Redox-responsive molecular helices with highly condensed π-clouds

Article abstract of DOI:10.1038/nchem.900

Helices have long attracted the attention of chemists, both for their inherent chiral structure and their potential for applications such as the separation of chiral compounds or the construction of molecular machines. As a result of steric forces, polymeric o-phenylenes adopt a tight helical conformation in which the densely packed phenylene units create a highly condensed π-cloud. Here, we show an oligomeric o-phenylene that undergoes a redox-responsive dynamic motion. In solution, the helices undergo a rapid inversion. During crystallization, however, a chiral symmetry-breaking phenomenon is observed in which each crystal contains only one enantiomeric form. Crystals of both handedness are obtained, but in a non-racemic mixture. Furthermore, in solution, the dynamic motion of the helical oligomer is dramatically suppressed by one-electron oxidation. X-ray crystallography of both the neutral and oxidized forms indicated that a hole, generated upon oxidation, is shared by the repeating o-phenylene units. This enables conformational locking of the helix, and represents a long-lasting chiroptical memory.

Full text of DOI:10.1038/nchem.900

ARTICLES
PUBLISHED ONLINE: 14 NOVEMBER 2010 | DOI: 10.1038/NCHEM.900
Redox-responsive molecular helices with highly
condensed p-clouds
Eisuke Ohta1, Hiroyasu Sato2, Shinji Ando1, Atsuko Kosaka1, Takanori Fukushima1 ,
*
Daisuke Hashizume1, Mikio Yamasaki3, Kimiko Hasegawa3, Azusa Muraoka4, Hiroshi Ushiyama4,
Koichi Yamashita4 and Takuzo Aida1,2,5
*
Helices have long attracted the attention of chemists, both for their inherent chiral structure and their potential for
applications such as the separation of chiral compounds or the construction of molecular machines. As a result of steric
forces, polymeric o-phenylenes adopt a tight helical conformation in which the densely packed phenylene units create a
highly condensed p-cloud. Here, we show an oligomeric o-phenylene that undergoes a redox-responsive dynamic motion.
In solution, the helices undergo a rapid inversion. During crystallization, however, a chiral symmetry-breaking phenomenon
is observed in which each crystal contains only one enantiomeric form. Crystals of both handedness are obtained, but in a
non-racemic mixture. Furthermore, in solution, the dynamic motion of the helical oligomer is dramatically suppressed by
one-electron oxidation. X-ray crystallography of both the neutral and oxidized forms indicated that a hole, generated upon
oxidation, is shared by the repeating o-phenylene units. This enables conformational locking of the helix, and represents a
long-lasting chiroptical memory.
elical motifs play crucial roles in many biological functions. prepared the octameric o-phenylene OP8NO2 (Fig. 1) and were
This notion has inspired chemists to design a wide variety able to show that it did not undergo oxidative cyclization.
H
of helical molecules1, not only to investigate their inherent Moreover, we coincidentally noted that OP8NO2 undergoes a
molecular chirality but also to use them for practical applications. chiral symmetry-breaking process during the purification process
For example, helical polymers with a high conformational stability by recrystallization, affording enantiomerically pure crystals
have been used as separation media for optically active com- (Fig. 2). Crystals of both enantiomeric helices (right- and left-
pounds2,3 and asymmetric catalysts4–7. Dynamic molecular helices handed) are formed, but not in equal quantities, so the overall
capable of responding to external stimuli by conformational sample is not a racemic mixture. Finally, we show that the confor-
changes can also serve as chiroptical sensors8 and actuators9. mational rigidity of oligomeric o-phenylenes can be dramatically
Examples of external stimuli that have been extensively studied so altered by electrical inputs (Fig. 2). In solution, the OP8NO2
far include chiral compounds8,10–16, solvents17,18, metal ions9,19 and helices undergo rapid helical inversion. Following one-electron oxi-
light20–23. Redox-active helical polymers that change conformation dation, however, they are converted into OP8NO2†þ helices, which
in response to an electrochemical signal have been proposed to adopt a more compact conformation and do not interconvert as
offer interesting potentials24, but examples of such polymers are cur- easily, thus locking the helix into one conformation.
rently limited to polyisocyanides with redox-active units in their
side chains25,26 that show a chiroptical response to oxidation.
Results and discussion
In this Article, we describe a helix with a redox-active main chain Polyphenylene is one of the most fundamental p-conjugated archi-
that responds to electrical inputs by a conformational change. We tectures. In sharp contrast to the extensively studied p- and m-con-
first took note of an old work by Wittig and colleagues on the syn- nected analogues9,32,33, however, polymeric o-phenylenes have been
thesis of oligomeric o-phenylenes27–29. This molecular motif con- recognized as an elusive class of p-conjugated compounds34.
sists of multiple phenylene units that are linked together at their Because they are difficult to synthesize, only a few reports on the
ortho positions (Fig. 1). Because of the heavily angled aromatic con- preparation of short-chain o-phenylene oligomers30,31,35 have been
nection, polymeric o-phenylenes are forced to adopt a helical struc- made since the first synthetic trial by Wittig in 195727–29
.
ture. In fact, a hexameric o-phenylene in the crystalline state was Furthermore, their physical and chemical properties have largely
proven to form a one-handed 31-helical conformation, in which remained unexplored. In the present work, we adopted an iterative
three repeating monomeric units constitute a single helical synthetic method involving copper-mediated oxidative coupling of
pitch30. However, it should be noted that oligomeric o-phenylenes, lithiated precursors. This method allowed for the incorporation of
on oxidation, are easily subject to cyclization31, which has made a variety of terminal functionalities, including Br (OP8Br), H
their chemistry difficult to study. Here we demonstrate that this (OP8H) and NO2 (OP8NO2) (Fig. 1). OP8Br can be converted
problem can be solved by nitration of the terminal aryl units. We into longer o-phenylenes that include 16, 24, 32, 40 and 48 aromatic
1RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan, 2Nanospace Project, Exploratory Research for Advanced
Technology–Solution Oriented Research for Science and Technology (ERATO–SORST), Japan Science and Technology Agency, National Museum of
Emerging Science and Innovation, 2-41 Aomi, Koto-ku, Tokyo 135-0064, Japan, 3Rigaku, 3-9-12 Matsubara-cho, Akishima, Tokyo 196-8666, Japan,
4Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan,
5Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
e-mail: fukushima@riken.jp; aida@macro.t.u-tokyo.ac.jp
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.900
ARTICLES
R
R R
R
R
H
R R
R
H
R
R R
R
1) n-BuLi, THF
2) MeOH
HNO3, CHCl3
Br
Br
O2N
NO2
R
R
6
R
R
R
R
6
6
OP8Br
OP8H
OP8NO2
R = OMe
1) t-BuLi, THF
2) CuCN
3) Duroquinone
~6nm
R
H
R R
R
OP16H (n = 14)
~5nm
OP24H (n = 22)
OP32H (n = 30)
OP40H (n = 38)
H
OP48H (n = 46)
R
R
n
~4nm
~3nm
~2nm
OP16H
OP24H
OP32H
OP40H
OP48H
Figure 1 | Synthesis and schematic structures of OPnH (n 5 8, 16, 24, 32, 40 and 48) and OP8NO2. Top: synthesis of the o-phenylene oligomers OPnNO2
from the brominated derivative OP8Br. Bottom: using the octameric oligomer as a precursor is crucial to obtain longer polymers that remain linear rather
than undergoing cyclization.
rings (Fig. 1). We would like to emphasize here that the use of this
However, a sample prepared by grinding together all the single
octameric derivative as the precursor has been essential for obtain- crystals of OP8NO2 formed in a single batch proved to be optically
ing such long o-phenylenes. When shorter-chain precursors such as active (Fig. 4a). Furthermore, we observed that the optical purity
the tetrameric homologue OP4Br were used (see Supplementary was significantly enhanced when the crystallization was carried
Information), the chain extension reaction was accompanied by out with mechanical stirring (Fig. 4b). The intensity and sign of
an undesired intramolecular cyclization.
the CD spectra of the samples thus obtained changed every
As described above, in an attempt to purify octameric o-pheny- time, but the CD spectral features were very intense, sometimes
lene derivatives OP8Br, OP8H and OP8NO2 by recrystallization, even almost comparable (ꢀ98%) to those of single crystals of
we noticed that OP8NO2 gives optically active single crystals OP8NO2. These chiroptical features clearly demonstrate the occur-
(Fig. 2). Each of the crystals formed was pulverized by rence of chiral symmetry-breaking in the crystallization of
grinding with KBr. The resulting solid was moulded into a OP8NO2 and the formation of enantiomerically pure crystals,
pellet and subjected to circular dichroism (CD) spectroscopy, which can consist of either enantiomer. This parity-breaking
which revealed a CD band at 265 nm (Fig. 3a). We initially event in crystallization, which leads to the emergence of optical
thought that this observation could be explained by spontaneous activity, has long been a subject of particular interest in physics
optical resolution, which allows for the formation of single crystals and has attracted attention in relation to the origin of homochir-
consisting of either right-handed or left-handed enantiomers with ality in nature. However, rational understanding of this phenom-
equal probability. In a spontaneous optical resolution process, enon still remains a challenge36–38. Although several examples of
each single crystal is therefore optically active, but the whole ‘chiral symmetry-breaking’ have been reported39–41, no molecular
system is a racemic mixture of the enantiomers and remains helices had so far been observed to undergo this parity-
optically inactive.
breaking event.
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69
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.900
ARTICLES
–e–
+e–
Slow
Rapid
In solution
Extended
Contracted
Long-lasting chiroptical memory
Chiral symmetry breaking
during crystallization
In the solid state
or
Permanent chiroptical memory
Figure 2 | Schematic representation of the helical inversion dynamics of OP8NO2 in solution and in the solid state. Dynamically inverting OP8NO2
crystallizes in either a right-handed (blue) or a left-handed helical form (orange) through chiral symmetry-breaking, affording crystals with a permanent
chiroptical memory in the solid state (left, top to bottom panels). Although OP8NO2 undergoes a rapid helical inversion in solution, this dynamic process
becomes very slow after one-electron oxidation of the helices (green, right-handed; red, left-handed), thereby allowing OP8NO2†þ to acquire a long-lasting
chiroptical memory (top, right and left panels).
100
a
b
50
0
–50
–100
–150
250
300
350
400
450
250
300
350
400
450
Wavelength (nm)
Wavelength (nm)
c
d
120
60
5
4
3
2
1
0
0
–60
–120
–180
250
300
350
400
450
0
3
6
9
Wavelength (nm)
Time (s 104)
×
Figure 3 | Chiroptical features and helical inversion dynamics of the neutral and oxidized forms of OP8NO2. a, Circular dichroism (CD) spectra in a KBr
pellet at 25 8C of single crystals of OP8NO2 with right-handed (blue) and left-handed (orange) helical conformations (determined by X-ray crystallography).
b, Time-dependent CD spectral change (135-s intervals) at 210 8C of a single crystal of OP8NO2 dissolved in MeCN. c, Time-dependent CD spectral change
(6-h intervals) at 210 8C of a single crystal of OP8NO2 dissolved in MeCN in the presence of the one-electron oxidant (4-BrC6H4)3N†þ[SbCl62]. The black
arrows in b and c represent the evolution of the CD intensities over time. d, Decay profiles of the CD intensities at 265 and 273 nm in spectra b (blue) and c
(green), respectively. The linear decay profiles indicate that the racemization events of OP8NO2 and its oxidized form follow a first-order kinetics, where the
helical inversion rates (kinv) at 210 8C are determined as 9.84 × 1024 and 2.18 × 1026 s21 in the absence and presence of the oxidant, shown in blue and
green, respectively.
Under optimized conditions, yellow-coloured prismatic crystals 31-helical conformation (Fig. 5a,b) with interplanar distances
suitable for X-ray crystallography were obtained. The crystals have between the cofacially oriented phenylene units of 3.232–3.285 Å
a chiral space group C2, where OP8NO2 adopts a perfect (Fig. 5c). For comparison, we note that these values are much
70
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.900
ARTICLES
a
100
50
0
100
50
0
b
Left-handed
Left-handed
Right-handed
50
100
50
100
Right-handed
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9 10
Sample
Sample
Figure 4 | Chiral symmetry-breaking in the crystallization of OP8NO2. a,b, Helical handedness data and enantiomeric excess (e.e.) values of various
samples of OP8NO2 crystals formed upon recrystallization from MeCN without (a) and with (b) mechanical stirring. Each dot represents a different single
batch. Sample 9 (black dot) did not show any CD signal. We observe that chiral symmetry-breaking is enhanced by mechanical stirring.
a
b
c
3.232 Å
3.282 Å
3.285 Å
3.256 Å
3.261 Å
d
e
f
3.201 Å
3.241 Å
3.252 Å
3.238 Å
3.192 Å
Figure 5 | Crystal structures of OP8NO2 and OP8NO2†1[SbF62] adopting a heavily twisted 31-helical structure. a–c, Top view (a) and side view (b) of
OP8NO2 with interplanar distances between cofacially oriented phenylene units (c). For comparison, the observed distances are much shorter than the
intersheet distance of graphite (3.35 Å). d–f, Top view (d) and side view (e) of OP8NO2†þ[SbF62] with interplanar distances between cofacially oriented
phenylene units (f). The observed distances are noticeably shorter than those for OP8NO2 due to a decrease in p-electronic repulsion, so that the
interconversion between helices of opposite handedness is much slower.
shorter than the intersheet distance of graphite (3.35 Å). When kinetics with a rate constant of 9.84 × 1024 s21 (Fig. 3d; blue). A
viewed along the longer axis of the molecule, the proximal pheny- detailed kinetic analysis at different temperatures revealed that the
lene units overlap with one another almost perfectly. As shown by activation energy for the helical inversion of OP8NO2 is
a crystal packing diagram (Supplementary Fig. S4b), a single 88.3 kJ mol21 (Supplementary Fig. S9a).
crystal consists exclusively of the right-handed or left-handed
Before the exploration of its redox-responsive conformational
helical conformer of OP8NO2. The crystallographic data (see behaviour, we investigated the electrochemical properties of
Supplementary Information) allowed us to confirm that a single helical OP8NO2. In cyclic voltammetry (CV), OP8NO2 showed a
crystal composed of the right-handed helical conformer of substantial stability against oxidation and released up to four elec-
OP8NO2 shows a negatively signed Cotton effect at 265 nm trons at þ0.62, þ0.82, þ1.01 and þ1.22 V (versus Ag/Agþ, in
(Fig. 3a; blue). As expected, a CD spectrum that is a perfect MeCN) in a stepwise manner. The resulting multiply oxidized
mirror image to the above was observed for a single crystal consist- species in turn accepted electrons to revert to its neutral form
ing of the left-handed helical conformer (Fig. 3a; orange).
(Supplementary Fig. S7). Encouraged by this unusual electrochemi-
The single crystals of OP8NO2, upon heating, remained optically cal stability, we attempted to oxidize OP8NO2 using a one-electron
active up to its phase transition temperature at 191 8C oxidant such as tris(4-bromophenyl)ammoniumyl hexachloroanti-
(Supplementary Fig. S6), indicating a high conformational stability monate [(4-BrC6H4)3N†þSbCl62]. We pulverized a single crystal of
of OP8NO2 in the crystal lattice. However, once the crystals were OP8NO2 and added the resulting powder at 220 8C to an MeCN sol-
dissolved in acetonitrile (MeCN) at 25 8C, the characteristic ution of (4-BrC6H4)3N†þ[SbCl62], whereupon an instantaneous
optical activity disappeared almost instantaneously, indicating the colour change from yellow to green took place. Electron spin reson-
occurrence of a rapid helical inversion in solution. The helical inver- ance (ESR) spectroscopy of the resulting solution showed a single
sion took place even at 210 8C in MeCN, where the half-life (t1/2) of broad line (Supplementary Fig. S2), indicating the formation of a
the CD intensity at 265 nm was only 352 s (Fig. 3b). The time- radical cation of OP8NO2 (OP8NO2†þ). This solution was CD
dependent CD spectral change thus observed obeyed a first-order active, with a partly different spectral pattern from that of neutral
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.900
ARTICLES
c
e
a
b
d
HO OH
NO2
O2N
6
OP8NO2m
Figure 6 | Density functional theory (DFT) calculation of a model compound of OP8NO2 (OP8NO2m) at the B3LYP/6-31G(d) level. a, Molecular structure
of OP8NO2m. b,c, An optimized geometry of the neutral form of OP8NO2m with its highest occupied molecular orbital (HOMO; b) and lowest unoccupied
molecular orbital (LUMO; c). d,e, Optimized geometry of the one-electron oxidized form (radical cation) of OP8NO2m with its singly occupied molecular
orbital (SOMO; d) and LUMO (e). These orbitals are drawn with an isosurface value of 0.04. The oxidized form of OP8NO2m adopts a more compact
conformation, where a hole generated in the molecule distributes at the outer six phenylene units. For b–e: grey, C; white, H; green, N; red: O.
OP8NO2 (Fig. 3c). Of particular interest was the fact that the optical electrons, a repulsive interaction that operates between condensed
activity lasted for a very long period of time (Fig. 3d; green); the half- p-clouds42 can be reduced, or can even become attractive43–50
.
life at 210 8C (t1/2 ¼ 44 h), for example, was 450 times as long as that This is a fundamental subject of molecular science and has been
observed for neutral OP8NO2 (t1/2 ¼ 352 s at 210 8C, vide ante) well documented in cofacially assembled noncovalent and covalent
(Fig. 3d; blue). Again, the racemization profile followed a first-order p-systems. However, these studies highlight only the static aspect of
kinetics with a rate constant at 210 8C of 2.18 × 1026 s21, where p-bonding interactions. In contrast, through investigations on the
the activation energy for the helical inversion of OP8NO2†þ was eval- conformational change of an electrochemically stable oligomeric
uated as 96.6 kJ mol21 (Supplementary Fig. S9b). This value is higher o-phenylene (OP8NO2) with helically condensed p-clouds, the
by 8.3 kJ mol21 than that of its neutral form.
present work sheds light on the dynamic aspect of p-bonding.
We also succeeded in obtaining the crystal structure of one-elec-
tron oxidized OP8NO2†þ. When ether vapour was allowed to diffuse
Methods
2
at 25 8C into an MeCN solution of OP8NO2†þ[SbF6 ], once isolated
Spontaneous symmetry breaking of OP8NO2 in crystallization. An MeCN
solution (5 ml) of OP8NO2 (30 mg, 25 mmol) was refluxed for 2 h, then allowed to
cool to 25 8C. The resulting solution was filtered with a polytetrafluoroethylene
membrane, and the filtrate was divided into five samples of 1 ml each and allowed to
stand. After 24 h, the formed crystals were thoroughly collected by filtration and
dried under reduced pressure. The crystals were then pulverized by grinding, and a
small portion (0.1–0.2 mg) of the resulting powder was pressed with KBr powder
(100–150 mg) into a transparent pellet, which was subjected to CD spectroscopy.
Following the CD spectral measurement, the pellet was dissolved in a mixture of
MeCN and water (5 ml; 1:1 v/v), and the resulting solution was subjected to
electronic absorption spectroscopy. The exact amount of OP8NO2 in the pellet was
quantified by an absorbance at 290 nm. The e.e. value for all the crystals (Fig. 4a) was
determined on the basis of a calibration curve (Supplementary Fig. S8), given by a
correlation between the CD intensity and absorbance of single-crystalline OP8NO2
(100% e.e.). Similarly, e.e. values of crystalline OP8NO2 samples, isolated by
recrystallization under mechanical stirring, were determined (Fig. 4b). For sample
preparation, an MeCN solution (10 ml) of OP8NO2 (59 mg, 50 mmol) was refluxed
for 2 h while stirring using a magnetic stirrer bar. The resulting solution was allowed
to cool to 25 8C with continuous stirring. After 2 h, the microcrystals that were
precipitated were corrected by filtration and dried under reduced pressure.
from the reaction mixture of OP8NO2 and NO†þ[SbF62], single
crystals suitable for X-ray crystallography formed. As is clearly
shown in Fig. 5d,e, OP8NO2, on oxidation, was able to preserve its
31-helical conformation. Importantly, the observed interplanar dis-
tances between the proximal phenylene units (3.192–3.252 Å,
Fig. 5f) in one-electron oxidized OP8NO2†þ are 0.02–0.07 Å
shorter than those of the neutral form of OP8NO2 (Fig. 5c). We
also noticed that all the C–O bonds in OP8NO2†þ (1.340–
1.365 Å) are noticeably shorter than the corresponding bonds in
the neutral form (1.358–1.381 Å) (Supplementary Table S1). The
structural characteristics thus observed for OP8NO2†þ allowed us
to conclude that a hole, generated by one-electron oxidation, is delo-
calized extensively over the helically twisted p-electronic array.
The geometrical shrinkage of OP8NO2 on oxidation was sup-
ported by a theoretical calculation of its simplified model
OP8NO2m (Fig. 6a) using density functional theory (DFT) at the
B3LYP/6-31G(d) level. Figure 6b,c shows the highest occupied mol-
ecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) distributions, respectively. This optimized structure of
OP8NO2m clearly showed that its neutral form adopts a
31-helical conformation, with a helical pitch of 4.35 Å (that is, the
average centre-to-centre distance between the proximal phenylene
units). We also calculated one-electron oxidized OP8NO2m†þ
and found that it adopts a 31-helical conformation with a reduced
helical pitch of 3.95 Å (Fig. 6d,e). Therefore, the geometrical shrink-
age of OP8NO2 on oxidation, as observed crystallographically, may
not be attributable to the effect of crystal packing, but is most prob-
ably an intrinsic feature of the molecule. Although the HOMO of
optimized OP8NO2m is localized on the four internal phenylene
units (Fig. 6b), the SOMO (singly occupied molecular orbital) of
OP8NO2m†þ distributes at the outer six phenylene units (Fig. 6d).
This result supports the suggestion from the crystal structure of
OP8NO2†þ that a hole, generated in the molecule by the removal
of one electron, is shared by the repeating o-phenylene units.
Kinetic study on helical inversion of OP8NO2. An arbitrarily chosen single crystal
of OP8NO2 was pulverized and dissolved in MeCN (3.5 ml) at a given temperature,
and the resulting solution was immediately subjected to CD spectroscopy at the
same temperature. For the preparation of its one-electron oxidized form
(OP8NO2†þ), a single crystal of OP8NO2 was pulverized and dissolved in MeCN
solution (3.5 ml) containing an excess amount of (4-BrC6H4)3N†þ[SbCl6
]
2
(typically 0.4 mg), and the resulting mixture was also subjected to CD spectroscopy.
The observed decay profiles of OP8NO2 and OP8NO2†þ both satisfied first-order
kinetics, where the rate constants of racemization (krac) and helical inversion (kinv
)
were obtained from equation (1):
ln[(CDt/CD0) × 100] = −kract = −2kinv
t
(1)
The half-life (t1/2) of the CD intensity was obtained from equation (2):
t1/2 = ln 2/krac = ln 2/2kinv = 0.693/2kinv
(2)
The kinv values obtained at various temperatures were plotted (Supplementary Fig.
S9a,b) and analysed according to the Arrhenius equation (3):
ln kinv = ln A − Ea/RT
(3)
Overall, we can conclude that the oxidized form of helical
OP8NO2 is conformationally locked by an attractive force operating
where A, Ea, R and T are frequency factor, activation energy, gas constant and
among the densely packed phenylene units (Fig. 2). On removal of absolute temperature, respectively.
72
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Acknowledgements
This work was supported by KAKENHI (21350108). The authors thank S. Ohkoshi and
K. Nakabayashi (University of Tokyo) for the measurement of the ESR spectrum.
Author contributions
T.F. and T.A. designed the work. E.O., T.F. and T.A. wrote the paper. E.O., H.S., S.A. and
A.K. performed the experiments. Single-crystal X-ray diffraction studies were carried
out through the collaboration of D.H., M.Y. and K.H., A.M., H.U. and K.Y. were
responsible for DFT calculations.
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The authors declare no competing financial interests. Supplementary information and
chemical compound information accompany this paper at www.nature.com/
naturechemistry. Reprints and permission information is available online at http://npg.nature.
com/reprintsandpermissions/. Correspondence and requests for materials should be addressed
to T.F. and T.A.
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