secondary, tertiary and
quaternary structures. In contrast, heterochiral and racemic compounds yield disordered crystals, amorphous solids or
liquids. Here, we report the self-assembly of perylene bisimide derivatives in a supramolecular helix that in turn self-
organizes in columnar hexagonal crystalline domains regardless of the enantiomeric purity of the perylene bisimide. We
show that both homochiral and racemic perylene bisimide compounds, including a mixture of 21 diastereomers that cannot
be deracemized at the molecular level, self-organize to form single-handed helical assemblies with identical single-crystal-
like order. We propose that this high crystalline order is generated via a cogwheel mechanism that disregards the chirality
of the self-assembling building blocks. We anticipate that this mechanism will facilitate access to previously inaccessible
complex crystalline systems from racemic and homochiral building blocks.
he origin of biological homochirality remains a fundamental enantiomerically rich, racemic and achiral assemblies may
question of natural science1 , even though the most advanced undergo deracemization, even in sufficiently mobile crystal states,
–5
T
functions of biological and non-biological systems emerge from when the transfer of a molecule between neighbouring assemblies
homochiral primary structures. Classic examples are the helical struc- is thermodynamically and kinetically allowed. Supramolecular
6
7
8,9
ture of proteins , carbohydrates , isotactic polypropylene , carbon assemblies of dendritic dipeptides held together via strong non-
10–12
13–15
nanotubes
and the double helix of DNA
. These secondary covalent interactions approaching the strength of a covalent bond
3
2
structures, together with other local conformations, are responsible precluded deracemization due to prohibited disassembly .
1
5,16
for the creation of tertiary and quaternary crystalline structures
Accordingly, their homochiral assemblies could crystallize while
3
2
and functions. Heterochiral primary structures such as syndiotactic their racemic assemblies could not .
polypropylene8 yield lower-order crystals than their homochiral
,17
A recent study of the self-assembly of a family of weakly interact-
counterparts, while racemic or atactic polymers generate amorphous ing, dynamic cyclotriveratrylene crowns substituted with 12
8
solids or liquids .
branched alkyl chains of various chiral compositions demonstrated
18
21
Since Pasteur’s seminal experiment , chiral self-sorting, or spon- the first example of deracemization in the crystalline state . The
taneous deracemization, of conglomerates during crystallization driving force for deracemization was the formation of a columnar
from solution1
8–25
has been used to generate homochirality at the hexagonal crystal, whose lattice symmetry demanded identical
single-crystal scale but not at the macroscopic level. This was single-handed helical columns in a crystal lattice, with a unit cell
demonstrated by conglomerates that had lower melting points containing fragments of four separate columns representing a
2
1
21
than crystals of the corresponding pure enantiomers . single column . This was possible due to the weak supramolecular
Spontaneous deracemization of helical assemblies produced from interactions between crowns. Homochiral and ‘racemic by mixture’
achiral molecules in thermotropic liquid crystals translated derace- samples were shown to deracemize at the molecular level to generate
mization from solution1
8–25
to the bulk liquid-crystal state26–28. crystalline order after annealing. However, a ‘racemic by synthesis’
This advance enabled access to monodomains of enantiomerically sample, which intrinsically cannot deracemize at the molecular
pure liquid crystals by spontaneous deracemization in melt liquid- level, was unable to generate high crystalline order and instead
crystal states2 . The same homochiral principles have been yielded poorly ordered columnar hexagonal crystals upon deracemi-
demonstrated during the self-organization of supramolecular bio- zation, with a much lower melting temperature than the corre-
6–28
29,30
21
logical assemblies, such as the tobacco mosaic virus , and of sponding enantiopure forms . This study also demonstrated that
non-biological assemblies3
1–41
.
deracemization at the supramolecular level occurs between enantio-
A study of all stereochemical permutations of self-assembling merically pure or enantiomerically rich supramolecular columns
dendritic dipeptides, including homochiral, heterochiral and rather than within a column.
various racemic variants in solution and the bulk state, demon-
Here, we report the discovery of a family of perylene bisimide
strated that the highest degree of stereochemical purity, enantiopure (PBI) derivatives containing six chiral side chains in various stereo-
homochiral, exhibits the most thermodynamically favoured self- chemical permutations including homochiral, ‘racemic by mixing’
assembly process in solution, corresponding to the greatest degree and ‘racemic by synthesis’, which self-assemble into single-handed
of order in the crystal state3 . These results showed that supramolecular columns to yield identical single-handed columnar
1–34
1
2
Roy & Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA. Department of
3
Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6396, USA. RIKEN Center for Emergent Matter Science (CEMS),
2
4
5
-1 Hirosawa, Wako, Saitama 351-0198, Japan. Max-Planck Institute for Polymer Research, Mainz 55128, Germany. Department of Materials Science
6
and Engineering, University of Sheffield, Sheffield S1 3JD, UK. Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China.
e-mail: percec@sas.upenn.edu
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2397
ARTICLES
a
HO
RO
RO
RO
RO
b
c
5
, 6 or 7
(
i)
(iv)
11, Φhk1
O
O
HO
RO
RO
1, Φhk2
1
62–88%
73–99%
N
N
NH2
HO
O
O
1
(S)-9, (R)-9, rac-9
(
S)-8, (R)-8, rac-8
RO OR
O
O
O
O
O
O
RO
O
N
O
O
N
(
v)
(
(
S)-9
R)-9
+
76–82%
rac-9
O
OR
OR
10
(
S)-11, (R)-11, rac-11, mix-11
RO
where mix-11 is (S)-11/(R)-11 (50:50)
(
ii)
(S)
Br
85%
Br
2
3
4
5
(
ii)
3%
iii)
95%
(
R)
RBr
Br
Br
Br
9
6
7
(
rac
HO
Figure 1 | Synthesis and supramolecular structure of PBI derivatives. a, Synthesis of chiral PBI derivatives (S)-11 and (R)-11, ‘racemic by synthesis’ rac-11,
which is a statistical mixture of all possible diastereomers synthesized from racemic bromide 7, and ‘racemic by mixing’ mix-11, which is a 50:50 mixture
of the two enantiomers (S)-11 and (R)-11. Reagents and conditions: (i) K
iv) NH NH ·H O, graphite, EtOH, reflux; (v) Zn(OAc) ·2H O, quinoline, 180 °C. b,c, Comparison of the helical structures of 11 in the low-order Φ
phase (b) and the high-order Φ phase (c). The supramolecular helix in the low-order Φ
exterior of the supramolecular column. In contrast, the supramolecular columns in the high-order Φ
2 3 2 2 2 4
CO , DMF, 70 °C; (ii) H , PtO , EtOAc, 25 °C; (iii) HBr, H SO (cat.), 120 °C;
k1
(
2
2
2
2
2
h
k2
h
k1
h
phase is slightly distorted, resulting in a wavy surface on the
k2
h
phase are crystallographically perfect helices with a
straight surface along the length of the column. Core of a PBI derivative: C, grey; O, red. The two branches are shown in blue and orange for clarity.
hexagonal crystalline domains, probably due to deracemization at demonstrated unexpected behaviour of the assemblies obtained
the molecular and supramolecular levels, irrespective of the chiral from the enantiopure ((S)-11, (R)-11) and racemic (rac-11, mix-11)
composition. A cogwheel model of crystallization was proposed to compounds. All assemblies exhibit a columnar hexagonal cry-
k1
42
facilitate this process and to explain the very high degree of order stalline phase (Φh ) formed under thermodynamic control , which
even at the interface between single-handed crystalline mono- melts at 200 °C with almost no hysteresis. This crystallizes in all
domains. A library of chiral PBI derivatives was screened in order samples, irrespective of their stereochemical composition, at 198 °C
–
1
–1
to discover the compounds reported here. The cogwheel mechanism and 199 °C with cooling rates of 10 °C min and 1 °C min ,
k1
responsible for the generation of single-crystal-like order from respectively. Low-order Φh is metastable at low temperatures,
k2
racemic compounds is expected to enable the prediction of libraries whereas high-order Φh , formed under kinetic control, can be
of building blocks that follow the same self-organization principles, detected only with special thermal treatment, such as (1) slow
–
1
via the design of other self-assembling molecules with structural heating and reheating (1 °C min ) via an exothermic transition
k1
k2
parameters congruent with the present PBI derivatives.
from Φh to Φh at ∼105 °C observed upon heating by DSC
(
–1
Supplementary Figs 2 and 10), (2) rapid heating (10 °C min ) fol-
Results and discussion
lowed by annealing at 110 °C for 3 h (Supplementary Fig. 2), or (3)
Synthesis of building blocks and their self-organization in bulk. an extended period of annealing at 23 °C (Supplementary Fig. 12).
k2
k1
The synthesis of four PBI derivatives substituted with two identical On heating, Φ
transforms into Φ
at 128–130 °C.
h
h
benzyl rings bearing three chiral dimethyloctyloxy chains in either
enantiopure ((R)-11 and (S)-11) or racemic form (rac-11 and Analysis of self-assembly in solution by circular dichroism and
mix-11) is outlined in Fig. 1a. The two helical models of the UV–vis. Circular dichroism (CD) and UV–vis spectra of solutions
k1
k2
supramolecular columns formed by 11 (Φh and Φh ) are of (S)-11 and (R)-11 in n-butanol/methylcyclohexane (85:15 vol/vol),
shown in Fig. 1b,c (Supplementary Sections 19 and 20). An recorded upon cooling from 50 °C to 10 °C, demonstrate that the
overview of previously reported helical assemblies of PBI is self-assembly of 11 proceeds via two distinct stages (Fig. 2a). In
provided in Supplementary Section 4.
the first stage, from 50 °C to 20 °C, (S)-11 and (R)-11 are
The length of the dimethyloctyl chains and the alkyl spacer CD-silent. However, a significant decrease in the intensity of their
between PBI and the benzyl ring were selected to achieve a thermo- UV–vis absorbance is observed. This suggests that (S)-11 and
dynamically controlled crystallization of their assemblies, as discov- (R)-11 are molecularly dissolved at 50 °C and form short
42
ered previously with linear achiral alkyloxy groups . (S)-8 and (R)-8 disordered stacks, such as dimers or trimers, on cooling to 20 °C.
are enantiomerically pure, whereas the racemic rac-8 is a mixture of The absence of Cotton effects between 50 °C and 20 °C excludes
six diastereomers with different chiralities of the side chains at the the formation of long, ordered helical assemblies. In the second
3-, 4- and 5-positions of the benzyl ring. Consequently, rac-11, also stage, from 20 °C to 10 °C, an intense Cotton effect emerges in
termed ‘racemic by synthesis’, contains 21 diastereomers whose the CD spectra, indicating the formation of an extended helical
chiralities are statistically scrambled. rac-11 differs from ‘racemic assembly (Fig. 2a, top). The spectra of (S)-11 are mirror images
by mixing’ (mix-11), which is a 50:50 mixture of the homochiral of those of (R)-11, confirming that the chirality of the peripheral
enantiomers (S)-11 and (R)-11.
alkyl chains selects the handedness of the helical assemblies. The
Differential scanning calorimetry (DSC) (Supplementary Fig. 2) intense Cotton effects in the visible region confirm the transfer
combined with X-ray diffraction (XRD; see later discussion) analysis and amplification of chirality from the dimethyloctyl chains to the
2
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2397
ARTICLES
2
,000
,500
a
1,000
b
d
CD
(+)
6
00
00
1
800
600
400
200
0
1
9–10 °C
100% S, 0% R
90% S, 10% R
4
1,000
500
8
0% S, 20% R
70% S, 30% R
0% S, 40% R
50% S, 50% R
0% S, 60% R
2
70
200
0
(
(
S)-11
R)-11
6
0
4
–
200
400
–500
–1,000
30% S, 70% R
20% S, 80% R
10% S, 90% R
0% S, 100% R
–200
–400
–600
–800
–
(
(
S)-11 – Film annealed at 110 °C for 3 h
S)-11 – Solution
(–)
74
–
1,500
2,000
19–10 °C
–600
(R)-11 – Film annealed at 110 °C for 3 h
(R)-11 – Solution
4
–
200
300
400
500
600
700
800
200 300 400 500 600 700 800 900 1,000
Wavelength (nm)
–1,000
Wavelength (nm)
3
2
2
1
1
0
0
.0 UV
(S)-11
c
e
1.0
0.8
0.6
0.4
Standard majority rules
(R)-11 + (S)-11
1
.0
.5
.0
.5
Modified majority rules
(R)- or (S)-11 + rac-11
4
90
50–20 °C
0.8
0.6
0.4
0.2
0.0
0.2
Te
he
Sample
(ºC)(kcal mol–1)
0.0
0.2
0.4
0.6
259
(S)-11 14 –19
(R)-11 15 –20
mix-11 12 –19
rac-11 14 –20
–
–
–
19–10 °C
.0
.5
.0
–0.8
1.0
–
–0.2
–100–80 –60 –40 –20
0
20 40 60 80 100
–
5
0
5
10
15
20
3
00
400
500
600
700
800
Enantiomeric excess of (R)-11 (%)
Temperature (ºC)
Wavelength (nm)
Figure 2 | Solution CD and UV–vis experiments. a, Solution CD and UV–vis spectra of (S)-11 and (R)-11 upon cooling. (S)-11: green (50 to 20 °C) and blue
(19 to 10 °C). (R)-11: red (19 to 10 °C). UV–vis spectra for (R)-11 and (S)-11 are identical. b, CD spectra of thin films (solid lines) of (S)-11 (blue) and
(R)-11 (red) (collected at 23 °C). Solution spectra are also given (broken lines). c, Degree of aggregation of (S)-11 (blue), (R)-11 omitted for clarity, mix-11
(
purple) and rac-11 (orange) calculated from UV–vis data on cooling and fitting with the cooperative elongation model (solid lines). Calculated46 values for
e e
the elongation enthalpy, h and elongation temperature, T are tabulated in the inset. d,e, Majority rules experiments: CD spectra collected from mixtures of
(R)-11 and (S)-11 (d) and net helicity dependence on the enantiomeric excess in mixtures of (R)-11 and (S)-11 (red) and (R)- or (S)-11 and rac-11 (blue) (e).
These experiments demonstrate not only that (R)-11 and (S)-11 generate supramolecular assemblies of opposite handedness in solution, but also that these
structures show some disregard for chirality during self-assembly in solution.
aromatic core of the assembly. The positive exciton coupling of elongation enthalpy h should reflect the non-zero mismatch
e
–1
(
R)-11 implies the formation of a right-handed helix, while the penalty (0.6 kcal mol ; Supplementary Table 2), but experimental
–1
negative exciton coupling of (S)-11 indicates a left-handed helix uncertainty in the value of h (±1 kcal mol ) obscures its influence.
(
e
2
1
Supplementary Section 7). The first stage of self-assembly, from Therefore, in contrast to previous studies , h shows that the
e
5
0 °C to 20 °C, observed only by UV–vis but not by CD, is thermodynamic stability and the supramolecular structure of
associated with a nucleation process, while the second part, from homochiral and racemic compounds do not strongly depend, at
0 °C to 10 °C, active in both CD and UV–vis, is associated with least in solution, on the enantiomeric purity of their side chains.
growth of the helix via a supramolecular helical polymerization Majority rules experiments probe whether a system exhibits
. CD experiments in thin film and in solution chiral amplification21,24,32–34,47–49. In a system with no chiral ampli-
solid and broken lines, respectively, in Fig. 2b) indicate fication, the net ellipticity of a mixture of two enantiomers changes
2
mechanism2
1,30,43–45
(
persistence of a similar helical structure in both states.
linearly with enantiomeric excess (Fig. 2e, broken black line). In
Variable-temperature CD/UV–vis experiments in solution per- contrast, a deviation from linearity suggests that the enantiomer
formed on all variants of 11 demonstrate a cooperative nucleation in excess has a disproportionate impact on the handedness of the
mechanism for their helical supramolecular polymerization supramolecular assemblies generated, as observed in majority
Fig. 2c)2
1,30,32,34,43–45
. Quantitative thermodynamic analysis from rules experiments with (R)-11 and (S)-11 (Fig. 2e, red) and with
(
UV–vis data demonstrated, within experimental error, identical (R)- or (S)-11 and rac-11 (Fig. 2e, blue). This moderate nonlinear
values for the elongation enthalpy h and the elongation temperature effect suggests that the enantiomers can co-assemble into a
e
Te, irrespective of the enantiomeric composition (Fig. 2c, inset). The single column in solution and that the majority enantiomer
elongation enthalpy h represents the net enthalpy change upon dictates the helical sense of the column. The similarity of the
e
elongation of the supramolecular column, while the elongation temp- deviation from linearity in both majority rules experiments suggests
erature T represents the temperature at which the nucleating assem- that elongation of a helical column with a monomer of the non-
e
4
6
blies begin to elongate to form extended helical segments . The preferred chirality is only slightly less unfavourable for a
slightly lower T of mix-11 may be due to heterogeneous nucleation monomer with all six stereocentres of the non-preferred chirality
e
mediated by the mismatched enantiomer impurity acting as a (that is, (R)- or (S)-11 than it is for a monomer with as little
32
seed . The negative sign of h confirms that the self-assembly is as one stereocentre of the non-preferred chirality (rac-11). Hence
e
enthalpically driven. These results agree with X-ray data (see later dis- a disregard for chirality is evident in the assembly of helical
cussion), which reveal identical structures in all cases, and suggest that supramolecular columns in solution.
the enthalpic energy gained by the addition of a PBI molecule to
the supramolecular structure is almost invariant to the chirality of Analysis of self-assembly in bulk by CD, micro-spot CD and
k1
the molecule being added. In other words, the additions of a match- optical polarized microscopy. The helicity of assemblies in Φ
h
k2
ing or mismatching monomer to a helical column are similarly and Φh was investigated in a thin film of (R)-11 monitored by
favourable, and the calculated enthalpy is an average value. The CD during annealing (Fig. 3a,b). No Cotton effect was observed in
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ARTICLES
a
c
(
R)-11
rac-11
Φhk1
4
2
00
00
0
2
5
Φhk2
20–100 °C
20
1
5
–
–
–
–
200
400
600
800
T (°C)
10
5
2
0
90
100
100, 1.5 h
100, 2.0 h
40
6
0
100, 0.5 h
100, 1.0 h
100, 2.5 h
100, 3.0 h
80
0
2
00 250 300 350 400 450 500 550 600 650
Wavelength (nm)
–5
10
15
20
25
b
800
–
–
–
–
(
R)-11
6
4
2
00
00
00
0
100–20 °C
–
–
–
–
200
400
600
800
T (°C)
1
00
90
60
50
20
10
3
00 350 400 450 500 550 600 650
Wavelength (nm)
–
1,000
1,200
80
40
30
0
7
0
–
2
00 250 300 350 400 450 500 550 600 650
Wavelength (nm)
d
mix-11
e
rac-11
P
P
A
A
20 μm
20 μm
20 μm
20 μm
20 μm
20 μm
Bright
Dark
Bright
Dark
5
μm
5 μm
5 μm
5 μm
5 μm
5 μm
Dark
Bright
Dark
Bright
Figure 3 | Thin-film CD and optical polarized microscopy studies. a,b, Thin-film CD spectra of (R)-11 during heating from 20 to 100 °C followed by
k1
h
k2
h
annealing at 100 °C (a) and cooling from 100 °C after annealing (b). c, Thin-film micro-spot CD spectra of rac-11 in Φ
(red) and Φ
(blue) phases. Each
spectrum was obtained from a different spot on the film at 23 °C. Thin-film micro-spot CD spectra of racemic by mixing mix-11 and enantiopure (S)- and (R)-11
k2
h
are provided in Supplementary Figs 7 and 8. d,e, Optical polarized micrographs of mix-11 (d) and rac-11 (e) in the Φ
phase, from the same films used for
micro-spot CD. The analyser was rotated ±5° from the crossed position in the right and left micrographs, respectively. Bright and dark areas (blue and brown,
respectively) are indicated. The directions of the polarizer (P) and analyser (A) are denoted by arrows. The emergence of distinct positive and negative CD
signals in a film of rac-11 demonstrates segregation into microdomains of supramolecular columns of a single handedness. Optical polarized micrographs show
these microdomains as distinct areas, the contrast of which changes from bright to dark, or vice versa, on rotating an analyser in different directions.
the as-prepared film at temperatures lower than 100 °C, indicating under the same conditions as for (R)-11, suggesting that both
k1
that assemblies in Φh either have no well-defined handedness or racemic samples contain a mixture of either an equal number of
exist as an equal mixture of right- and left-handed columns helical columns with opposite handednesses (racemic between
k1
(
Fig. 3a). Heating and annealing at 100 °C transformed Φh into columns) or columns with a helix inversion and an equal amount of
k2
Φh , and the Cotton effect increased dramatically, demonstrating right- and left-handed helical segments (racemic within a column).
that assemblies in Φh exhibit a well-defined helical structure that Micro-spot CD experiments on films of rac-11 (Fig. 3c) and
is stable upon cooling to 20 °C (Fig. 3b). Neither rac-11 nor mix-11 (Supplementary Fig. 7) in the Φh and Φh phases were
mix-11 exhibit a CD signal in solution or in a thin film annealed performed to discriminate between these two possibilities . The
k2
k1
k2
2
1
4
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2397
ARTICLES
a
(S)-11
b
(R)-11
c
mix-11
d
rac-11
L = 6
L = 5
L = 4
L = 3
L = 2
L = 1
L = 0
L = 6
L = 6
L = 6
L = 5
L = 4
L = 3
L = 2
L = 1
L = 0
L = 5
L = 4
L = 3
L = 2
L = 1
L = 0
L = 5
L = 4
L = 3
L = 2
L = 1
L = 0
(
004)
(101)
(110)
(
100) (200)
Φhk1
T = 33 °C
Φhk1
T = 20 °C
Φhk1
T = 20 °C
Φhk1
T = 33 °C
a = b = 29.1 Å, c = 26.7 Å
a = b = 29.2 Å, c = 26.6 Å
a = b = 28.9 Å, c = 26.5 Å
a = b = 29.1 Å, c = 26.7 Å
e
L = 3
(S)-11
f
(R)-11
Experimental Simulated
g
L = 3
mix-11
h
L = 3
rac-11
L = 3
(
002)
L = 2
L = 1
L = 0
L = 2
L = 1
L = 0
L = 2
L = 1
L = 0
L = 2
L = 1
L = 0
(
101)(111)
(110)
(100)(200)
Φhk2
T = 105 °C
Φhk2
T = 114 °C
Φhk2
T = 105 °C
Φhk2
T = 105 °C
a = b = 27.6 Å, c = 14.4 Å
a = b = 27.4 Å, c = 14.4 Å
a = b = 27.4 Å, c = 14.4 Å
a = b = 27.8 Å, c = 14.4 Å
k1
Figure 4 | Oriented fibre XRD. a–h, XRD patterns collected from oriented fibres of (S)-11 (a,e), (R)-11 (b,f), mix-11 (c,g) and rac-11 (d,h) in the Φ
h
(a–d)
k2
and Φ
h
(e–h) phases, respectively. In f, an experimental XRD pattern is compared with an XRD pattern simulated from the molecular model of (R)-11
presented in Fig. 5d,f. Fibre axis, temperature, phase and lattice parameters (a = b = D, where D is the column diameter, Supplementary Table 1) are
k1
indicated. Fully indexed patterns for (R)-11 (b,f) showing L = 8 and L = 4 layer lines are provided in Supplementary Fig. 12. The XRD patterns of the Φ
h
and
k2
Φ
h
phases are, respectively, identical, irrespective of the chiral composition of 11. These patterns demonstrate that homochiral (S)- and (R)-11 and both
k1
k2
racemic derivatives, mix-11 and rac-11, self-assemble to give identical supramolecular structure with low order in the Φ
h
phase and high order in the Φ
h
phase. The cross-like pattern of diffraction peaks in all diffraction patterns indicates that the assembled structures are helical.
k1
birefringence of these films was confirmed to be negligibly small for shorter times at higher temperatures transformed Φ
(
into
h
k2
Supplementary Section 11), as also supported by thin-film micro- Φh (Fig. 4e–h, Supplementary Figs 10–12). Crystallographic
spot CD spectra of enantiopure (S)- and (R)-11 (Supplementary layer lines, L = l, are labelled in Fig. 4, and selected reflections are
k2
Fig. 8). Micro-spot CD experiments demonstrated that the Φh of identified in Fig. 4a,e. Irrespective of the chiral composition, and
k1
k2
mix-11 (Supplementary Fig. 7) and surprisingly also rac-11 as suggested by DSC (Supplementary Fig. 2), Φh and Φh are
Fig. 3c, blue) consists of domains containing columns of a single identical in all samples of 11 (Fig. 4). Sharp reflections on layer
(
k1
k1
handedness. A control experiment on the Φh of rac-11 (Fig. 3c, line L = 0 of Φh (Fig. 4a–d) indicate well-ordered columns
red) shows no deracemization in the low-order Φh phase. arranged on the projection of the three-dimensional hexagonal
k1
Therefore, deracemization takes place between left- and right- lattice along the c axis (the a–b plane of the three-dimensional
handed homochiral supramolecular columns in the crystal state, as unit cell, Supplementary Fig. 12). Diffuse streaks extending from
21
demonstrated for the first time recently . Optical segregation of the meridian and quadrants demonstrate a helical arrangement of
1
3,14,21
k1
k2
enantiopure domains was also identified by uncrossing the the columns
. The lattice parameters of Φh and Φh are
polarizers of an optical polarized microscope. Enantiomeric listed in Fig. 4 and Supplementary Table 1. The c-axis length in
k1
k2
domains are distinguished as bright and dark regions that Φh is almost double that in Φh (26.6 Å versus 14.4 Å). The unit
–3
interchange on rotating the analyser clockwise or anticlockwise cell dimensions and experimental density (1.05 g cm ) indicate
Fig. 3d,e). These experiments support a chiral self-sorting or that eight molecules form the unit cell of Φh and only four form
k1
(
18,21
k2
k1
deracemization process
occurring for assemblies of mix-11 and the unit cell of Φh . The (008) reflection of Φh (Supplementary
rac-11 during the transition to the columnar hexagonal crystal phase, Fig. 12) corresponds to a 3.3 Å π–π stacking distance between
k2
Φh . A control experiment with homochiral (S)- and (R)-11 successive PBI units. The meridional (004) reflection suggests a
k1
(
Supplementary Fig. 9) shows no relative change in the contrast of four-fold repeat stacking along the c axis in Φh . The presence of
k1
individual microdomains, but instead an overall change across the the (101) reflection at L = 1 in Φh suggests a certain degree of
entire film. This indicates the presence of microdomains of a single intercolumnar crystalline order. However, the lack of other off-
k2
handedness throughout Φh , as expected from homochiral samples. meridional reflections indicates that this intercolumnar order is
k1
k2
We conclude that in Φh , the supramolecular columns generated weak and therefore Φh has only short-range helical order, in line
from all four homochiral and racemic compounds are single-handed with CD (Fig. 3c) and NMR (Fig. 6 and Supplementary Section 10)
k2
and homochiral.
experiments. In contrast, Φh is a highly ordered columnar
hexagonal crystal with long range intra- and intercolumnar order,
Structural analysis of supramolecular assemblies by XRD. X-ray as evidenced by numerous sharp reflections in its XRD pattern
k1
fibre patterns of Φh of all chiral compositions of 11 were (Fig. 4e–h) and solid-state NMR analysis (Fig. 6). The absence of
collected from oriented fibres extruded41 from non-annealed (001) coupled with the observation of a nonzero (002) reflection
k2
powders (Fig. 4a–d). Annealing for more than 12 days at 23 °C or in Φ
indicates that the PBI molecules may be dimerized, as
h
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ARTICLES
a
Top view
c
Column
e
Side view
Top view
g
Dimer
3.3 Å
Side view
Methyl group
of chiral
45º
centre
b
Single handedness
Low-order Φhk1 phase – a 41 distorted helix with off-centred stacking of PBI dimers
d
Column
f
Side view
Top view
h
Dimer
3.6 Å
Reversed handedness
45º
Conflict
High-order Φhk2 phase – a 21 perfect helix with centred stacking of PBI dimers
k1
h
k2
h
k2
h
Figure 5 | Model of supramolecular packing in the Φ
and Φ
phases. a, Single molecular conformation in Φ
of (R)-11 (top and side views).
b, Simplified models of four molecules (two dimers) stacked in a column with single handedness (top) and reversed handedness (bottom), showing conflict
k1
h
k2
between periphery alkyl chains. c,d, Side views of stick column models. e,f, Space-filling models for Φ
and Φ
h
. g,h, Schematic intracolumnar arrangement
of dimers in the two phases. The high steric interaction between alkyl groups upon helix reversal ensures that an assembling column maintains the same
k1
k2
helicity along its entire length. Molecules of 11 organize into dimers, which then aggregate to form the supramolecular column in both the Φ
h
and Φ
h
h
k1
phases. The dimers in both phases are identical, but their relative arrangement defines the difference between the two phases: in the low-order Φ phase,
phase, the dimers are arranged co-
k2
h
the dimers are arranged off-axis, leading to a staggered surface on the exterior of the column; in the high order Φ
axially, generating a smooth column exterior and mediating higher-order packing of the supramolecular columns. Core of the PBIs: C, green; O, red. Branches
of the PBIs: aromatic ring, yellow; O, red; aliphatic chains are shown in blue and orange with the methyl group of the chiral centre in pink.
supported by CD and UV–vis experiments (Fig. 2) and that this that chiral self-sorting or deracemization occurs at the supramole-
aggregation consists at least in part of distortions along the c axis.
cular level during or after assembly of mix-11 and rac-11 to
k1
The diameter of the Φ column is only slightly larger than that produce hexagonal crystals containing enantiopure domains of
h
k2
k2
of the Φh column (29.2 Å versus 27.4 Å). Modelling shows that a single-handed columns. This high order in Φ
is unexpected,
h
2
1
supramolecular column assembled from 11 with alkyl chains because only low crystalline order or no crystallization has
extending perpendicular to the column has a maximum diameter previously been observed in related assemblies comprising single-
of 44 Å (Supplementary Fig. 15b,c). The experimental column handed monodomains produced from non-deracemizing racemic
k2
building blocks32,34.
diameter (27.4 Å) can only be explained by the model of Φh
from Fig. 5d,f with a diameter of 27.7 Å (Supplementary
Models of supramolecular single-handed helical columns
k1
k2
Fig. 15a), in which the alkyl groups are extended parallel to the forming the low-order Φh and high-order Φh crystals are
column axis. Polarized infrared experiments in aligned films of shown in Fig. 5. In the columns of both phases, two neighbouring
k2
rac-11 in the Φh phase support the arrangement of alkyl chains molecules are stacked and rotated by 45° with respect to each
k2
parallel to the column axis (Supplementary Section 24).
other to form dimers (Fig. 5g,h). In Φh , the helical axis corre-
k2
Each unit cell of Φh contains only one column (Fig. 7b, brown sponds to the centre of each column, whereas the axis is off-
k1
k1
area). Because the unit cell repeats itself three-dimensionally across centre in the columns comprising Φh . Consequently, in Φ , a
the entire crystal, this implies that every column in the crystal must 41-helix with a pitch of 26.6 Å is formed, and in Φh , a 2 -helix
h
k2
1
be identical with the same helical handedness in order to form a with a pitch of 14.4 Å is formed. This structure can also be crystal-
21
perfect hexagonal crystal . This is true even for rac-11 and lographically defined as a double helix that is different from that
k2
mix-11. The Φh crystal of the racemic compounds show no of DNA; in DNA the strands are covalent, whereas here they are
CD signal in thin film, but exhibits signals in micro-spot CD exper- supramolecular (Supplementary Section 19).
iments (Fig. 3c and Supplementary Fig. 7). These micro-spot CD
The sense of the 45° rotation between neighbouring molecules is
data, in combination with the changes in contrast in the optical selected by their chirality. The rotation of racemic dimers is also
polarized micrographs (Fig. 3d,e) and the identical patterns and single-handed, but statistically forms racemic crystals with large
k2
Φh lattice symmetry demonstrated by fibre XRD (Fig. 4g,h), single-handed domains. The alkyl chains extend parallel to the
provide three complementary types of evidence demonstrating column axis, with the methyl stereocentres pointing into internal
6
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ARTICLES
Stacking of PBI core
Alkyl chain conformation
Aliphatics
crystal. The theoretical and experimental values for the column diam-
eter in Φh are 28.9 and 29.1 Å, respectively. The diameters of the
k1
O–CH2
columns as modelled are supported by X-ray data (Fig. 4 and
Supplementary Fig. 15). The helix reversal penalty (1.5 kcal mol ,
(
i) Low-order Φhk1 phase, 120 °C
–1
Supplementary Table 2) is higher than the chiral mismatch
–
1
penalty (0.6 kcal mol , Supplementary Table 2), that is, the ener-
getic penalty for a chiral monomer to add to a helix of its non-
preferred handedness (compare bottom and top, Fig. 5b). This
provides the mechanism by which a monomer with non-preferred
chirality or even mixed chiral character is incorporated into a
single-handed helix. The handedness of all dimers is the same
within a column (Fig. 5b) and therefore crystals of racemic com-
pounds have a racemic mixture of left- and right-handed columns,
segregated into crystalline domains of single handedness columns
(
ii) Low-order Φhk1 phase, 50 °C
(Figs 3c and 7). This allows the generation of columns with indistin-
guishable cogwheel shapes from non-deracemizing rac-11 and the
formation of hexagonal crystals with as high a degree of order as
those formed from enantiomerically pure compounds (Fig. 7).
(
iii) High-order Φhk2 phase, 50 °C
Solid-state NMR experiments. Solid-state NMR experiments of
assemblies of 11 support the structure determined by XRD (Fig. 6
13
and Supplementary Figs 4 and 5). C cross-polarization magic
k1
angle spinning (CP–MAS) spectra of Φh of rac-11 exhibit
broadening of the aromatic signals on cooling to 120 °C and 50 °C
due to molecular motions mediated by the irregular packing of
the PBI core (Fig. 6i, ii). In contrast, well resolved signals are
k2
(
iv) High-order Φh phase, 50 °C
(
after annealing for 30 days at 23 °C)
k2
obtained in Φh at 50 °C, indicating a regular packing of the
aromatic part of the PBI (Fig. 6iii). The –CH O– and aliphatic
2
chain signals follow the same trend with respect to order,
k1
indicating a liquid-like molecular motion in Φh and almost
k2
perfect conformational order in Φh that is unexpected for the
1
60 150 140 130 120 110
70
40
30
20
5
0
13C chemical shift (ppm)
short and branched dimethyloctyl chains . Repeating the CP-MAS
experiment with rac-11 after extended annealing at ambient
temperature (30 days, 23 °C) improves the spectral resolution due
to substantially improved packing in the aromatic and aliphatic
side chains (Fig. 6iv). This indicates that very well-defined
packing of the alkyl groups in the highly ordered Φh phase of
rac-11 is achieved due to sufficient molecular fluctuations that
refine and improve the molecular structure of the periodic array
on very long timescales. Two-dimensional C{ H} heteronuclear
Lee–Goldburg cross-polarization (LGCP) correlation experiments and
additional CP-MAS experiments with (R)-11 and rac-11 provided
further details on these observations (Supplementary Section 10).
The proposed cogwheel model (Fig. 7a) requires the helical
packing of molecules with alkyl chains parallel to the column axis
and methyl groups pointed towards the centre of the column.
Within each supramolecular column, the teeth (alkyl chains,
13
13
Figure 6 | C CP-MAS solid-state NMR studies. C CP-MAS solid-state
NMR spectra of rac-11. (i) Low-order Φ
phase at 50 °C. (iii) High-order Φ
phase at 50 °C after annealing the sample for 30 days at 23 °C. Green
highlighted areas correspond to signals arising from the PBI core. Yellow
highlighted areas correspond to signals arising from the aliphatic chains at
the periphery of the structure. The broad aromatic peaks in the low-order
phase at high temperature sharpen significantly on cooling the Φ
phase, in the Φ
anneal at 23 °C for 30 days. This indicates that the optimal molecular
packing is very well defined for rac-11 and that even in the high-order phase,
sufficient molecular fluctuations are present to refine and improve the
supramolecular structure of the crystal on very long timescales.
k1
h
phase at 120 °C. (ii) Low-order
phase at 50 °C. (iv) High-order
k1
k2
Φ
Φ
h
h
k2
k2
h
13
1
k1
h
k1
Φ
h
k2
h
k2
h
phase and, unexpectedly, by allowing the Φ
phase to
gaps defined by the rotation of the supramolecular helical backbone Fig. 7a, bottom right) are hydrophobic while the grooves are more
of the PBI cores (Fig. 5a–d). This arrangement, demonstrated by the polar due to the ether linkages between the alkyl chains and aro-
column diameter (Fig. 4 and Supplementary Fig. 15), by the simu- matic rings. During self-assembly, the columns seek to maximize
21
lation of XRD (Fig. 4f) of the supramolecular model from Fig. 5d,f, the intercolumnar electrostatic and hydrophobic interactions by
by UV–vis analysis in solution (Fig. 2a, bottom) and thin film in vertically aligning the alkyl chains at the interface between
k2
Φh (Fig. 3c) and by solid-state NMR (Fig. 6), explains the mech- columns. Only packing with the same handedness columns
anism by which the chirality of mismatched enantiomers or racemic simultaneously optimizes the interactions with all neighbours by
molecules neither affects neighbouring columns (Fig. 7) nor distorts positioning the grooves of neighbouring columns at the same
the highly ordered hexagonal crystal structure (Fig. 5a,c–f). The height (Fig. 7b,c,e and Supplementary Section 21). The grooves of
length of the alkyl chains matches perfectly with the half-pitch of columns of the same handedness (Fig. 7e, left) will always align in
the helix, thus optimizing organization into helices and providing all directions. In contrast, the grooves of columns of different
a principle for the design of additional building blocks following handedness will align in some instances (Fig. 7e, centre) but not
the same self-assembly and crystallization mechanisms.
in others (Fig. 7e, right; see also Fig. 7c). It is a requirement of
k2
This model resembles a cogwheel (Figs 5c–f and 7), with 24 alkyl Φh that all columns have the same handedness, as there is only
chains on the periphery of the column forming teeth. Slight inter- one column in the crystallographic unit cell (Fig. 7b). Aligning
locking (0.3 Å) with adjacent single-handed columns reduces the the grooves and teeth of adjacent columns provides a mechanism
column diameter from
a
theoretical value of 27.7 Å through which only columns with the same handedness pack
(
(
Supplementary Fig. 15) to the experimental value of 27.4 Å together to form highly ordered crystals, even in fully racemized
k2
Fig. 4e–h) and thus generates the high order observed in the Φh
rac-11 (Fig. 7d).
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ARTICLES
a
(
R)-11
b
c
d
rac-11
Top view
Top view
Single molecule
Unit cell
1
/3
1/3
2
/3
2/3
Methyl
group
1
/3
0
0
2/3
1/3
2/3
1/3
0
0
2/3
1/3
of chiral
centre
2
/3
0
0
0
0
Alkyl chain
(teeth)
0
0
2/3
1/3
2/3
0
0
1/3
2/3
1/3
0
Tilt view
Side views
1/3
0
0
2/3
0
1
/3
2/3
2/3
1/3
1/3
2/3
1/3
2
/3
2/3
1/3
2/3
1/3
Side view
3/3
2/3
1/3
3/3
2/3
1/3
e
Figure 7 | Cogwheel model of self-assembly and its role in generating microdomains of single-handed supramolecular columns. a, Schematic
representation of a right-handed column produced by (R)-11 (top and side views). The supramolecular column (left) can be considered as a cogwheel (right).
The conformation of a single molecule within the column is also shown. Colour code as in Fig. 5. b,c, Schematic packing of helical columns with single
handedness (b) and a mixture of two handednesses (c) (Supplementary Section 21). Right- and left-handed helical columns are denoted in blue and orange,
respectively. The relative heights of the grooves at the interfaces between adjacent columns are indicated as a fraction of column length (0, 1/3, 2/3).
k2
h
Grooves with matching and mismatching interfacial heights are presented in green and red, respectively. In b, the unit cell of the Φ
lattice containing a
single helical column is marked in brown. d, Columns of rac-11 packed in a right-handed crystal domain (top and side views). e, Schematic representations of
the alignment of the grooves of neighbouring columns. Left: matching between columns of the same handedness. Centre: matching between columns of
different handedness. Right: mismatching between columns of different handedness. In d and e, blue and orange teeth represent chiral alkyl chains with
different chiralities.
Conclusion
optical microscopy (POM) and density measurements are described in
Supplementary Section 2.
The cogwheel columnar model from Figs 5 and 7 and its role in
enabling deracemization in supramolecular assemblies is demon-
strated herein by three key complementary techniques: variable-
temperature XRD of oriented fibres (Fig. 4 and Supplementary
DSC. Thermal transitions were determined on a TA Instruments Q100 differential
–1
scanning calorimeter equipped with a refrigerated cooling system with 10 °C min
–1
and 1 °C min heating and cooling rates. Indium was used as a calibration standard.
Figs 10–12 and 16), micro-spot CD of thin films (Fig. 3c and The transition temperatures were calculated as the maxima and minima of their
Supplementary Figs 7 and 8) and optical polarized microscopy endothermic and exothermic peaks. An Olympus BX51 optical microscope (×100
magnification) equipped with a Mettler FP82HT hot stage and a Mettler Toledo
(
Fig. 3d,e), as well as a diversity of other supporting techniques
FP90 Central Processor was used to verify thermal transitions.
including solid-state NMR (Fig. 6 and Supplementary Section 10),
solution CD (Fig. 2), DSC (Supplementary Fig. 2), molecular mod-
elling (Supplementary Fig. 15), and infrared (Supplementary
CD spectroscopy in solution. CD and UV–vis spectroscopy measurements were
carried out on a Jasco J-720 spectropolarimeter. The temperature was controlled by a
Fig. 17) and UV–vis (Supplementary Fig. 4) analysis. This cogwheel Peltier temperature controller (Jasco PTC-423). Methylcyclohexane, cyclohexane
(
both 99%, spectrophotometric grade, Aldrich) and 1-butanol (99.5%,
model provides a mechanism for perfect packing of single-handed
columns irrespective of the chirality of their molecular building
blocks (Fig. 7b), as well as near-perfect packing for helical
spectrophotometric grade, Aldrich) were used as solvents. Solution spectra were
recorded in 10 mm quartz cuvettes stirred at 1,000 r.p.m. with a small magnetic
stirring bar, and corrected by subtracting the spectrum of the pure solvent at the
–1
columns with different handednesses (Fig. 7d). We anticipate that same temperature. A scan rate of 100 nm min with a response time of 1 s and a
the principle of matching the helical half-pitch with the length of bandwidth of 1 nm was used to measure the CD spectra, which were recorded in
low-sensitivity mode between 800 and 200 nm (three accumulations). Further
details are provided in Supplementary Section 2.3.
the side chains will give access to extended libraries of compounds
with similar helical structures. This packing mechanism yields a
higher level of hexagonal crystal order in the assemblies of homo-
chiral, racemic and irreversibly racemized building blocks than
CD spectroscopy in thin film. Thin-film CD/UV–vis experiments were
conducted as follows. A 2% wt/vol solution of 11 in n-butanol/methylcyclohexane
any homochiral biological or non-biological single or double (85:15 vol/vol) was prepared by heating 2 mg of sample in 100 μl of solvent until
helix. The findings reported here challenge the established require- dissolved. The hot solution was spin-coated on a round quartz plate (22 mm
diameter) at 2,500 r.p.m. for 6 s and 7,000 r.p.m. for 30 s, using a Chemat
Technology Spin Coater KW-4A. The thin film was then annealed at 110 °C for
ment that highly ordered crystalline systems are achievable only
with enantiopure building blocks and hence provide new access to
3
h and cooled to room temperature, whereupon measurements were recorded.
high-order assemblies of chiral PBIs, which have higher charge
CD/UV–vis spectra of thin films were recorded using the same parameters as those
35
carrier mobility than their achiral counterparts . We therefore envi- used for solution experiments and the spectra were corrected by subtracting the
spectra of the quartz plate recorded before spin-coating. The presence of linear
sage that the cogwheel mechanism of self-assembly and self-organ-
ization described herein will find practical application in the design
of functions in complex materials.
dichroism was negated by recording CD spectra of the thin film at various rotations
perpendicular to the beam and ensuring that the CD spectra were the same,
regardless of the rotation.
Methods
XRD. XRD measurements were performed using Cu-Kα1 radiation (λ = 1.542 Å) on2
a Bruker-Nonius FR-591 rotating anode X-ray source equipped with a 0.2 × 0.2 mm
filament and operated at 3.4 kW. The Cu radiation beam was collimated and focused
Syntheses, materials, characterization data and methods for matrix-assisted laser
desorption/ionization–time of flight (MALDI-TOF), micro-spot CD, polarized
8
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2397
ARTICLES
by a single bent mirror and sagittally focused through a Si(111) monochromator,
generating a 0.3 × 0.4 mm spot on a Bruker-AXS Hi-Star multiwire area detector.
21. Roche, C. et al. Homochiral columns constructed by chiral self-sorting during
supramolecular helical organization of hat-shaped molecules. J. Am. Chem. Soc.
136, 7169–7185 (2014).
22. González-Campo, A. & Amabilino, D. Biomolecules at interfaces: chiral,
naturally. Top. Curr. Chem. 333, 109–156 (2013).
2
To minimize attenuation and background scattering, an integral vacuum was
maintained along the length of the flight tube and within the sample chamber.
Samples were held in quartz capillaries (0.7–1.0 mm in diameter), mounted in a
temperature-controlled oven (temperature precision, ±0.1 °C; temperature range
from −120 to 270 °C). Further details are provided in Supplementary Section 2.5.
23. Fletcher, S. P., Jagt, R. B. C. & Feringa, B. L. An astrophysically-relevant mechanism
for amino acid enantiomer enrichment. Chem. Commun. 43, 2578–2580 (2007).
24. Sato, K., Itoh, Y. & Aida, T. Homochiral supramolecular polymerization of bowl-
Preparation of aligned fibres for XRD. Aligned samples for fibre XRD experiments
were prepared using a custom-made extrusion device . The powdered sample
shaped chiral macrocycles in solution. Chem. Sci. 5, 136–140 (2014).
4
1
25. Safont-Sempere, M. M., Fernández, G. & Würthner, F. Self-sorting phenomena
in complex supramolecular systems. Chem. Rev. 111, 5784–5814 (2011).
26. Link, D. R. et al. Spontaneous formation of macroscopic chiral domains in a fluid
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27. Nagayama, H. et al. Spontaneous deracemization of disc-like molecules in the
columnar phase. Angew. Chem. Int. Ed. 49, 445–448 (2010).
(
∼10 mg) was placed inside the extrusion device and the fibre was extruded in the
k1
as-prepared state (Φ
Typically, the aligned fibre samples have a thickness of 0.3–0.7 mm and a length of
–7 mm. All XRD measurements were done with the aligned sample axis
perpendicular to the beam direction.
h
phase) at room temperature without any heat treatment.
3
2
8. Jeong, H. S. et al. Spontaneous chirality induction and enantiomer separation in
liquid crystals composed of achiral rod-shaped 4-arylbenzoate esters.
J. Am. Chem. Soc. 131, 15055–15060 (2009).
1
3
1
Solid-state NMR spectroscopy. Variable-temperature C{ H} CP-MAS NMR
spectra (Fig. 6) were recorded with a Bruker Avance III console operating
1
13
at 700.23 MHz H Larmor frequency (176.1 MHz C Larmor frequency) using a
commercial double resonance probe supporting zirconia rotors with 2.5 mm outer
diameter. Measurements were performed at 25 kHz MAS spinning frequency using
a CP contact time of 1 ms. A RF nutation frequency of 100 kHz was used on both the
29. Klug, A. From macromolecules to biological assemblies (Nobel lecture). Angew.
Chem. Int. Ed. Engl. 22, 565–582 (1983).
30. Klug, A. The tobacco mosaic virus particle: structure and assembly. Phil.
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31. Percec, V. et al. Self-assembly of amphiphilic dendritic dipeptides into helical
pores. Nature 430, 764–768 (2004).
32. Rosen, B. M. et al. Programming the supramolecular helical polymerization of
dendritic dipeptides via the stereochemical information of the dipeptide. J. Am.
Chem. Soc. 133, 5135–5151 (2011).
33. Percec, V. & Leowanawat, P. Why are biological systems homochiral? Isr. J.
Chem. 51, 1107–1117 (2011).
34. Rosen, B. M., Roche, C. & Percec, V. Self-assembly of dendritic dipeptides as a
model of chiral selection in primitive biological systems. Top. Curr. Chem. 333,
213–254 (2013).
1
13
1
H and C channels, as well as for H heteronuclear decoupling during acquisition
using the SPINAL64 scheme. The given temperatures were corrected for known
deviations due to frictional heating under fast spinning conditions based on the
temperature-dependent chemical shift of lead nitrate. To compare the temperature
dependence of the CP-MAS signal intensities, spectra with 5,120 transients and
identical experimental parameters were recorded in the temperature range
13
1
4
0–100 °C. Two-dimensional C{ H} heteronuclear LGCP correlation experiments
1
(
Supplementary Fig. 4) were recorded at 18 kHz MAS and 850 MHz H Larmor
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The authors acknowledge financial support from the National Science Foundation (DMR-
1066116, DMR-1120901 and OISE-1243313), the Humboldt Foundation and the P. Roy
Vagelos Chair at Penn (all to V.P.). G.U. and X.Z. acknowledge support from the joint NSF-
EPSRC PIRE project ‘RENEW’ (EPSRC grant EP-K034308). B.E.P. thanks the Howard
Hughes Medical Institute for an International Student Research Fellowship. The authors
thank I.U. Rehman (University of Sheffield) for recording polarized infrared spectra.
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©
2015 Macmillan Publishers Limited. All rights reserved

NATURE CHEMISTRY DOI: 10.1038/NCHEM.2397
ARTICLES
Author contributions
Additional information
C.R., P.L., B.E.P. and D.A.W. synthesized and characterized compounds. C.R., B.E.P.,
D.A.W. and M.E.P. performed solution-state CD and UV–vis analysis. F.A. performed
micro-spot CD analysis. H.-J.S., M.P. and B.E.P. collected and processed X-ray diffraction
data. X.Z., H.-J.S., P.A.H. and G.U. generated the molecular model. H.-J.S. and B.E.P.
simulated X-ray data. R.G. and H.W.S. performed solid-state NMR analysis. V.P. designed
the study. V.P., B.E.P., C.R. and H.-J.S. analysed data and prepared the manuscript. All
authors discussed the results and commented on the manuscript.
Supplementary information and chemical compound information are available in the
online version of the paper. Reprints and permissions information is available online at
www.nature.com/reprints. Correspondence and requestsfor materialsshouldbe addressed toV.P.
Competing financial interests
The authors declare no competing financial interests.
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2015 Macmillan Publishers Limited. All rights reserved