Stimuli responsive chiral liquid crystal phases of phenylboronic acid functionalized rodlike viruses and their interaction with biologically important diols

Article abstract of DOI:10.1039/c4cc04639k

The rodlike M13 viruses with chemically decorated phenylboronic acid moieties form pH responsive chiral nematic liquid crystal (LC) phases. Binding with biologically important diols results in LC phases with microstructures that closely correlate with the molecular structure of the diols and can be conveniently discerned by visual cues. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc04639k

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Stimuli responsive chiral liquid crystal phases of
phenylboronic acid functionalized rodlike viruses
and their interaction with biologically important
diols†
Cite this: Chem. Commun., 2014,
50, 10402
Received 18th June 2014,
Accepted 22nd July 2014
Jun Cao,ab Shuaiyu Liu,ab Jie Xiong,ab Yingjun Chenab and Zhenkun Zhang*ab
DOI: 10.1039/c4cc04639k
www.rsc.org/chemcomm
The rodlike M13 viruses with chemically decorated phenylboronic
acid moieties form pH responsive chiral nematic liquid crystal (LC)
phases. Binding with biologically important diols results in LC
phases with microstructures that closely correlate with the mole-
cular structure of the diols and can be conveniently discerned by
visual cues.
Viruses have recently attracted tremendous interest in versatile
fields ranging from nanomaterials, biomedicines to fundamental
research. Such versatility is due to their genetic and chemical
addressability, nanoscale size and defined spherical or rodlike
shape.1 Genetically and chemically modified viruses have found
applications in many fields.2 Rodlike viruses are also ideal
building blocks for lyotropic liquid crystal (LC) materials due to
their monodisperse and anisotropic shape.3 Most of the rodlike
particles normally form nematic LC phases, in which the long
axis of the rodlike particles points to one direction while no
positional ordering exists. However, the prestigious feature of the
rodlike viruses is that some of them, such as the M13 and fd
viruses, can form chiral nematic LC (CLC) phases.3a,f In such a
phase, the long axis of the rodlike virus arranges in a helical way
around a director, resulting in stripe-like fingerprints. The peri-
odicity of the fingerprints is in the micrometer range and can be
Fig. 1 (A) Schematic representation of the chemical modification of the
rodlike M13 virus with an N-hydroxysuccinimide ester (NHS) derivative of
phenylboronic acid (3). (B) and (C) pH dependent liquid crystal behaviour
of M13–PBA. The chiral nematic LC phase (B) was formed at pH = 10.2
(200 mM carbonate buffer) while the pure nematic LC phase (C) was
formed at pH = 7.2 (100 mM phosphate buffer). The concentration of
M13–PBA is 30 mg mLÀ1. Insets: schematic representation of the 3D
ordering of the rodlike virus in each phase.
conveniently observed by polarized optical microscopy (POM) lyotropic liquid crystals (LCs) formed by rodlike nanocolloidal
(Fig. 1B). This is an elegant example from nature that three particles are limited. Especially, in the case of rodlike viruses,
dimensional hierarchal self-assembled structures can be although understanding of the physical origin of their CLC phase
achieved by subtle inter-particle interactions and has fascinated has advanced greatly,3f,5 wise use of such phases lags behind.6
researchers from various fields. Compared to their small mole-
Herein, we present a stimuli responsive chiral nematic LC
cular counterparts that have been widely used in many fields system based on the rodlike virus–M13 bacteriophage modified
such as display technology, sensors, optical materials, etc.,4 with phenylboronic acid (PBA) moieties. We further exploit the
dynamic LC response to specific binding with biologically
a Key Laboratory of Functional Polymer Materials of Ministry of Education, and
Institute of Polymer Chemistry, Nankai University, Tianjin, 300071, China.
E-mail: zkzhang@nankai.edu.cn
important diols, which manifest as simple visual cues. The
M13 virus, a natural nanorod with a length of 880 nm and a
diameter of 6.6 nm, is basically a rodlike protein capsid
consisting of ca. 2700 coat proteins (P8) in which the viral gene
is wrapped.7 On the surface of M13, ca. 5400 amino groups
of the Lys 8 and N-terminal arrange with nanoscale precision
on the virus surface and offer tremendous opportunities for
b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin, 300072, China
† Electronic supplementary information (ESI) available: Materials, synthetic
procedures, characterization and many other results. See DOI: 10.1039/
c4cc04639k
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chemical modifications.2 PBA is pH sensitive and can transform buffer (Table S1, ESI†), indicating that M13–PBA and M13 have
between the hydrophobic neutral trigonal form and the hydro- similar physical properties such as flexibility and surface
philic tetrahedral boronate anion through tuning the environ- charge.3f,5 However, the 3D ordering of the virus in the nematic
mental pH lower or above its pKa (Fig. 1).8 We conjecture that LC phase of M13–PBA strictly depends on the environmental
the pH-dependent amphiphilicity of PBA moieties on the surface pH. At pH o pKa of the PBA (ca. 8.9), M13–PBA only formed the
of the M13 virus can be exploited to fine-tune the 3D arrange- pure nematic LC phase (Fig. 1C). At pH 4 pKa, the classic
ment of the viral particles in the nematic LC phase. Furthermore, fingerprint of the CLC phase recovered (Fig. 1A), and the CLC
boronic acid can form reversible, dynamic covalent bonds with phase exists in the whole virus concentration range of the
compounds containing the cis-1,2 and cis-1,3 diol groups, such nematic LC phase. It is noted here that the pristine M13 virus
as saccharides. There are tremendous applications in sensors formed the CLC in the same buffers, consistent with previous
based on such high affinity of boronic acids toward polyols, results.3g All the results obtained so far point to the fact that
which generally rely on either fluorescent or conductivity M13–PBA and unmodified M13 have similar secondary struc-
changes upon binding of diols with a boronic acid moiety.9,15
ture of the coat proteins, packing style of the capsid, flexibility
For our purpose, an N-hydroxysuccinimide ester (NHS) and surface charges, which are the factors known to influence
derivative – DDPCP (3 in Fig. 1), was synthesized and coupled the nature of the nematic LC phase of M13.5 Therefore, the pH
to the virus surface under mild conditions (Fig. 1A, see the ESI dependent nematic LC phase of M13–PBA must be due to the
for details†).10 It is well established that coupling of such coupled PBA moieties. The PBA moieties are hydrophobic when
compounds only occur to the surface amino groups of the virus pH o pKa.8 While the virus can still remain colloidally stable
due to the compact packing of the coat protein in the capsid.2b due to the electrostatic repulsion of the large amount of surface
MALDI-TOF mass spectroscopy analysis confirmed that part of carboxyl groups as indicated by DLS (Fig. S5, ESI†), hydropho-
the coat proteins was coupled with PBA (Fig. S3 in ESI†). bic PBA moieties probably confer attractive interactions among
Around 700 PBA groups are installed on each virus. Atomic the M13–PBA viruses. Such interactions may induce the viruses
force microscopy (AFM) and transmission electron microscopy to align in parallel so as to maximize the attraction, leading to
(TEM) revealed the rodlike conformation of the modified the formation of the pure nematic phase. It is noted here that
viruses (Fig. S6 in ESI†). From dynamic light scattering (DLS), the attraction brought by the PBA moieties is not strong enough
it is observed that the M13–PBA hybrid virus has an apparent to induce gelation at such high concentrations for the LC phase
hydrodynamic diameter (Dh) distribution similar to the pristine as indicated by the liquid-like rheological behavior of the
M13 virus under the conditions where the PBA moieties concentrated M13–PBA suspension (Fig. S8, ESI†). When pH 4
are either in the hydrophobic state or in the hydrophilic state pKa, the PBA moieties become hydrophilic and charged, and
(Fig. S5, ESI†). This result indicates that the M13–PBA virus is intervirus repulsion is the dominant interaction which helps in
still colloidally stable and no inter-virus aggregation exists in the formation of the CLC phase. To the best of our knowledge,
aqueous media. Such excellent colloidal stability stems from this is the first example that has pH responsive reconfigurable
the negative surface charges offered by the large amount of 3D arrangement of rodlike particles in the nematic LC phase.
carboxyl groups (see Table S1 in the ESI for the apparent zeta
As stated before, boronic acid is the classic receptor for diol
potential of the viruses†).11 In addition, M13–PBA and pristine containing compounds. The above results also highlight the
M13 viruses have a similar circular dichroism (CD) spectrum sensitivity of the microstructure of the nematic LC phase to the
and endogenous fluorescence behaviors of the Try26 of the P8 chemical state of PBA moieties in the case of M13–PBA. We
coat protein (Fig. S4, ESI†).12 Therefore, coupling PBA to part of conjecture that diol compounds of various molecular structures
the coating protein does not perturb the helical structure of the may induce different intervirus interactions after binding to the
coat proteins and their helical packing in the capsid. This is surface PBA groups. Such binding may result in LC phases with
important since such factors are known to influence the nature a microstructure that closely correlate with the molecular
of the nematic LC phase of the filamentous viruses.5 Finally, structure of the diols and can be conveniently discerned by
the solvent exposure and specific binding capability of the viral POM or the naked eye. As a proof of concept, two classes of
surface PBA moieties to diols were confirmed by the standard biologically important diols were tested: (i) aromatic diols such
fluorogenic method based on Alizarin red S (ARS). ARS turns as catechol and its derivatives, which are 1,2-dihydroxyphenyl
from the non-fluorescent into the fluorescent state after binding containing compounds (see Scheme S3 in the ESI†); (ii) aliphatic
with PBA moieties (Fig. S7, ESI†).13 The apparent binding con- diols such as glucose and fructose, two typical monosaccharides.
stant, Kb,app, of the PBA groups on the virus surface to several The pH was kept above the pKa where M13–PBA will form the
typical diol compounds was estimated following literature methods perfect CLC phase (Fig. 1B). In the presence of enough amount
(Table S1, ESI†).13 The different affinity to the various diols is of aromatic diols (above ca. 40 mM), the perfect CLC phase of
consistent with the literature but Kb,app is much higher than those M13–PBA was completely quenched and only pure nematic LC
of free PBA and diols under the same conditions.13
phases formed, regardless of the chemical structure of the diols
Similar to unmodified M13, M13–PBA suspended in aqu- (Fig. 2 and Fig. S9, ESI†). As a control, unmodified M13 was
eous buffer turned from the isotropic into the nematic LC mixed with the same amount of aromatic diols under identical
phase above a certain threshold concentration. The concen- conditions and the CLC phase always formed. DLS also revealed
tration range is comparable to the unmodified M13 in the same that no inter-virus aggregation existed as indicated by a similar
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follows the order dopamine 4 catechol 4 Tiron. Above 10 mM
of dopamine, the inhomogeneity approaches such a degree that
determination of the pitch is impossible. Such specific power
of dopamine might be relative to its chemical structure
(Scheme S3, ESI†). Besides the binding of the 1,2-dihydroxyphenyl
moieties with the surface coupled PBA, the amino group of
dopamine can electrostatically interact with the rich surface
carboxyl groups of the virus.14 This effect will further complicate
intervirus interactions that facilitate the formation of the pure
nematic LC phase. Tiron has two anionic sulfonate groups
which might counteract the attractive interaction brought by
the aromatic benzene rings. Therefore, Tiron has the lowest
efficacy to quench the CLC phase of M13–PBA. The general trend
is that reagents, if containing aromatic moieties and/or groups
which can interact with the virus surface to introduce subtle
attractive interactions, have stronger power to induce changes in
the phase behaviour of the virus.
In contrast to the aromatic diols, in the presence of mono-
saccharides such as fructose, M13–PBA can still form perfect
CLC phases, regardless of the amount of the added sugar. This
can be ascribed to the hydrophilicity of boronate ester formed
by binding the monosaccharide to PBA (Scheme S2 in ESI†).
Glucose is expected to follow the same trend as that of fructose.
However, in the presence of glucose, the M13–PBA suspension
turned into an opaque suspension or gel, depending on the
Fig. 2 Solution behavior of M13–PBA in the presence of diols. (A) Sche-
matic representation of the binding of catechol and its derivatives with PBA
moieties on the surface of the M13 virus. (B) Influence of the amount of
catechol and its derivatives on the pitch of the CLC phase of M13–PBA.
The buffer and the concentration of M13–PBA are the same as in Fig. 1B.
Dihydroxybenzene(catechol), dopamine, and 4,5-dihydroxy-1,3-bezene-
disulfonate(Tiron) were investigated (see Scheme S3, ESI† for the chemical
structure of these diols). (C) Photos of the three states of the M13–PBA
suspension in the presence of glucose: transparent (C1), opaque suspen-
sion (C2) or gel (C3). (D) Schematic illustration of the intervirus crosslinking
by the furanose form of D-glucose.
hydrodynamic diameter distribution between M13 and M13– relative ratio of the glucose to M13–PBA (Fig. 2C and D and
PBA in the absence or presence of catechol (Fig. S5C and D, Fig. S10A, ESI†). It is known that the furanose form of the
ESI†), excluding the possibility that the pure nematic LC phase monosaccharides is favored by the binding with boronic acids
may stem from catechol induced aggregation. Therefore, the (Fig. S2B in ESI†). The furanose form of D-glucose, a-D-gluco-
pure nematic LC phase in the presence of catechol derivatives furanose, has a pair of cis-diol units in the 1,2- and 5,6-positions
must be due to the subtle intervirus interactions brought by which act as two binging sites and render glucose with the
binding of the aromatic diols to BPA receptors on the virus unique property to form the 1 : 2 complex with boronic acids
surface. With synperiplanar arrangement of the aromatic hydroxyl (Fig. S2B in ESI†).15 Therefore, the glucose molecule can
groups and their electro-donating character, catechol and its covalently react with two PBA moieties from two viruses and
derivatives can easily react with boronic acid moieties, resulting crosslink them together (inter-virus crosslinking), as indicated
in five membered cyclic esters linked to two aromatic benzene rings by bundles of several viruses (Fig. S10B in ESI†). In addition, the
(Fig. 2A). Probably, such aromatic dominant groups may bring linear polymer like conformation of the semi-flexible rodlike
hydrophobic, or p–p stacking into the intervirus interactions which virus can facilitate such crosslinking. Therefore, by visual inves-
favor the formation of the pure nematic LC phase.
tigation of the nature of the nematic phase formed by M13–PBA
The 3D arrangement of M13–PBA in the nematic LC phase is in the presence of different kinds of diols, one can discriminate
indeed very sensitive to the amount of catechol and its deriva- between aromatic diols such as catechol derivatives and alipha-
tives. Although the CLC phase can still form in the M13–PBA tic monosaccharides. The distinct solution behavior of M13–PBA
suspension in the presence of a small amount of catechol and in the presence of fructose and glucose might help in selective
its derivatives such as 5 mM Tiron, the fingerprint texture is less sensing of glucose over fructose.
regular compared to that of pure M13–PBA (Fig. S9 in ESI†).
Finally, to demonstrate potential applications, a simple
With increasing amount of catechol, the homogeneity of the device was designed to visualize the interaction of catechol
CLC phase further decreased with increasing areas corre- derivatives with the surface BPA moieties by monitoring the
sponding to the pure nematic LC phase (Fig. S9 in ESI†). The fingerprint texture of the CLC phase of M13–PBA (Fig. 3).
twist periodicity of rodlike viruses in the CLC phase was Concentrated M13–PBA was sealed in a glass capillary, in which
characterized by the cholesteric pitch (P) (highlighted in the perfect chiral cholesteric LC phase formed. The open end of
Fig. 1B). Under otherwise identical conditions, a smaller pitch the capillary was blocked with a 3.0% agar gel which allowed
means stronger tendency to form the CLC phase and vice versa. small molecules like diols to diffuse through freely while the
As listed in Fig. 2B, it is clear that, with increasing amount of virus was blocked. The capillary was immersed in aqueous
diols, the pitch of the CLC phase also increased, suggesting buffer containing diols such as dopamine and the fingerprint
unwinding of the helically ordered rodlike viruses. Among the texture of the CLC phase was monitored by POM (Fig. 3A). Upon
three aromatic diols, the power of quenching the CLC phase diffusing of the diols inside the capillary, the cholesteric LC
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R. Y. Sweeney, A. Hayhurst, G. Georgiou, B. Iverson and A. M. Belcher,
Science, 2004, 303, 213–217; (e) N. Gandra, D. D. Wang, Y. Zhu and
C. Mao, Angew. Chem., Int. Ed., 2013, 125, 11488–11491;
( f ) R. J. Tseng, C. Tsai, L. Ma, J. Ouyang, C. S. Ozkan and Y. Yang,
Nat. Nanotechnol., 2006, 1, 72–77.
2 (a) F. C. Wu, H. Zhang, Q. Zhou, M. Wu, Z. Ballard, Y. Tian,
J. Y. Wang, Z. W. Niu and Y. Huang, Chem. Commun., 2014, 50,
4007–4009; (b) K. Li, Y. Chen, S. Li, H. G. Nguyen, Z. Niu, S. You,
C. M. Mello, X. Lu and Q. Wang, Bioconjugate Chem., 2010, 21,
1369–1377; (c) Y. S. Nam, T. Shin, H. Park, A. P. Magyar, K. Choi,
G. Fantner, K. A. Nelson and A. M. Belcher, J. Am. Chem. Soc., 2010,
132, 1462–1463; (d) J. K. Pokorski, K. Breitenkamp, L. O. Liepold,
S. Qazi and M. Finn, J. Am. Chem. Soc., 2011, 133, 9242–9245;
(e) Z. M. Carrico, M. E. Farkas, Y. Zhou, S. C. Hsiao, J. D. Marks,
H. Chokhawala, D. S. Clark and M. B. Francis, ACS Nano, 2012, 6,
6675–6680; ( f ) Z. Zhang, J. Buitenhuis, A. Cukkemane, M. Brocker,
M. Bott and J. K. Dhont, Langmuir, 2010, 26, 10593–10599.
3 (a) Z. Dogic and S. Fraden, Langmuir, 2000, 16, 7820–7824;
(b) J. Lapointe and D. Marvin, Mol. Cryst. Liq. Cryst., 1973, 19,
269–278; (c) R. Oldenbourg, X. Wen, R. Meyer and D. Caspar, Phys.
Rev. Lett., 1988, 61, 1851; (d) Z. Dogic and S. Fraden, Phys. Rev. Lett.,
1997, 78, 2417; (e) E. Grelet, Phys. Rev. Lett., 2008, 100, 168301;
( f ) E. Grelet and S. Fraden, Phys. Rev. Lett., 2003, 90, 198302;
(g) K. R. Purdy and S. Fraden, Phys. Rev. E: Stat., Nonlinear, Soft
Matter Phys., 2004, 70, 061703.
Fig. 3 (A) Schematic illustration of a prototype device to monitor the
fingerprint texture change of the cholesteric LC phase of M13–PBA under
the influence of catechol. (B)–(F) Typical texture of the nematic LC phase
of M13–PBA recorded at the noted time. M13–PBA was filled in a glass
capillary which was immersed in a carbonate buffer containing 40 mM
catechol. Scale bar: 500 mm.
phase slowly became irregular starting from the interface close
to the gel at the time scale of several days. Eventually, the
fingerprint texture in the whole capillary disappeared comple-
tely. When a similar capillary was immersed in the same
aqueous buffer containing no diol, no change occurred to the
fingerprint texture (Fig. S11 in ESI†).
4 (a) N. Tamaoki, Adv. Mater., 2001, 13, 1135–1147; (b) A. M. Lowe and
N. L. Abbott, Chem. Mater., 2011, 24, 746–758; (c) P. S. Noonan,
R. H. Roberts and D. K. Schwartz, J. Am. Chem. Soc., 2013, 135,
5183–5189; (d) S. J. Woltman, G. D. Jay and G. P. Crawford, Nat.
Mater., 2007, 6, 929–938.
5 (a) S. Tomar, M. M. Green and L. A. Day, J. Am. Chem. Soc., 2007, 129,
3367–3375; (b) E. Barry, D. Beller and Z. Dogic, Soft Matter, 2009, 5,
2563–2570; (c) Z. Zhang and E. Grelet, Soft Matter, 2013, 9, 1015–1024.
6 (a) J.-W. Oh, W.-J. Chung, K. Heo, H.-E. Jin, B. Y. Lee, E. Wang,
C. Zueger, W. Wong, J. Meyer and C. Kim, Nat. Commun., 2014,
5, 3034; (b) K. E. Shopsowitz, H. Qi, W. Y. Hamad and
M. J. MacLachlan, Nature, 2010, 468, 422–425.
7 D. Marvin, R. Hale, C. Nave and M. H. Citterich, J. Mol. Biol., 1994,
235, 260–286.
8 (a) K. Kataoka, H. Miyazaki, M. Bunya, T. Okano and Y. Sakurai,
J. Am. Chem. Soc., 1998, 120, 12694–12695; (b) D. Roy and
B. S. Sumerlin, ACS Macro Lett., 2012, 1, 529–532.
9 (a) S. D. Bull, M. G. Davidson, J. M. Van den Elsen, J. S. Fossey,
A. T. A. Jenkins, Y.-B. Jiang, Y. Kubo, F. Marken, K. Sakurai and
J. Zhao, Acc. Chem. Res., 2012, 46, 312–326; (b) X. Wu, Z. Li,
X.-X. Chen, J. S. Fossey, T. D. James and Y.-B. Jiang, Chem. Soc.
Rev., 2013, 42, 8032–8048.
10 M. L. Stolowitz, C. Ahlem, K. A. Hughes, R. J. Kaiser, E. A. Kesicki,
G. Li, K. P. Lund, S. M. Torkelson and J. P. Wiley, Bioconjugate
Chem., 2001, 12, 229–239.
11 (a) J. Buitenhuis, Langmuir, 2012, 28, 13354–13363; (b) K. Zimmermann,
H. Hagedorn, C. C. Heuck, M. Hinrichsen and H. Ludwig, J. Biol. Chem.,
1986, 261, 1653–1655.
12 G. E. Arnold, L. A. Day and A. K. Dunker, Biochemistry, 1992, 31,
7948–7956.
In conclusion, we prepared a phenylboronic acid (PBA)
decorated M13 virus by convenient surface chemical modifica-
tions. The M13–PBA hybrid virus can form either a pure nematic
or chiral LC (CLC) phase, due to the pH-dependent amphiphi-
licity of PBA. Under the conditions suitable for the formation of
the CLC phase, aromatic diols such as catechol, dopamine etc.
can further influence the 3D ordering of the hybrid virus and
induce the formation of the pure nematic LC phase. In contrast,
monosaccharides such as fructose do not interfere with the
formation of the chiral LC phase of M13–PBA. Such responsive
arrangement of the rodlike virus in the presence of varied diols
might find applications in the development of sensing materials.
This work was supported by the National Natural Science
Foundation of China (No. 21274067, 91127045, 51390011),
the Fundamental Research Funds for the Central Universities,
the Natural Science Foundation of Tianjin, China (No.
12JCQNJC01800) and PCSIRT (IRT1257). We would like to
thank Prof. Linqi Shi for his generous support.
13 G. Springsteen and B. Wang, Tetrahedron, 2002, 58, 5291–5300.
14 H. Ueno, T. Iwata, N. Koshiba, D. Takahashi and K. Toshima, Chem.
Commun., 2013, 49, 10403–10405.
15 (a) Y.-J. Huang, W.-J. Ouyang, X. Wu, Z. Li, J. S. Fossey, T. D. James
and Y.-B. Jiang, J. Am. Chem. Soc., 2013, 135, 1700–1703; (b) W. Wu,
T. Zhou, A. Berliner, P. Banerjee and S. Zhou, Angew. Chem., Int. Ed.,
2010, 122, 6704–6708; (c) Y. Liu, C. Deng, L. Tang, A. Qin, R. Hu,
J. Z. Sun and B. Z. Tang, J. Am. Chem. Soc., 2010, 133, 660–663.
Notes and references
1 (a) W.-J. Chung, A. Merzlyak and S.-W. Lee, Soft Matter, 2010, 6,
4454–4459; (b) D. Ghosh, Y. Lee, S. Thomas, A. G. Kohli, D. S. Yun,
A. M. Belcher and K. A. Kelly, Nat. Nanotechnol., 2012, 7, 677–682;
(c) T. Gibaud, E. Barry, M. J. Zakhary, M. Henglin, A. Ward, Y. Yang,
C. Berciu, R. Oldenbourg, M. F. Hagan and D. Nicastro, Nature, 2012,
481, 348–351; (d) C. Mao, D. J. Solis, B. D. Reiss, S. T. Kottmann,
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