Chiral Conflict as a Method to Create Stimuli-Responsive Materials Based on Dynamic Helical Polymers

Article abstract of DOI:10.1002/anie.201907069

A new multi-sensor material based on helical copolymers showing the chiral conflict effect have been prepared. It can successfully detect and identify diverse metal cations in solution. The design of this material has taken into account not only the oppos

Full text of DOI:10.1002/anie.201907069

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Accepted Article
Title: “Chiral Conflict” as a New Tool to Create Stimuli-Responsive
Materials Based on Dynamic Helical Polymers
Authors: Felix Freire, Riguera Ricardo, Emilio Quiñoa, Sandra Arias,
Rafael Rodríguez, and Mohammad Alzubi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201907069
Angew. Chem. 10.1002/ange.201907069
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10.1002/anie.201907069
Angewandte Chemie International Edition
COMMUNICATION
“Chiral Conflict” as a New Tool to Create Stimuli-Responsive
Materials Based on Dynamic Helical Polymers
Mohammad Alzubi, Sandra Arias, Rafael Rodríguez, Emilio Quiñoá, Ricardo Riguera, Félix Freire*
Abstract: A new multi-sensor material based on helical copolymers
showing “Chiral Conflict effect” have been prepared, which can
successfully detect and identify diverse metal cations in solution.
The design of this novel material has taken into account not only the
opposite helical senses induced by the two chiral monomers in the
copolymer, but also their dynamic behavior. So, the induced helical
sense can be enhanced, diminished or inverted by interaction with
different stimuli (i.e., metal ions). Thus, depending on both the
copolymer compositions (i.e., monomer ratios and absolute
configurations) and the nature of the metal ion, the response of
these dynamic helical copolymers to adopt a single-handed P or M
helix is unique, making it possible not only to detect their presence,
but also to identify them individually. New multi-sensors materials
based and inspired on this effect should arise in the future choosing
appropriate monomers and stimuli.
copolymer (i.e., its M or P helicity) is commanded by the
absolute configuration of the Sergeant (minor component), the
chirality at the periphery of the helix depends on the intrinsic
chirality of the Soldier (major component).
Those studies showed that this chiral to chiral communication
phenomenon —operative along
a
copolymer chain and
responsible of the enhancement of a single-handed helicity—
works efficiently when the two participant chiral monomers
comprise the following features:
1) The two monomers (Sergeant and Soldier) must promote
similar scaffolds in their corresponding homopolymers. For
instance, if the homopolymer of the Soldier adopts a cis-cisoidal
structure, the homopolymer of the Sergeant must also show a
cis-cisoidal backbone.
2) The monomer used as chiral Soldier must display, in the
corresponding homopolymer, a conformational equilibrium at
least between two conformers providing a highly dynamic
polymer with null CD at the vinylic region.
3) The monomer used as chiral Sergeant must show a preferred
conformation in the pendant moiety, with a large energy barrier
with respect to other conformers, resulting in restricted rotations
around its bonds.
The helical structures of dynamic polymers, such as
poly(phenylacetylene)s (PPAs), are usually stabilized by
supramolecular interactions between monomer repeating units
along the polymer chains, with steric effects also playing a
role.[1-10]
Controlled conformational changes at the pendant groups
modulate these supramolecular and/or steric interactions
resulting in variations on the helical structure of the polymer —
i.e., elongation[11-13] and/or helical sense[14-15]—. Thus, chiral
amplification, helical enhancement or helix inversion can be
induced by the action of appropriate external stimuli.[1-10]
In other cases, the communication between chiral monomers is
not in force. As a result, both monomers induce —independently
of each other— the scaffolds that they previously induced in
their corresponding homopolymers which can be of equal
(“Chiral Accord”) or opposite (“Chiral Conflict”) helical senses.
Green and coworkers described the latter phenomenon in
polyisocyanates, where forces favoring left- and right-handed
helices arise from structurally different chiral monomers.[29-30]
However, as far as we know and despite its potential interest, no
practical application of this effect has been reported.
Herein, we show how it is possible to take advantage of this
“Chiral Conflict” effect in a dynamic helical copolymer to control
its helical sense after interaction with an external stimulus. In
this way, the copolymer becomes its sensor.
Several strategies can be envisaged to make use of this effect.
In a simple first approximation, it should be possible to tune the
helical sense in a copolymer featuring “Chiral Conflict”, and
where one comonomer induces a P helix while the other
promotes an M helix, acting on the conformational composition
of one of the comonomers, and thus transforming an axially
racemic (or partially racemic) helix into single-handed one after
stimulation (Figure 1a).
In the case of copolymers formed by the combination of chiral
and achiral monomers, the modulation of the helical structure
may occur through a conformational communication mechanism
—going from the chiral to the achiral pendants— that originates
a chiral amplification phenomenon denoted as the “Sergeants
and Soldiers effect.”[16-28] In such a case, the presence of a small
amount of the chiral monomer (minor component, Sergeant)
commands the secondary structure of the whole copolymer
chain —i.e., either M or P helices— by inducing a certain spatial
orientation in the achiral units (major component, Soldier).
Recently, our group extended the concept of "Sergeants and
Soldiers effect" to a more complex situation of PPA copolymers
—specifically, bipolymers, i.e., copolymers derived from two
monomers— in which the two components are chiral.[16-17] In this
way, one of them acts as a chiral Sergeant, while the other acts
as a chiral Soldier. Hence, while the helical sense of the
In a more elaborate application, if each monomer responds in a
different and individualized way to the external stimuli in the
copolymer showing “Chiral Conflict”, it should be possible, acting
selectively on either one or the other, to tune the helical content
inducing any of the two helicities (M or P) (Figures 1a and 1b).
Finally, in a borderline case, it should be possible to transform
an axially racemic helix (P/M) into its enantiomeric helix (M/P),
also racemic, acting simultaneously on the two monomers using
one or more stimuli (Figure 1c).
[a]
M. Alzubi, Dr. S. Arias, Dr. R. Rodríguez, Prof. E. Quiñoá, Prof. R.
Riguera, Prof. F. Freire
Centro Singular de investigación en Química Biolóxica e Materiais
Moleculares (CiQUS) and Departamento de Química Orgánica,
Universidade de Santiago de Compostela, E-15782 Santiago de
Compostela, Spain.
E-mail: felix.freire@usc.es
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201907069
Angewandte Chemie International Edition
COMMUNICATION
In all these cases, not only the of the stimuli and the structure
of the pendants would be main factors in the final results, but the
relative composition (% of each comonomer) would also be an
important variable to be taken into account due to the different
percentage contribution of each pendant to the final structural
and optical outcomes.
Sergeants and Soldiers effects) should occur.[17] Moreover,
these two types of homopolymers respond in different ways to
monovalent and divalent metal ions.[31-38] Hence, if each
monomer retains independently its ability to respond to these
stimuli within the copolymer without being affected by the
presence of the other chiral monomer, a novel sensor material
with better properties than the parent homopolymers could be
generated based on the “Chiral Conflict” effect depicted above.
poly-[(m-1)x-co-(m-2)1-x
]
a)
Phelix
m-1 (M helix)
Mhelix
m-1 (P helix)
a)
b)
EXTERNAL
STIMULUS A
O
O
(on m-1)
O
O
Mhelix
Mhelix
m-2 (M helix)
O
m-2 (M helix)
NH
N
H
m-(R)-1
O
axially racemic 1 P/M
M helix
Active optical response
H
r
O
Null / Low optical response
b)
NH
m-1 (P helix)
Phelix
Phelix
m-1 (P helix)
1-r
H
n
EXTERNAL
STIMULUS B
m-(S)-1
O
O
(on m-2)
N
L2
Mhelix
m-2 (M helix)
H
m-2 (P helix)
Phelix
L1
HN
O
O
O
L1 = H, L2 = Ph Poly-[(R)-1-co-(R)-2]
L1 = Ph, L2 = H Poly-[(S)-1-co-(R)-2]
P helix
Active optical response
axially racemic 1 P/M
Null / Low optical response
m-(R)-2
c)
c)
m-1 (P helix)
m-1 (M helix)
Phelix
Mhelix
n
n
H
H
EXTERNAL
STIMULI C
O
cis-cisoidal
cis-transoidal
N H
O
N
(on m-1 and m-2)
L2
m-2 (M helix)
m-2 (P helix)
H
Mhelix
Phelix
L1
O
O
L1 = H, L2 = Ph Poly-(R)-1
L1 = Ph, L2 = H Poly-(S)-1
O
Poly-(R)-2
mirror image
helices
axially racemic 1 P/M
axially racemic 2 M/P
Null / Low optical response
Null / Low optical response
Figure 2. Chemical structure of (a) monomers, (b) copolymer series and (c)
homopolymers used in these studies.
“Chiral Conflict” between chiral pendants
Without stimuli
m-1 chiral pendant:
it induces a P helix
chiral pendant:
it induces a M helix
m-2
Thus, homopolymers poly-(R)-1 and poly-(R)-2 and the
Figure 1. a) Conceptual representation of the “Chiral Conflict” effect in a
dynamic helical copolymer and its transformation from an axially racemic helix
(P/M) to an optically active helix (M), by means of an appropriate stimulus (“A”)
acting just on one monomer (m-1). b) Idem to the enantiomeric helix (P) by
means of a different stimulus (“B”) acting just on the other monomer (m-2). c)
Idem from an axially racemic helix (P/M) to another axially racemic helix (M/P),
by means of appropriate stimulus/stimuli (“C”) acting simultaneously on both
monomers (m-1 and m-2).
copolymer series poly-[(R)-1x-co-(R)-21-x] (x= 0.1-0.9) were
prepared by using a Rh(I) catalyst, [{Rh(nbd)Cl}2 (nbd=2,5-
norbonadiene)]. All the copolymers showed by NMR (vinyl
protons, d= 5.7–5.8 ppm) and Raman (1580, 1343, 1006 cm-1) to
possess a cis polyene backbone.[39-45] Their molecular weights
were estimated by GPC (THF as eluent with polystyrene
standards as calibrants; see SI for details) to be between Mn=
7832 and 20325 with Mn/Mw values around 1.58-1.68. The
monomer ratios [(R)-1/(R)-2] in the copolymers were shown by
NMR to be coincident with the feeding mixtures (see SI). Also,
the Kelen-Tüdos method using varied monomer feed ratios and
termination at low conversions[46-47] demonstrated their random
distribution along the polymer chain (see SI).
The CD spectra of poly-(R)-1 and poly-(R)-2 (0.3 mg/mL in THF)
at room temperature showed the presence of opposite helical
senses —M helix for poly-(R)-1 (CD negative at 370 nm, Figure
3a)[35] and P helix for poly-(R)-2 (CD positive at 380 nm, Figure
3b)[38]—. In the poly-[(R)-1x-co-(R)-21-x] copolymer series, the CD
signature is cancelled when the ratio between monomers reach
a value close to 50% (Figure 3c). This fact indicates that a
“Chiral Conflict” effect is taking place, and therefore, while the
pendants from m-(R)-1 induce a M helix, those from m-(R)-2
As a proof of concept of application of the “Chiral Conflict” effect,
we envisioned a number of experiments where different metal
ions could be appropriate as external stimuli acting on a series
of two-component copolymers with different percentage
compositions. As a result, we would expect a set/combination of
different and characteristic responses that would allow us to
differentiate/identify the diverse stimuli used.
It is known from previous studies that homopolymers from (R)-
and (S)-N-(4-ethynylphenyl)-2-methoxy-2-phenylacetamide [m-
(R)-1 and m-(S)-1; enantiomers] and methyl (R)-2-(4-
ethynylbenzamido)-2-phenylacetate [m-(R)-2] (Figure 2), adopt
different types of polyene scaffolds —poly-(R)-1 and poly-(S)-1
cis-cisoidal; poly-(R)-2 cis-transoidal— and therefore, no chiral
communication (neither Chiral Coalition nor Chiral to Chiral
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10.1002/anie.201907069
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COMMUNICATION
poly-(R)-1/M+
poly-(R)-1
poly-(R)-1/M2+
induce the opposite P helical sense (Figure 3c). In THF, the
"strengths" (i.e., the capacities to induce a certain helical sense)
of the pendants from m-(R)-1 and m-(R)-2 are similar, being this
the reason for a null CD in a copolymer with a monomer ratio of
approximately 50% —i.e., poly-[(R)-10.5-co-(R)-20.5]—.
M+
M2+
a)
O
O
O
O
NH
O
NH
O
NH
(ap)
(ap)
(sp)
10
a)
8
poly-(R)-1
6
poly-(R)-1/M2+
(P helix, P1)
poly-(R)-1/M+
(M helix, M1)
poly-(R)-1
(M helix, M1)
4
c)
2
poly-[(R)-1x-co-(R)-21-x
]
poly-(R)-2/M+
poly-(R)-2/M2+
30
20
10
0
poly-(R)-2
0
-2
(R)-2(%)
b)
-4
100
90
80
70
60
50
P
M2+
M+
M
-6
-8
O
O
O
O
-10
O
250
300
350
400
450
500
O
O
HN
HN
b)
Wavelength [nm]
HN
30
20
10
0
40
30
20
10
0
O
(syn)
(anti)
(syn)
P
O
-10
M
poly-(R)-2/M2+
(M helix, M2)
poly-(R)-2/M+
(M helix, M2)
poly-(R)-2
(P helix, P2)
poly-(R)-1-co-(R)-2/M+
poly-(R)-1-co-(R)-2/M2+
poly-(R)-1-co-(R)-2
250
300
350
400
450
500
-10
-20
Wavelength [nm]
M+
M2+
poly-(R)-2
c)
250
300
350
400
450
500
O
O
O
O
Wavelength [nm]
NH
(ap)
O
NH
NH
(ap)
O
g value (370 nm)
g value (377nm)
poly-[(R)-1x-co-(R)-21-x
d)
(sp)
15
M+
M2+
]
O
O
O
O
O
10
5
O
O
HN
HN
HN
(syn)
O
O
(anti)
(syn)
0
poly-[(R)-1-co-(R)-2]/M2+
(Axially racemic 2, P1/M2)
poly-[(R)-1-co-(R)-2]/M+
(M helix, M1/M2)
poly-[(R)-1-co-(R)-2]
(Axially racemic 1,M1/P2
poly-(R)-2 (P Helix)
)
-5
0
50
100
M1 to P1
P2 to M2
M1 = M1
P2 to M2
Axially racemic mirror images
Excess of M helicity
-10
% (R)-2
Figure 4. Established helical senses in (a) poly-(R)-1/M+ complex, poly-(R)-1
and poly-(R)-1/M2+ complex. (b) Idem in poly-(R)-2/M+ complex, poly-(R)-2 and
poly-(R)-2 /M2+ complex. (c) Expected helical senses in poly-[(R)-1-co-(R)-
2]/M+ complex, poly-[(R)-1-co-(R)-2] and poly-[(R)-1-co-(R)-2]/M2+ complex.
poly-(R)-1 (M Helix)
Figure 3. CD spectra of (a) poly-(R)-1, (b) poly-(R)-2 and (c) poly-[(R)-1x-co-
(R)-21-x] copolymer series, including (d) g values. [copolymers] = 0.3 mg/mL in
THF.
We found, as expected, that the two components responded to
the different ions independently, just as they did in the parent
homopolymers. Therefore, depending on the nature of the metal
ion and the copolymer composition, it was possible to play with
the content of a helical sense excess in the copolymer. For
Therefore, once the presence of the “Chiral Conflict” was
established, we decided to explore the dynamic behavior of
these copolymers by studying their interaction with metal ions
(i.e., external stimuli) in THF. In literature, it is found that poly-
(R)-1 adopts an M helix in the presence of monovalent metal
ions, while a P helix is induced by divalent ones (Figure 4a).[33-37]
On the other hand, poly-(R)-2 stabilizes an M helix in the
presence of both types of ions (Figure 4b).[38-39]
Due to the active “Chiral Conflict” in the copolymer, it was
expected that the different interactions of the comonomers with
the ions could either enhance a certain helical sense —e.g., m-
(R)-2 inverts its helical sense from P2 to M2, while m-(R)-1 keeps
its M1 helical structure; i.e., an excess of M helicity (Figure 4c)—
or generate another axially racemic copolymer mirror image of
the original one —e.g., m-(R)-1 goes from M1 to P1 and m-(R)-2
from P2 to M2; i.e., from axially racemic 1 to axially racemic 2
(Figure 4c)— through the mechanisms shown in Figure 1.
To test this hypothesis, we studied the interaction between the
copolymer series —poly-[(R)-1x-co-(R)-21-x] (x= 0.1-0.9); a total
of 9 copolymers—, with different metal perchlorates —Mn+= Li+,
Na+, Ag+, Ba2+, Fe2+, Ca2+, Mn2+, Co2+, Ni2+, Zn2+, Pb2+, Cu2+,
Hg2+—. These metal ions were added to THF solutions of the
above copolymers (0.3 mg/mL) in Mn+/mru (monomer repeating
unit) ratios= 10 mol/mol.
instance, Ba2+ induced an excess of a P helix in poly-[(R)-10.8
-
co-(R)-20.2] (CD380nm> 0), while an excess of an M helix was
obtained in poly-[(R)-10.3-co-(R)-20.7] (CD380nm< 0) with the same
ion (Figure 5a). On the contrary, when Li+ is used, the opposite
helical sense excess is induced in both copolymers: M helix for
poly-[(R)-10.8-co-(R)-20.2] (CD380nm< 0) and P helix for poly-[(R)-
10.3-co-(R)-20.7] (CD380nm> 0) (Figure 5b).
The CD data from the titration of the nine copolymers —poly-
[(R)-1x-co-(R)-21-x] (x= 0.1-0.9)— with the different perchlorates
showed that five metals —Li+, Na+, Ag+, Fe2+ and Ba2+— control
the helical content inducing specific combinations of P and M
helices —detected as positive, negative or null CD responses—
along the series. The other divalent ions tested induce always a
P helix (SI).
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10.1002/anie.201907069
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COMMUNICATION
a)
10
b)
poly-[(R)-10.8-co-(R)-20.2]/Ba2+
poly-[(R)-10.3-co-(R)-20.7]/Ba2+
poly-[(R)-10.8-co-(R)-20.2]/Li+
poly-[(R)-10.3-co-(R)-20.7]/Li+
15
10
5
series described in this article, and therefore "mirror image"
results must be obtained —i.e., opposite helicities and CD
responses—.
5
0
P
P
M
-5
0
Table 1. P, M and null helical sense induction by different metal perchlorates
in the poly-[(R)-1x-co-(R)-21-x] series.
-10
-15
-5
380 nm
380 nm
M
400
-10
250
300
350
400
450
500
250
300
350
450
500
Copolymer series
poly-[(R)-1x-co-(R)-21-x
Metal perchlorates
Wavelength [nm]
Wavelength [nm]
]
Li+
Na+
Ag+ Ba2+ Fe2+
Figure 5. CD traces for poly-[(R)-10.8-co-(R)-20.2] and poly-[(R)-10.3-co-(R)-20.7
]
- -
- -
- -
- -
- -
- -
- -
- -
- -
M
M
M
M
M
M
M
M
M
M
P
P
poly-[(R)-10.9-co-(R)-20.1
poly-[(R)-10.8-co-(R)-20.2
poly-[(R)-10.7-co-(R)-20.3
poly-[(R)-10.6-co-(R)-20.4
poly-[(R)-10.5-co-(R)-20.5
poly-[(R)-10.4-co-(R)-20.6
poly-[(R)-10.3-co-(R)-20.7
poly-[(R)-10.2-co-(R)-20.8
poly-[(R)-10.1-co-(R)-20.9
]
]
]
]
]
]
]
]
]
after the addition of (a) Ba(ClO4)2 and (b) LiClO4. Mn+/mru ratios= 10 mol/mol.
[copolymers]= 0.3 mg/mL in THF, [M(ClO4)n] = 10 mg/mL in THF.
M
M
M
M
M
P
M
M
M
M
- -
P
M
M
M
M
M
P
So, those five metal ions can be identified from their
unambiguous CD patterns. For instance, Fe2+ produces a null
CD in all the copolymer series (Table 1 and SI). The other four
—Li+, Na+, Ag+ and Ba2+— induce an M helix in poly-[(R)-
1
0.7/0.6/0.5-co-(R)-20.3/0.4/0.5] and a P helix in poly-[(R)-10.1-co-(R)-
20.9]. However, they induce specific patterns in the CD spectra of
other copolymers that allow distinguishing among them (Table 1
and SI). Thus:
1) Na+ is the only metal that induces a null CD in the vinylic
region of poly-[(R)-10.4-co-(R)-20.6]. Consequently, it is possible
to identify Na+ from the other monovalent ions.
2) Ba2+ is the only one from these five metals that induce a P
helix in poly-[(R)-10.9/0.8-co-(R)-20.1/0.2].
P
P
P
P
P
P
In conclusion, the work presented here constitutes the proof of
concept that establishes that copolymers showing “Chiral
Conflict” —where their helical structure is cancelled by
appropriate chiral comonomers— can adopt an excess of a
single-handed helix by acting, through an appropriate external
stimulus, on the conformational composition of the constituting
comonomers. In this way it is possible to tune the excess of the
helical content by playing with the nature of the external stimulus
(e.g., metal ion) and the copolymer composition. Another
interesting phenomenon is observed when the external stimuli
act on the conformational composition of the two comonomers at
the same time. In this case, an axially mirror image structure is
generated, where the helical structure is again cancelled by the
two opposite helicities induced by the constituent monomers.
This control in the helical content of copolymers based on
“Chiral Conflict” effect opens new horizons to inspire the design
of new polymeric sensor materials based on chiral recognition
ability —choosing appropriate comonomers and stimuli— and to
deepen the knowledge of the role of the secondary structure in
applications such as asymmetric catalyst, chiral recognition,
chiral chromatography or sensory devices among others.
3) Ag+ is the only metal that induces an M helix in poly-[(R)-10.2
co-(R)-20.8].
-
4) Li+ is the only one that induces a P helix in poly-[(R)-10.3-co-
(R)-20.7] and an M helix in poly-[(R)-10.4-co-(R)-20.6].
In summary, the signature combinations of these 5 cations using
nine copolymers are different and allow their identification (Table
1).
Similar studies were carried out with mixtures of the two
homopolymers —poly-[(R)-1 and poly-[(R)-2— in ratios ranging
from 0.9/0.1 to 0.1/0.9 to compare their behavior with the poly-
[(R)-1x-co-(R)-21-x] series. It was found that the responses of the
homopolymer mixtures to the metal ions were different to those
from the copolymers, being the latter more sensitive to a larger
number of metal ions (see full description in the SI).
A
different and complementary copolymer series can be
obtained from other diastereomeric combination of monomers —
e.g., poly-[(S)-1x-co-(R)-21-x]—, which can help to discriminate
more metals.
Thus, stimuli-responsive studies on the poly-[(S)-1x-co-(R)-21-x
]
series indicated, as in the previous case, that those copolymers
are dynamic and respond to the presence of metal ions as
external stimuli (see SI). The results obtained reveal that in this
series is also possible to control the folding of the copolymers
according to both the nature of the metal ion and the
composition ratio of the two constituent monomers. As a result,
and together with the five ions also identified by the poly-[(R)-1x-
co-(R)-21-x] series —Li+, Na+, Ag+, Fe2+, Ba2+—, three other
divalent cations can be individually recognized —Ca2+, Hg2+,
Cu2+— (see SI).
Acknowledgements
Financial support from MINECO (CTQ2015-70519-P), Xunta de
Galicia (ED431C 2018/30, Centro singular de investigación de
Galicia accreditation 2016-2019, ED431G/09) and the European
Regional Development Fund (ERDF) is gratefully acknowledged.
The other two possible series based on the combination of other
stereoisomers —i.e., poly-[(R)-1x-co-(S)-21-x] and poly-[(S)-1x-co-
(S)-21-x]— present the enantiomeric composition of the two
Keywords: “Chiral Conflict” • external stimuli • dynamic helical
polymer • poly(phenylacetylene) • circular dichroism
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10.1002/anie.201907069
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COMMUNICATION
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10.1002/anie.201907069
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
“Chiral Conflict effect” in chiral helical copolymers becomes a new instrument for
the development of multi-sensor materials and demonstrates its potential identifying
monovalent and divalent metal ions
Mohammad Alzubi, Sandra Arias, Rafael
Rodríguez, Ricardo Riguera, Emilio
Quiñoá, Félix Freire*
“Chiral Conflict”
Page No. – Page No.
m- 1 (P helix)
M
P
P
M
M
“Chiral Conflict” as a Tool to Create
Stimuli-Responsive Materials from
Dynamic Helical Polymers
EXTERNAL
STIMULUS
EXTERNAL
STIMULI
M
m- 2 (M helix)
axially racemic P/M
axially racemic M/P
M helix
This article is protected by copyright. All rights reserved.