Poly-γ-p-Biphenylmethyl-Glutamate as Enantiodifferentiating Alignment Medium for NMR Spectroscopy with Temperature-Tunable Properties

Article abstract of DOI:10.1002/chem.201802921

The use of anisotropic NMR parameters—especially residual dipolar couplings (RDCs)—offers access to additional structural information and, therefore, alignment media required for this approach are under a continuous development. Here, we present poly-γ-p-biphenylmethyl-glutamate (PBPMG) as a new versatile enantiodifferentiating alignment medium. The thermoresponsive properties of this polymer allowed for RDC measurements of more than one orientation within the same sample at different temperatures. Moreover, the outstanding enantiodifferentiation along with excellent spectral quality even offered the opportunity to differentiate enantiomers in mixtures.

Full text of DOI:10.1002/chem.201802921

A Journal of
Accepted Article
Title: Poly-gamma-p-biphenylmethyl-glutamate as
Enantiodifferentiating Alignment Medium for NMR-Spectroscopy
with Temperature Tunable Properties
Authors: Sharon Jeziorowski and Christina Thiele
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To be cited as: Chem. Eur. J. 10.1002/chem.201802921
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Poly-γ-p-biphenylmethyl-glutamate as Enantiodifferentiating
Alignment Medium for NMR-Spectroscopy with Temperature
Tunable Properties
Sharon Jeziorowski, and Christina M. Thiele*[a]
Abstract: The use of anisotropic NMR parameters - especially
residual dipolar couplings (RDCs) – offers access to additional
structural information and therefore alignment media required for this
approach are under a continuous development. Herein, we present
diastereomorphous interactions with the alignment medium
resulting in different orientations with respect to the magnetic
field.[11]
The long term objective to possibly enable absolute
configuration via RDCs in the future,[12] as well as a continued
development in structure elucidation, ask for an in-depth
understanding of the alignment process itself. Also, it is
favourable to enhance the pool of available alignment media to
achieve a higher diversity of analyte and solvent compatibility
and induced orientation. In the course of this progress, several
novel alignment media with new properties are described in
literature and offer an innovative design strategy.
poly-γ-p-biphenylmethyl-glutamate (PBPMG) as
a new versatile
enantiodifferentiating alignment medium. The thermoresponsive
properties of this polymer allow for RDC measurements of more than
one orientation within the same sample at different temperatures.
Moreover, the outstanding enantiodifferentiation along with excellent
spectral quality even offer the opportunity to differentiate
enantiomers in mixtures.
Recently, it was shown that the additional independent
stereogenic centre located in the sidechain in poly-γ-S-2-
Introduction
methylbutyl-
L-glutamate
has
an
effect
on
the
enantiodifferentiating
capabilities
causing
a possible
enhancement.[13] In another project of our group it was shown
that poly-β-phenethyl-aspartate (PPLA/PPDA), the constitutional
isomer of PBLG, can be used as a thermoresponsive alignment
medium. The ability to invert the helix sense in the backbone of
the polymer at a certain temperature changes the orientation of
the analyte. Therefore two sets of RDCs as a result of two
independent orientations in one sample are obtained.[14]
Within classical NMR spectroscopy in isotropic solution there are
only few established possibilities to elucidate the spatial
structure in organic solvents, e.g. NOE[1] or J-coupling[2]. Over
the last two decades the use of anisotropic NMR parameters –
especially residual dipolar couplings (RDCs) – attracts more and
more attention in the field of structure elucidation.[3] In general, a
partial alignment of an analyte is necessary to observe RDCs.
For non-water-soluble small molecules two major classes of so
called alignment media that are able to orient the analyte, are
mainly reported in literature:[3e, 3f, 4] anisotropic swollen polymer
gels (SAG, strain-induced alignment in a gel) or lyotropic liquid
crystalline (LLC) phases. Due to the one-handed α-helical
In consideration of these findings, a combination of an additional
stereogenic element as well as thermoresponsive behaviour
seems promising for further development of innovative
alignment media. In this regard we were wondering whether
poly-γ-p-biphenylmethyl-glutamate (PBPMG) could be used as
an appropriate system. While the α-helical structure of
homopolypeptides causes the formation of rodlike mesogens in
solution, the addition of a biphenyl group as a further small
mesogen in the sidechain could influence the liquid crystalline
behaviour. Moreover, the biphenyl group contains a potentially
chiral axis as additional stereogenic element which could also
affect the enantiodifferentiating abilities.
structure of homopolypeptides, poly-γ-benzyl-
(PBLG),[5] poly-γ-ethyl- -glutamate (PELG)[6] or poly-ε-
carboxybenzoyl-
-lysine (PCBLL)[6a] are the most prominent LLC
L-glutamate
L
L
phase based alignment media which are used for the RDC
approach. Apart from that, polymeric LLCs such as
polyguanidines,[7] polyacetylenes[8] and polyisocyanides,[9] which
also form a helical structure, can be used. The helicity of these
polymers is - in contrast to homopolypeptides - not induced by
centro-chiral elements in the backbone but by chiral information
in the sidechain. All helical polymers potentially offer different
orientations for the enantiomers of an analyte.[10] In general,
enantiodifferentiation of analytes is based on different
PBPMLG was first synthesised by Feijen ET AL. and investigated
towards its stability and viscosity behaviour with regard to the
transition between helix and random coil in presence of
dichloroacetic acid.[15] Although the additional p-phenyl
substituent as compared to PBLG decreases the stability of the
helix, PBPMLG still forms a stable helix in helicogenic solvents.
More interestingly, PBPMLG exhibits a significant temperature-
dependent effect on its optical behaviour. Dilute solutions of the
polymer in several solvents show a sudden change of the
specific rotation below 273 K.[16], [17] The corresponding circular
dichroism (CD) spectra confirm that the α-helical structure
remains while a strong CD band associated with the biphenyl
chromophore emerges.[16] It is proposed that this transition is
caused by association caused by a phase transition at lower
[a]
S. Jeziorowski, Prof. Dr. C. M. Thiele
Clemens-Schöpf-Institut für Organische Chemie und Biochemie
Technische Universität Darmstadt
Alarich-Weiss-Str. 4, 64287 Darmstadt (Germany)
E-mail: cthiele@thielelab.de
Homepage: www.thielelab.de
Supporting information for this article can be found under XXX
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temperatures with an ordered supramolecular structure where
the chains of the polymer aggregate in parallel.[18] While
literature experiments were performed at comparably low
concentrations with regard to critical concentrations of lyotropic
liquid crystals, we were interested whether these characteristics
can also be observed for possible LLC phases. Moreover, only
After successful synthesis of the homopolymers Poly-
Poly- -1 we investigated their chiroptical properties by CD
spectroscopy (Figure 1). The general course and the
temperature dependence of CD spectra of Poly- -1 could be
reproduced in accordance to the literature.[16] With decreasing
temperature in the range from 323 K to 273 K only a small
increase of intensity is observed in the region of 222 nm which
generally is assigned to the homopolypeptide backbone.
D-1 and
L
L
the
whereas there are no insights on the properties of the
corresponding -polymer so far. To study and exploit the
observed effects for the application as an alignment medium we
synthesised and -poly-γ-p-biphenylmethyl-glutamate
L-polymer starting from L-glutamate is literature-known
D
However, decreasing the temperature further to 263 K a
significant increase in intensity for the band at 222 nm as well as
the reformation of the CD-band at 260 nm associated to the
D
-
L
(PBPMDG/PBPMLG). Therefore, as we apply the polymers in a
different context, we examined whether PBPMG is able to build
stable LLC phases and if the thermoresponsive behaviour is
also observable for such high concentrations required for LLCs.
Furthermore, we studied different influences e.g. temperature or
concentration variation on the orientation of an analyte via RDC
analysis.
biphenyl region occurs. The spectrum for Poly-D-1 is almost the
mirror image indicating that both polymers show enantiomeric
behaviour in their optical properties. After verifying the chiral
optical properties for Poly-L-1 as well as Poly-D-1 the question is
now whether the polymers form stable LLC phases and
discriminate enantiomers.
Results and Discussion
Synthesis and characterisation of PBPMG
Synthesis of the polymer PBPMLG poly-L-1 was achieved
according to literature[15] in a six step synthesis with a slightly
modified procedure with respect to literature based on our
experience with PBLG.[5d] We used the same protocol (see
Scheme 1) for the hitherto unknown Poly-D-1 starting from D-
glutamic acid 2 using alcoholysis of the N-phthaloyl-protected
cyclic anhydride 3 of glutamic acid followed by removal of the
protection group for a convenient access to the desired glutamic
acid ester 5. The corresponding N-carboxyanhydride (NCA) 6
was synthesised with phosgene (as 20% solution in toluene),
yielding highly pure monomer 6. The ring opening
polymerisation was then successfully carried out with
triethylamine as initiator (Scheme 1, for experimental details see
SI).
Figure 1. Temperature dependent CD spectra at a concentration of ca. 1.5
mg/mL in tetrahydrofuran (THF) with steps of 10 K between each
measurement. Orange: Poly-D-1; blue: Poly-L-1.
Alignment properties of LLC phases
A good indication whether a polymer is applicable as an
alignment medium is its liquid crystalline behaviour. For PBPMG
birefringence was observed in chloroform or tetrahydrofuran
(THF) with
a concentration of 9.5 % w/w and 15 % w/w,
respectively. It is noteworthy that the LLC phase prepared from
THF shows the usual birefringence of a crystalline phase if the
sample is inspected through crossed polarisation filters (see
Figure 2a). The texture of the same sample after sufficient time
in an NMR magnet changes to a more ordered and uniform
optical appearance (Figure 2).
Scheme 1. Synthesis of PBPMDG via esterification of
D-glutamic acid 2,
followed by phosgenation to give NCA 6 and polymerisation to Poly-D-1.
This change in optical appearance before and after the
application of a magnetic field can also be tracked with 2H-NMR
spectroscopy. After bringing the sample into the magnetic field a
Pake pattern[19] like signal shape of deuterated chloroform is
formed, indicating uniform distribution of multiple ordered
domains (see Figure 2b lower trace). After some time in the
Yields of the analogous synthesis route for the
(Phth = Phthaloyl).
L-polymer are listed in grey
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magnet the expected doublet with two sharp lines expected for
anisotropic LLC phases in chloroform develops. The time the
liquid crystal requires to orient itself depends on multiple factors:
First of all, using a higher magnetic field reduces the alignment
time, which is probably related to the reorientation und structural
conversion process of a cholesteric to a nematic phase known
for PBLG.[20] In addition, there are differences between individual
polymer batches. Some polymer batches form a stable phase
within minutes, while others take hours until no more line shape
changes occur. As one might expect, we observed that samples
with a lower viscosity also orient faster. However, we do not see
a reasonable correlation with the molecular weight of the
polymers, their poly dispersity index (PDI) or the critical
concentration yet. Further investigations on this behavior are in
progress. Furthermore, initial investigations with respect to
concentration dependence in correlation with equilibration time
revealed no clear trend. Usually a fast equilibration is favorable
for alignment media, but on the other hand such a slow
reorientation could open up possibilities for further investigations
on the various factors that have an influence on the orientation
process of the polymer as well as a potential influence of the
mesogenic side chain.
explanation of this observation is not possible with the present
data and is currently under investigation.
A
similar phase behaviour can also be observed for
tetrahydrofuran (THF) as solvent. The observed reversal point[21]
at 280 K is lower than for chloroform but still higher than the
expected value observed from CD spectra with approximately
270 K. This suggests that the point is solvent as well as
concentration dependent. Moreover, when comparing several of
our synthesised polymers small deviations between different
batches are observed. For the sake of clarity we only use two
polymers with very similar reversal points throughout this paper
and will further investigate these phenomena in the future. With
2H-NMR-experiments we show that PBPMG forms stable LLC
phases with uniform orientation. Further, the quadrupolar
splitting ∆νQ shows a dependency on temperature. These
findings point towards an applicability of PBPMG as alignment
medium. This is investigated as a next step with small organic
molecules as analytes.
Figure 3. 2H-NMR spectra of CDCl3 in PBPMDG (9.5 % w/w) at various
temperatures at a 700 MHz spectrometer (5 hours equilibration time after each
temperature change). The signs were determined in additional Q.E. COSY
experiments (see SI).
Figure 2. a) PBPMG-LLC samples between crossed polarisation filters. Left:
in CDCl3; middle: in [D8]THF; right: in [D8]THF after 12 h in the magnetic field
(400 MHz). 2b) 2H-NMR-spectra: PBPMLG in CDCl3 (400 MHz spectrometer)
with 9.5 % w/w at 315 K and an acetone-d6-capillary as reference.
[D8]THF as analyte
As a next step, the temperature dependency of LLC phases of
PBPMG was investigated. We studied the quadrupolar splitting
∆νQ of the deuterated solvent chloroform with 2H-NMR spectra in
the LLC phase as function of temperature (Figure 3). For
temperatures above 303 K the splitting is equivalent to the outer
edges of the Pake pattern indicating a parallel orientation of the
chloroform quadrupole moment to the magnetic field B0. At
temperatures below 303 K the inner tips of the Pake pattern
build the quadrupolar splitting along with a sign change of ∆νQ.
This points towards a perpendicular orientation to B0 at this
temperature and therefore for a reorientation of the solvent
molecules within the LLC phase. According to literature[16] there
are no changes detected by optical methods for solutions of
PBPMLG in chloroform. If there was a connection between this
phenomenon and the occurrence of the powder pattern of the
LLC phase we would not expect any sudden changes for the line
2H-NMR-spectra can also be used to examine chiral properties
of the LLC phase and with that performing 2H-NMR experiments
in [D8]THF as a solvent offers the advantage of using this as a
reliable probe for chiral alignment.[5c, 22] For isotropic samples of
THF two signals, one for each methylene group is observed. In
achiral anisotropic phases the signals split into two doublets and
due to the loss of symmetry in chiral anisotropic phases four
doublets are observed. The spectrum of THF in PBPMLG at
300 K reveals four doublets for four pairs of enantiotopic nuclei
and thus confirm a chiral LLC phase. For a closer examination
and analysis of the quadrupolar splittings ∆νQ including their sign
we utilised Q.E. COSY-spectra[23] (Figure 4). The compounds
studied with Q.E. COSY-NMR-experiment reported in literature
are fairly simple so far. THF proved to have a rather complex
spin system due to diastereotopic interactions and the additional
signals tend to overlap in the Q.E. COSY spectra. Nevertheless,
the expected signal squares can be related with each other due
splitting for chloroform in the 2H-NMR-spectra.
A precise
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1
to different JC-D couplings. Signal changes can clearly be
calculated using the software RDC@hotFCHT[26]. Aforesaid
tensors can then be compared and the generalised angle
β[27] between the eigenvectors obtained was used to quantify
either the enantiodifferentiation or the difference in orientation
between the different temperatures compared.[28] Figure 5 shows
the outstanding quality of the spectra with narrow lines, allowing
the extraction of all possible one bond C-H RDCs. This results in
an excellent correlation of the experimentally determined RDCs
with those of the calculated RDCs received from the structural
proposal of the DFT-optimised structure.[28]
tracked and only if couplings are very similar the spectra are not
evaluable. Decreasing the temperature from 300 K close to the
reversal point results in an increase of the absolute value of ∆νQ
.
At the reversal point the signal pattern and several signs change
abruptly. By lowering the temperature further a steady change is
again observed. Noticeable, at lower temperature only a very
small splitting for the C2-D2 pairs is observed and the
enantiodifferentiation is observed predominantly for the C1-D1
groups.
Figure 5. CLIP-HSQC-spectra of (-)-IPC in CDCl3 (isotropic, blue) and (-)-IPC
in PBPMDG/CDCl3 (9.5 % w/w, anisotropic, red) at 300 K.
Figure 4. [D8]THF in PBPMLG (20 % w/w) at various temperatures. Top: Q.E.
COSY at 300 K used for assignment and sign determination for the C1-D1 pair.
Bottom: 2H-NMR spectra at different temperatures.
Quantification of orientational properties
For future application of PBPMG as a new alignment medium in
structure elucidation we were interested in the overall
enantiodifferentiating abilities. To evaluate these we performed a
series of experiments with both enantiomers of isopinocampheol
((+)- and (-)-IPC). We chose IPC as an analyte since both
enantiomers are commercially available and it has proven as a
reliable test substance for new alignment media.[5d, 6c, 7, 8b, 14] We
prepared the anisotropic samples above the critical
Figure 6. RDCs of (-)-IPC in PBPMDG/CDCl3 (9.5 % w/w) as a function of
temperature.
The thermoresponsive properties were further investigated via
temperature series around the reversal point observed in 2H-
NMR-spectra. Therefore we prepared eight samples – both
enantiomers of IPC in each polymer-configuration in chloroform
as well as in THF as solvent. For each temperature a CLIP-
HSQC[25] spectrum accompanied with a 2H-NMR spectrum
before and after the CLIP-HSQC spectra as well as a 2H-
image[24] were recorded to guarantee a stable, homogeneous
and unchanged phase during the measurement. Plotting the
2
concentration and verified the stability of the LLC-phase by H-
NMR-spectra until no line shape change occurred. The
homogeneity was always confirmed with 2H-NMR-imaging[24]
.
The RDCs (1DCH) were extracted from CLIP-HSQC[25] spectra
using the relation 1TCH = 1JCH+ 2 1DCH. The order tensors S were
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extracted RDCs against the temperature displays a surprising
course as demonstrated in Figure 6. While a representative
sample is shown here similar behaviour was observed for the
other samples. The abrupt change of quadrupolar splitting
observed earlier can also be retrieved for the RDCs – even
crossing zero at the reversal point (Figure 6). The abrupt change
of the RDCs, especially the sign changes appearing, also
One crucial point here is to note that different sample
concentrations were used whereby an inter-polymer comparison
is hardly reasonable. Moreover, different molecular weights of
the polymers lead to different critical concentrations of the LLC
phases.[5d] Due to these different critical concentrations, it would
be necessary to prepare samples at the corresponding higher
concentration. Therefore, we studied the concentration
dependence of our system.
suggests
a reorientation of the analyte. To quantify the
orientational difference within the individual samples we
compared the order tensors of (-)-IPC with respect to different
temperatures. We observed, that the angle
β of (-)-IPC at the
different temperatures of the LLC phase in chloroform increases
steadily with increasing temperature difference (See SI Table
19). It is noteworthy that two samples each change their
orientation with a likewise tendency. This would be expected for
enantiomeric cases as in theory, the diastereomorphous
interaction of (-)-IPC with D-polymer must be the same as (+)-
IPC with L-polymer and vice versa if both polymers are the
enantiomers of each other.
Temperature dependency of enantiodifferentiation
To evaluate the impact of temperature on enantiodifferentiation
we compared the alignment tensors of (-)- and (+)-IPC at each
temperature (Table 1). For all cases temperature dependence
for the enantiodifferentiation is observable. For chloroform as
solvent the effect decreases steadily and becomes higher again
Figure 7. Comparison of the order tensors of (+) and (-)-IPC with
generalised angle = 89.1° (PBPMDG, [D8]THF, 278 K).
a
β
at lower temperatures below the reversal point.
A direct
correlation between the reversal point determined via the zero
crossing of the RDCs and the enantiodifferentiation cannot be
established. In contrast, the enantiodifferentiation of the LLC
phases in THF is not only higher but also reaches a value of
Concentration dependency
For this, we prepared samples of (-)-IPC in PBPMLG/chloroform
with a concentration of 12.3 %, 15 % and 20 % w/w, respectively.
Extraction of RDCs is still possible for higher concentrations with
a comparatively good correlation between experimental and
calculated RDC values although spectral quality and emerging
1H-1H-coupling artefacts increase the extraction error (for
experimental details see SI). The point where all RDCs cross
zero still remains the same and at first glance quadrupolar
splitting and RDCs scale with respect to each other. For closer
proximately 90° for the generalised angle
β close to the reversal
point (Figure 7). This indicates outstanding enantiodifferentiation.
On closer examination it is noticeable that in fact the values for
both polymers are not the same as would be expected for
enantiomeric polymers. This indicates that the polymers, which
were determined to be enantiomeric by optical spectroscopy,
apparently do not behave like ideal enantiomeric alignment
media.
examination we calculated the generalised angle
β at each
temperature and observed differences up to 10°. This value
exceeds our presumed experimental error and therefore we
have drawn the conclusion that our system is not independent of
concentration. This is not necessarily a disadvantage since
selective concentration variation could also be used for
adjustments in a structure elucidation process.
Table 1. Generalised angle
β between (+) and (-)-IPC in LLC phases of
PBPMLG/PBPMDG in CDCl3 and [D8]THF.
PBPMDG
CDCl3
PBPMLG
CDCl3
PBPMDG
[D8]THF
PBPMLG
[D8]THF
9.5 % w/w 12.3 % w/w
15 % w/w 20 % w/w
T [K]
315
310
305
300
295
290
280
270
T [K]
300
290
284
282
280
278
275
270
|
β
| [°] | [°]
|
β
|
β
| [°] | [°]
|β
19.7
18.0
16.6
13.0
12.0
8.8
23.0
22.8
21.7
19.7
18.0
15.5
11.8
13.3
30.0
34.0
36.4
37.2
50.2
89.1
62.1
53.5
37.2
37.3
35.1
36.6
55.0
76.6
60.7
53.8
Investigation of the influence of solutes on LLC behavior
In fact, the system reacts highly sensitive to temperature
changes and also is concentration dependent. Comparisons
between different samples therefore are challenging because
they require the same sample concentration and the same
quadrupolar splitting needs to be accomplished. Furthermore,
we wanted to ensure that the different enantiomers of the
analyte do not change the LLC phase behaviour of the polymer
15.6
24.1
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like in the anneling process with chiral agents known e.g. from
polyacetylenes or polyisocyanides.[29] For this reason we studied
mixtures of the enantiomers of IPC. The racemic mixture of (+)/(-
)-IPC (1:1) verifies that the LLC phase behaves the same way
as the corresponding enantio-pure samples. The sample also
shows the same quadrupolar splitting as the samples of the
individual pure enantiomers. This proofs that the LLC phase
behaviour is independent of analyte (see SI for spectra). With
the enantio-enriched mixture of (+)/(-)-IPC (2:1) we wanted to
demonstrate that a) the individual contributions of each
enantiomer can be obtained from one sample due to the
outstanding enantiodiffentiating properties and b) that the
extracted RDCs coincide with the RDCs of the pure samples.
feasible. In general, such a procedure is rather challenging since
the RDCs need to be sufficiently different and an adequate
spectral resolution is required. In our case the F1-coupled BIRD-
filtered HSQC[30] offers the desired resolution with excellent
spectral quality (Figure 8). For 300 K the enantio-enriched
mixture in PBPMLG/[D8]THF provides distinctability for 10 out of
11 one bond coupling pairs for the enantiomers of IPC. The
corresponding assignment was obvious based on signal
intensities.
The comparison of the RDCs for the enantio-pure samples and
the enantio-enriched mixture are in remarkable accordance
(Figure 9). Slight discrepancies might be explained by
concentration differences of the samples.
Analyte compatibility
Finally, we were interested in the compatibility and applicability
of PBPMG as a practicable alignment medium for organic
molecules. Therefore, we selected four additional analytes with
different functional groups (Figure 10). Sample preparation was
straightforward and the extraction of the RDCs was easy and
precise. For example, for strychnine we were able to extract 19
of 22 possible RDCs of the CLIP-HSQC-spectrum and received
excellent correlation of experimental and calculated data with a
Q-factor of 0.07 (See SI for more details and spectra). This low
value indicates an excellent fit of RDCs to the proposed
structure and highlights the high quality of data obtained.
18
N
16
20
10
1
17
Figure 8. Section of F1-coupled BIRD-filtered HSQC of an enantio-enriched
15
14
2
3
2
mixture of (+)/(-)-IPC (2:1) in PBPMLG/[D8]THF at 300 K (scaling factor of 8).
7
6
1
3
4
8
21
7
5
6
N
9
13
4
22
10
12
5
23
11
8
O
O
(-)-β-Pinene
(-)-Strychnine
12
10
8
7
6
9
10
11
1
2
9
O
2
5
3
1
7
8
NH
O
S
4
6
5
(-)-Camphor
O
(-)-10,2-Camphorsultam
Figure 10. Molecular structures of test analytes used for compatibility
experiments in NMR measurements.
Figure 9. Comparison between the RDCs of IPC in the pure samples versus
the enantio-enriched mixture.
Conclusions
The huge advantage of such an enantio-enriched mixture is that
all external influences are excluded as well as the environment
of each analyte is equal. Consequently, the comparison is
In summary, we successfully synthesised
L
- and
D
- version of
The
PBPMG and obtained stable LLC
phases.
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[6]
a) C. Aroulanda, M. Sarfati, J. Courtieu, P. Lesot, Enantiomer 2001, 6,
281-287; b) C. M. Thiele, J. Org. Chem 2004, 69, 7403-7413; c) S.
Hansmann, T. Larem, C. M. Thiele, Eur. J. Org. Chem. 2016, 2016,
1324-1329.
enantiodifferentiating power of the presented alignment medium
and its tuneability with concentration and temperature for IPC is
quite unique. We demonstrated that these properties also allow
investigation of mixtures of enantiomers. The outstanding
spectral quality along with the compatibility towards different
organic solutes is promising for future structure elucidation
challenges.
[7]
[8]
L. Arnold, A. Marx, C. M. Thiele, M. Reggelin, Chem. Eur. J. 2010, 16,
10342-10346.
a) N. C. Meyer, A. Krupp, V. Schmidts, C. M. Thiele, M. Reggelin,
Angew. Chem. Int. Ed. 2012, 51, 8334-8338; b) A. Krupp, M. Reggelin,
Magn. Reson. Chem. 2012, 50, 45-52.
In addition, the occurring Pake pattern like shape and its change
over time in the magnetic field reveals new aspects with respect
to the alignment process. Moreover, we are interested in the
origin of the observed reversal point. The temperature
dependent changes which seem connected with the
characteristic orientations known from the Pake pattern can be
seen as a sign of a special rearrangement within the sample.
Further insights in this behaviour could also contribute to a
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Funding by the DFG (FOR 1583) is gratefully acknowledged.
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Keywords: alignment medium • NMR spectroscopy • residual
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10.1002/chem.201802921
Chemistry - A European Journal
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Tune your RDCs! Lyotropic liquid
crystalline phases of poly-γ-p-
biphenylmethyl-glutamate are
presented as new versatile
Sharon Jeziorowski, Christina M.
Thiele*
Page No. – Page No.
enantiodifferentiating alignment
media for the structure elucidation
of small organic molecules. The
thermoresponsive properties allow
for RDC measurements of more
than one orientation within the
same sample.
Poly-γ-p-biphenylmethyl-glutamate
as Enantiodifferentiating
Alignment Medium for NMR-
Spectroscopy with Temperature
Tunable Properties
This article is protected by copyright. All rights reserved.