Temperature-triggered switchable helix-helix inversion of poly(phenylacetylene) bearing L-valine ethyl ester pendants and its chiral recognition ability

Article abstract of DOI:10.3390/molecules21111583

A phenylacetylene containing the L-valine ethyl ester pendant (PAA-Val) was synthesized and polymerized by an organorhodium catalyst (Rh(nbd)BPh₄) to produce the corresponding one-handed helical cis-poly(phenylacetylene) (PPAA-Val). PPAA-Val showed a unique temperature-triggered switchable helix-sense in chloroform, while it was not observed in highly polar solvents, such as N,N′-dimethylformamide (DMF). By heating the solution of PPAA-Val in chloroform, the sign of the CD absorption became reversed, but recovered after cooling the solution to room temperature. Even after six cycles of the heating-cooling treatment, the helix sense of the PPAA-Val's backbone was still switchable without loss of the CD intensity. The PPAA-Val was then coated on silica gel particles to produce novel chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). These novel PPAA-Val based CSPs showed a high chiral recognition ability for racemic mandelonitrile (α = 2.18) and racemic trans-N,N′-diphenylcyclohexane-1,2-dicarboxamide (α = 2.60). Additionally, the one-handed helical cis-polyene backbone of PPAA-Val was irreversibly destroyed to afford PPAA-Val-H by heating in dimethyl sulfoxide (DMSO) accompanied by the complete disappearance of the Cotton effect. Although PPAA-Val-H had the same L-valine ethyl ester pendants as its cis-isomer PPAA-Val, it showed no chiral recognition. It was concluded that the one-handed helical cis-polyene backbone of PPAA-Val plays an important role in the chiral recognition ability.

Full text of DOI:10.3390/molecules21111583

molecules
Article
Temperature-Triggered Switchable Helix-Helix
Inversion of Poly(phenylacetylene) Bearing L-Valine
Ethyl Ester Pendants and Its Chiral Recognition Ability
1
1,
1
1,
1
1
Yanli Zhou , Chunhong Zhang *, Yuan Qiu , Lijia Liu *, Taotao Yang , Hongxing Dong ,
2
1,3
Toshifumi Satoh and Yoshio Okamoto
1
Polymer Materials Research Center, Key Laboratory of Superlight Materials and Surface Technology,
Ministry of Education, College of Materials Science and Chemical Engineering,
Harbin Engineering University, Harbin 150001, China; zhouyanli0808022@163.com (Y.Z.);
wukong.s@163.com (Y.Q.); az106321@126.com (T.Y.); dhongxing@hrbeu.edu.cn (H.D.);
okamoto@apchem.nagoya-u.ac.jp (Y.O.)
Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan; satoh@eng.hokudai.ac.jp
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
Correspondence: zhangchunhong97@163.com (C.Z.); liulijia@hrbeu.edu.cn (L.L.);
Tel.: +86-451-8256-8191 (C.Z.); Fax:+86-451-8256-8191 (C.Z. & L.L.)
2
3
*
Academic Editor: Derek J. McPhee
Received: 10 October 2016; Accepted: 16 November 2016; Published: 21 November 2016
Abstract: A phenylacetylene containing the L-valine ethyl ester pendant (PAA-Val) was
synthesized and polymerized by an organorhodium catalyst (Rh(nbd)BPh ) to produce the
4
corresponding one-handed helical cis-poly(phenylacetylene) (PPAA-Val). PPAA-Val showed a unique
temperature-triggered switchable helix-sense in chloroform, while it was not observed in highly polar
0
solvents, such as N,N -dimethylformamide (DMF). By heating the solution of PPAA-Val in chloroform,
the sign of the CD absorption became reversed, but recovered after cooling the solution to room
temperature. Even after six cycles of the heating-cooling treatment, the helix sense of the PPAA-Val’s
backbone was still switchable without loss of the CD intensity. The PPAA-Val was then coated on
silica gel particles to produce novel chiral stationary phases (CSPs) for high-performance liquid
chromatography (HPLC). These novel PPAA-Val based CSPs showed a high chiral recognition ability
0
for racemic mandelonitrile (
α
= 2.18) and racemic trans-N,N -diphenylcyclohexane-1,2-dicarboxamide
(α = 2.60). Additionally, the one-handed helical cis-polyene backbone of PPAA-Val was irreversibly
destroyed to afford PPAA-Val-H by heating in dimethyl sulfoxide (DMSO) accompanied by the
complete disappearance of the Cotton effect. Although PPAA-Val-H had the same L-valine ethyl
ester pendants as its cis-isomer PPAA-Val, it showed no chiral recognition. It was concluded that
the one-handed helical cis-polyene backbone of PPAA-Val plays an important role in the chiral
recognition ability.
Keywords: helical poly(phenylacetylene); temperature-triggered; helix-helix inversion; chiral
stationary phases; cis-to-trans isomerization
1
. Introduction
Biomacromelocules, such as DNA and proteins, possess helical structures, which are known to
play key roles in nature and closely linked with specific activities in living systems [
by nature, a number of helical polymers, including polyisocyanates [ ], polyisocyanides [
polytriarylmethyl methacrylates [ ], etc. [10 12], have been synthesized and their properties widely
1,2
]. Inspired
3
,4
5–7],
8
,9
–
studied. One of the unique features of helical polymers is the helix-helix transition from the
one helical conformation to another which is induced by external stimuli, including solvent, salt
Molecules 2016, 21, 1583; doi:10.3390/molecules21111583
www.mdpi.com/journal/molecules

Molecules 2016, 21, 1583
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concentration, pH, temperature, or light irradiation. These switchable helical polymers have broad
applications in liquid crystals, data storage, and switching devices [10 13]. cis-Poly(phenylacetylene)
,
prepared by an organorhodium catalyst is a typical dynamic helical polymer [14]. Their helix-helix
inversions based on the dynamic cis-backbone were studied by Yashima, Freire, and other groups [15].
Generally, to realize this helix-helix inversion, an extraneous small molecule was necessary. However,
only a few studies have reported results regarding the switchable endogenous helix-helix inversion of
polyacetylene without the assistance of extraneous molecules.
Another attractive feature of optically-active poly(phenylacetylene)s is their chiral recognition
ability as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC).
Some high stereoregular helical poly(phenylacetylene)s bearing chiral pendants, synthesized utilizing
an organorhodium catalyst, have proved to be prospective CSPs for the HPLC separation of
enantiomers [16–20]. Amino acids are naturally-occurring building block and have been widely
applied in polymer science. Kakuchi reported an anion sensor based on poly(phenylacetylene)s
bearing the L-leucine methyl ester [21]. Tang investigated the hierarchical structures of amphiphilic
poly(phenylacetylene)s bearing L-valine pendants [22]. In our previous studies, we reported that
the CSP based on poly(phenylacetylene)s bearing amino acid pendants, such as L-phenylalanine,
L-phenylglycine and L-leucine pendants, that showed good chiral recognition abilities [19,20,23]. It was
determined that as the linkage group between the polyene main chain and the chiral pendant amide
is effective for improving the chiral recognition of the poly(phenylacetylene)s; the polymers having
an amide linkage group showed a higher chiral recognition than the ones having urea or sulfonamide
linkage groups [19,23]. However, there exist no reports regarding the chiral recognition ability of
poly(phenylacetylene)s bearing L-valine pendants as CSPs, and the chiral recognition mechanism of
these helical poly(phenylacetylene)s bearing amino acid pendants as CSPs, especially the contributions
of their one-handed helical cis-polyene backbone to the chiral recognition ability, is still not very clear.
In this study, a novel one-handed helical cis-poly(phenylacetylene), in which the L-valine ethyl
ester pendants are linked with the polyene main chain by an amide group (PPAA-Val), was designed
and synthesized, and the effects of solvent and temperature on their helical structure and CD signals are
discussed. To evaluate the chiral recognition ability of PPAA-Val and its potential for chiral separation,
PPAA-Val was coated on silica gel particles to produce a novel type of CSP for HPLC and a batch of
chiral separations occurred on these CSPs. In addition, PPAA-Val was transformed into PPAA-Val-H
by heat treatment [24–30], and the contribution of the one-handed helical cis-structure of PPAA-Val for
its chiral recognition was discussed.
2
. Results and Discussion
2
.1. Synthesis and Polymerization of PAA-Val
Scheme 1 illustrates the synthetic and polymerization routes for PAA-Val. The amidation
reaction of 4-ethynylbenzoic acid with the L-valine ethyl ester proceeded by using 4-(4,6-dimethoxy-
,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM) as a condensation agent in a good yield
to produce the novel phenylacetylene, PAA-Val. The polymerization of PAA-Val was carried out in
1
tetrahydrofuran (THF) at room temperature for 24 h using Rh(nbd)BPh as the catalyst to produce
4
5
PPAA-Val as a yellow solid in 76.8% yield. The M
n
and M
w
/M
n
of PPAA-Val were 1.1
×
10 and
0
2
.23, respectively. PPAA-Val was soluble in CHCl , MeOH, acetone, THF, N,N -dimethylformamide
3
(
DMF), N,N’-dimethylacetamide (DMAc), and partially soluble in dimethyl sulfoxide (DMSO) at room
−
1
temperature. As shown in Figure 1, the absorption bands at 3259 and 2108 cm associated with the
C-H and C C stretching vibrations of PAA-Val were not observed in the spectra of its polymer
≡
≡
(
PPAA-Val) (Figure 1b), suggesting that the acetylene triple bond of PAA-Val was consumed by the
1
polymerization. The H-NMR spectrum of PPAA-Val showed the characteristic signal of the cis-proton
at 5.75 ppm, indicative of the propensity of the cis-configuration in the poly(phenylacetylene) main
chain (Figure 2a) [21].

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Scheme 1. Synthesis of poly(phenylacetylene) derivative PPAA-Val.
(a)
C C
C H
(b)
4000
3500
3000
2500
2000
1500
1000
500
-
1
Wavenumber (cm )
Figure 1. IR spectra of (a) monomer PAA-Val and (b) its polymer PPAA-Val.
DMSO-d6
Cis-configuration
main chain
(a)
(b)
9
8
7
6
5
4
3
2
1
0
Chemical Shift (ppm)
Figure 2. 1H-NMR spectra of (
a
◦
) PPAA-Val and (b) PPAA-Val-H in DMSO-d at 80 C (the polymers
6
◦
are soluble in DMSO-d at 80 C).
6

Molecules 2016, 21, 1583
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2
.2. Secondary Structure of PPAA-Val
The specific rotations ([ 2
α
0
]) of PAA-Val and its polymer PPAA-Val in various solvents at room
D
20
temperature are summarized in Table 1. The [
of its corresponding monomer in all the solvents; e.g., the [
thirty-fold greater than that of PAA-Val (+10.3 ). This means that the high optical activity of PPAA-Val
is not only from its chiral pendants. Since a one-handed helical polymer chain can exhibit a very high
α ] values of PPAA-Val were much higher than those
D
2
0
◦
α ] of PPAA-Val in MeOH (+334 ) was
D
◦
optical activity [31,32], the one-handed helicity in the polymer is suspected to be the reason for the
higher optical activity of the polymer.
Table 1. Specific optical rotations of PAA-Val and PPAA-Val in different solvents.
2
0
◦ a
]/
[
α
D
Compound
CHCl3
MeOH DMF
DMAc
THF
Acetone
PAA-Val
PPAA-Val
a
+57.6
+433
+10.3
+334
+40.6
+515
+40.0
+453
+25.3
+557
+39.3
+457
Measured by polarimetry at room temperature (c = 1 mg/mL).
To confirm the helical structure of the PPAA-Val, its chiroptical properties were investigated in
◦
different solvents at 25 C by CD spectroscopy (Figure 3). The polymer showed similar split Cotton
effects in MeOH, acetone, THF, DMF, DMAc, and CHCl in the range from 300–500 nm, i.e., the first
3
positive Cotton effect at 375 nm and the second negative Cotton effect at 320 nm, indicating the
one-handed helical main chain of PPAA-Val [33]; in other words, the one-handed helical PPAA-Val
bearing L-valine ethyl ester pendants was successfully synthesized.
The induced helical sense in the backbone of PPAA-Val by the chiral pendants was thought to
be maintained by the intramolecular hydrogen bonding and steric repulsion between the chiral side
chains [34]. Generally, a highly polar solvent can weaken the intra- and/or inter- molecular hydrogen
bonds, and decrease the stability of the induced helicity in the polymer [35,36]. However, PPAA-Val
showed stronger CD absorptions in the highly polar solvents, such as DMF and methanol, than those
◦
in chloroform and THF at 25 C. It is possible that the interior stress in the polymer may relax in the
highly polar solvents which can weaken the intramolecular hydrogen bonding between the adjacent
side chains.
30
20
10
0
DMF
MeOH
Acetone
THF
DMAc
CHCl3
-10
-20
-30

1
8
6
4
2
0
0

300
350
400
450
500
550
Wavelength (nm)
◦
Figure 3. CD (upper) and UV-Vis (lower) spectra of PPAA-Val in variable solvents at 25
c = 1 mg/mL).
C
(

Molecules 2016, 21, 1583
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These speculations have been supported by the FT-IR and UV analyses. The intramolecular
hydrogen bonding in PPAA-Val was investigated based on the IR spectra of PAA-Val and PPAA-Val
in the solid- and liquid-states (Table 2). In the solid state, both PAA-Val and PPAA-Val exhibited the
−
at 1644 cm , indicating that the hydrogen-bonded amide groups were formed. In CHCl ,
C=O 3
1
amide
ν
− −1
of the amide group at 1648 cm , which was 14 cm lower than that of
C=O
1
PPAA-Val exhibited the
PAA-Val (1662 cm ). On the other hand, PAA-Val and PPAA-Val exhibited the
ν
−
1
ν
of the ester group
C=O
at a similar wavenumber in both the solid state and their solution (Table 2). These results suggested
that, in CHCl , the intramolecular hydrogen bonding was formed between the amide groups in the
3
valine ethyl ester side chains, while the ester group had no contribution to the hydrogen bonding.
The UV-VIS absorptions of PPAA-Val in different solvents are shown in Figure 3. In DMF and MeOH,
the absorption of the polyene backbone of PPAA-Val appeared at a higher wavelength (around 420 nm)
than those in CHCl (around 390 nm). This may be associated with the fact that the longer conjugated
3
length in the polyene backbone in the more polar solvent is shortened by the intramolecular hydrogen
bonding in the polymer than in a nonpolar solvent [37].
Hence, in chloroform, the intramolecular hydrogen bonding in the polymer may disturb the
chiral induction to the main chain by the chiral valine ethyl ester pendants and may cause the lower
regularity of the helical structure of PPAA-Val. In the more polar solvents, the interior stress in the
polymer is relaxed due to the breaking of the intramolecular hydrogen bonding; therefore, the chirality
in the polyene backbone can be efficiently induced to afford the stronger CD absorptions of PPAA-Val.
a
Table 2. νC=O of PAA-Val and PPAA-Val measured in CHCl and solid state .
3
Wavenumber (cm 1)
−
Sample
Condition
ν
(Amide) (Ester)
ν
C=O
C=O
CHCl3
solid
1662
1644
1735
1734
PAA-Val
CHCl3
solid
1648
1644
1735
1734
PPAA-Val
a
IR spectra were taken at room temperature in solution (20 mM) or in solid state.
2
.3. Temperature-Triggered Helix-Helix Inversion of PPAA-Val
To investigate the thermostability of the helix sense of PPAA-Val in different solvents, the CDs
of PPAA-Val were separately measured in CHCl and DMF at different temperatures (Figure 4).
3
Interestingly, in chloroform, PPAA-Val showed CD signals with opposite signs at the low and high
◦
temperatures. At
−
10 C, PPAA-Val showed a negative first Cotton effect at around 365 nm and
a positive second Cotton effect at a wavelength around 310 nm, and the signs of these CD signals were
opposite to the CD signals measured in DMF (Figures 3 and 4). With the increasing temperature, the CD
intensities initially decreased, then the CD signals became inverted to the ones with the opposite signs,
and the CD signals’ pattern at high temperature became the same as the CD in DMF (Figure 5a,b).
Figure 4b shows the temperature dependence of the first Cotton effect at 365 nm in chloroform.
◦
From
−
10–10 C, the intensity of the negative [
θ
]365 value gradually decreased, then sharply inverted
◦
to be positive from 10–30 C. The positive [
the solution to 50 C. Since the Cotton effect around 365 nm is due to the one-handed helical main
chain conformation of PPAA-Val, the inversion of the CD from
the inversion of the helix-sense of the main chain. In addition, this temperature-triggered helix-helix
θ]365 value then slightly decreased by further heating
◦
◦
−
10–30 C in chloroform indicates
inversion in the polyene main chain of PPAA-Val was reversible and switchable, even after six cycles
◦
of thermocycling (
−
10–30 C) (Figure 5). In summary, a temperature-triggered switchable helix-helix
inversion of PPAA-Val was realized in chloroform without the assistance of any other extraneous
compounds. However, this interesting temperature-triggered inversion of the CD signals of PPAA-Val
was not observed in DMF, and the CD intensity only showed a one-way decrease in DMF with the
increasing temperature (Figure 4c).

Molecules 2016, 21, 1583
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Generally, hydrogen bonding is strengthened at low temperature and weakened at high
temperature [35 38 39]. Compared to the relaxed structure of PPAA-Val in DMF due to the weakened
,
,
intramolecular hydrogen bonding, in chloroform, the low temperature strengthened the intramolecular
hydrogen bonding and caused the tight hydrogen-bonding strands of the side chains in the polymer.
These might afford the different spatial positions of chiral pendants and the opposite helix sense
compared to those in DMF. With the increasing temperature, the intramolecular hydrogen bonding
was weakened and the inner stress in the polymer chain was relaxed, so that the spatial positions of
the side chains and the main chain helix recovered to be the same as the relaxed state in DMF. When
the solution was cooled again, the intramolecular hydrogen bonds recovered and the helix inverted
to be the same as the original direction. This is a unique temperature-triggered helix-helix inversion
based on the dynamic polyene backbone of PPAA-Val without the co-existence of any other extraneous
molecules. The one-way decrease of the CD in DMF with the increasing temperature should be caused
by the violent thermo motion of the molecules at high temperature, which caused the irregularity of
the polymer’s helical structure.
(a)
60
50
40
30
20
10
0
2
2
o
o
21
30
C
(b)
2
0
9
40 C
1

18
20
10
o
5
0 C
1
7
6
1
360
365
370
375
380
385
390
Wavelength (nm)
o
2
1
0 C
o
0
5 C

-
10
20
30
40
o
-10
C
o
0
1
C
-10
-20
-30
-40
-
-
-
o
o
o
o
o
o
o
0
C
o
15
C
C
C
C
C
C
-
10 C
20

25
12
30
40
50
1
8
6
4
2
0
-10
0
10
20
30
40
50
o
Temperature ( C)

300
350
400
450
500
550
Wavelength (nm)
(
c)
4
4
3
2
1
0
o
-
10
C
o
0
C
o
o
o
o
o
o
o
o
3
2
1
10
C
2
3
4
5
6
7
8
0
0
0
0
0
0
0
C
C
C
C
C
C
C

-11
-
22
33
44
-
-
10
8
6
4
2
0

3
00
350
400
450
500
550
Wavelength (nm)
Figure 4. Temperature dependence of CD and UV-VIS spectra of PPAA-Val in CHCl3 (
a); plots of [
θ]365
of PPAA-Val in CHCl versus temperature (
b); and CD and UV-VIS spectra of PPAA-Val in DMF at
3
different temperature (c) (c = 1 mg/mL).

Molecules 2016, 21, 1583
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40
30
20
10
0
o
3
0 C
-10
-20
-30
-40
-50

o
C
-
10
0
1
2
3
4
5
6
Cycle (times)
Figure 5. Plots of the [
θ
]365 values of PPAA-Val in chloroform upon continuous thermocycling
◦
(
−10–30 C) in CHCl (c = 1 mg/mL).
3
2
.4. Chiral Recognition Ability of PPAA-Val
Since PPAA-Val not only has chiral pendants, but also a one-handed helical polyene backbone,
the polymer was expected to be effective for chiral recognition. Its chiral recognition ability was
evaluated as CSPs by HPLC using the eight tested racemates (Figure 6). Since PPAA-Val showed
different chiroptical properties in different solvents, DMF, THF, and CHCl were separately selected
3
as the coating solvent to coat the PPAA-Val on silica gel. The influences of the coating solvents on
the resolutions of the racemates on PPAA-Val were evaluated by HPLC, and the coating solvents
clearly influenced the chiral recognition abilities of the PPAA-Val-based CSPs (Table 3). Figure 7
shows the chromatograms for racemates
are completely separated with the high separation factors of 2.18 for
2
and
6
by the CSP coated with DMF, and their enantiomers
and 2.60 for . Therefore,
2
6
a novel CSP based on the one-handed helical poly(phenylacetylene) having L-valine ethyl ester
pendants has been successfully prepared and showed a good chiral recognition ability in HPLC.
However, our previous CSPs based on the helical poly(phenylacetylene)s bearing L-phenylalanine,
L-phenylglycine or L-leucine pendants did not show any chiral recognition for racemates
2 and 6,
although they showed chiral recognitions for the other racemates [19 20 23]. The novel PPAA-Val-based
,
,
CSPs can be complementary with the our previous poly(phenylacetylene)s-based CSPs.
In addition, the chiral recognition abilities of the PPAA-Val-based CSPs coated with THF and
CHCl were lower than the CSPs coated with DMF (Table 3). The higher chiral recognition ability
3
of the CSP coated with DMF should be correlated to the stronger Cotton effects, i.e., the higher
◦
one-handedness of its cis-backbone of the polymer in DMF at the coating temperature (25 C), and this
more one-handed helical structure may be retained in the CSP which may result in the higher chiral
recognition ability.
Figure 6. Structures of racemates.

Molecules 2016, 21, 1583
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Table 3. Resolution of racemates on PPAA-Val and PPAA-Val-H a.
PPAA-Val
PPAA-Val-H
Coating solvent:
DMF
Coating Colvent:
THF
Coating Solvent:
CHCl3
Coating Solvent:
DMF
Racemates
1
0
0
0
0
k1
α
k1
α
k1
α
k1
α
0.19
1.00
2.18
1.00
1.00
~1
2.60
1.00
1.00
0.05
0.13 (−)
0.18
1.00
2.15
1.00
1.00
~1
2.25
1.00
1.00
0.13
0.24
0.33
0.84
4.33
1.00
1.00
1.00
1.00
1.00
2.01
1.00
1.00
0.11
1.14
0.36
0.99
5.80
0.92
1.99
1.19
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
2
3
4
5
6
7
8
0.44 (−)
0.65
1.67
0.58
13.54 (+)
0.36 (−)
4.83
13.12 (+)
0.12 (−)
1.58
0.29 (−)
2.18
2.26
0.67
1.12
a
Column: 25 cm
×
0.2 cm i.d.; Eluent:hexane-2-propanol (90/10, v/v); Flow rate: 0.1 mL/min; The signs in
parentheses represent the optical rotation of the first-eluted enantiomer.
(b)
(a)
t1 (-)
6
t1 (-)
O
C
H
N
t2 (+)
2
OH
C
O
N
H
t2 (+)
CN
t0
t0
0
5
10
15
20
25
0
5
10
15
20
25
30
35
40
Eluent time (min)
Eluent time (min)
Figure 7. Chromatograms for the resolution of (a) 2 and (b) 6 on PPAA-Val coated with DMF with
hexane/2-propanol (90/10) as eluent.
2
.5. Effects of One-Handed Helical cis-Polyene Main Chain of PPAA-Val on its Chiral Recognition Ability
To investigate the influence of the regularity and configuration of the helical polyene backbone in
poly(phenylacetylene) on their chiroptical properties and chiral recognition ability, a DMSO solution of
◦
PPAA-Val was heated to 150 C for 10 min under nitrogen to destroy its one-handed helical cis-polyene
backbone to afford PPAA-Val-H. After the heat treatment, the color of the polymer solution changed
1
from orange to red brown. In the H-NMR of the PPAA-Val, the signal of the cis-proton in PPAA-Val
1
at 5.75 ppm disappeared after the heat treatment, and the sharp H-NMR signals of the main chain
became broader along with a decreasing peak intensity (Figure 2) [40,41]. These results suggest that
during heat treatment, the one-handed helical cis-polyene backbone of PPAA-Val collapsed. The M
n
5
4
and M
w
/M
n
of PPAA-Val-H were 3.2
×
10 and 1.98, respectively. The decreased M
n
(from 1.1
×
10
4
to 3.2 × 10 ) indicates that degradation of the polymer occurred by the heat treatment [28].
After the heat treatment of PPAA-Val with the cis-polyene backbone, the CD absorptions
completely disappeared, although the polymers had the same chiral pendants. This indicated the
preferred one-handed helical structure of PPAA-Val irreversibly collapsed after the heat treatment.
As shown in the lower part of Figure 8, the broad band up to 500 nm due to the π-π* transition of
the main chain of PPAA-Val weakened after the heat treatment. Therefore, it is concluded that the
weakened absorption should be caused by the cleavage or deflection of the main chain and PPAA-Val-H
lost its coplanar property of the conjugated double bonds [28,42].

Molecules 2016, 21, 1583
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30
20
10
0
PPAA-Val
PPAA-Val-H
-10
-20
-30
-40

10
8
6
4
2
0

300
350
400
450
500
550
Wavelength (nm)
◦
Figure 8. CD and UV-VIS spectra of PPAA-Val and PPAA-Val-H measured in DMF at 25
c = 1 mg/mL).
C
(
The chiral recognition ability of the CSP coated by PPAA-Val-H with DMF was also evaluated
using the tested racemates by HPLC, however, the PPAA-Val-H based CSP had no recognition
ability for all the tested racemates although it took the same chiral pendants with the cis-PPAA-Val
(Table 3). These results indicated that as the CSP, the regular one-handed helical cis-polyene backbone
of PPAA-Val played key roles in the chiral recognition by HPLC. In summary, a regular helical
cis-backbone is essential for the high chiral recognition ability of the poly(phenylacetylene) derivatives.
3
. Materials and Methods
3
.1. Materials
L-Valine (purity 99%) and 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride
(DMT-MM) (purity 98%) were purchased from Sahn Chemical Technology Co., Ltd. (Shanghai, China).
Hydrochloride in ethanol (30%–40%) was purchased from Chengdu Xiya Chemistry Technology Co.,
Ltd. (Chengdu, China). Triphenylphosphine (purity 99%) was purchased from J and K Chemical
Co., Ltd. (Beijing, China). 4-Ethynylbenzoic acid was synthesized according to a previously reported
+
6
method [43]. Rh (2,5-norbornadiene)((
a previous report [44]. All of the solvents used in the reactions were of analytical grade, carefully dried,
and distilled before use. Silica gel with a mean particle size of 37–56 m for column chromatography
was purchased from Qingdao Haiyang Chemical Co., Ltd. (Qingdao, China). The porous spherical
silica gel with a mean particle size of 7 m and a mean pore diameter of 100 nm (Daiso gel
η -C H )B(C H ) ) [Rh(nbd)BPh ] was prepared based on
6 5 6 5 3 4
µ
µ
SP-1000-7) for HPLC was kindly supplied by Daiso Chemicals (Osaka, Japan), then silanized with
◦
(
3-aminopropyl)triethoxysilane in toluene at 80 C before use. All of the solvents used in the
preparation of the chiral stationary phases were of analytical grade. Hexane and 2-propanol used in
chromatographic experiments were of HPLC grade. The racemates were commercially available or
were prepared by the usual methods.
3
.2. Instruments
The H- and 13C-NMR spectra (500 MHz) were recorded using a Bruker AVANCE III-500
1
(Fällanden, Switzerland) instrument at room temperature. The IR spectra were obtained using
a Perkin-Elmer FTIR-100 spectrophotometer (Fremont, CA, USA). The number-average molecular

Molecules 2016, 21, 1583
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n w w n
weight (M ), the weight-average molecular weight (M ), and the polydispersity (M /M ) of
the polymers were determined by size exclusion chromatography (SEC) calibrated with standard
◦
polystyrenes at 40 C using a JASCO SEC system (Tokyo, Japan) (PU-980 Intelligent pump, CO-965
column oven, RI-930 Intelligent RI detector, and Shodex (Tokyo, Japan) DEGAS KT-16) equipped
with a Shodex Asahipak GF-310 HQ column (linear, 7.6 mm × 300 mm; pore size, 20 nm; bead size,
4
5
µ
m; exclusion limit, 4
×
10 ) and a Shodex Asahipak GF-7 M HQ column (linear, 7.6 mm × 300 mm
;
7
pore size, 20 nm; bead size, 9
0.01 M) at the flow rate of 0.4 mL/min. The optical rotation was measured at room temperature
µ
m; exclusion limit, 4
×
10 ) in DMF containing lithium chloride
(
using a Perkin-Elmer Model 341 polarimeter (Fremont, CA, USA). The circular dichroism (CD) and
ultraviolet visible (UV-VIS) spectra were measured in a 1-mm path length cell using a JASCO J-815
spectropolarimeter (Tokyo, Japan). All the enantioseparation experiments were performed using
a JASCO PU-2089 high performance liquid chromatograph (HPLC) system (Tokyo, Japan) equipped
with UV-VIS (JASCO-UV-2070) and circular dichroism (JASCO-CD-2095) detectors. A solution of
a racemate (3 mg/mL) was injected into the chromatographic system through an intelligent sampler
(JASCO AS-2055).
3
.3. Synthesis of L-Valine Ethyl Ester
The L-valine ethyl ester was synthesized via the esterification reaction of the L-valine in
a hydrochloride ethanol solution. A typical procedure is described as follows: L-Valine (8.00 g,
8.0 mmol) and hydrochloride in ethanol (1.50 mol/L, 160 mL) were added to a 500 mL round-bottomed
flask. The mixture was refluxed with stirring for 12 h and turned yellow. It was then cooled to room
temperature, and extracted with a saturated NaHCO aqueous solution and DCM. The organic layer
6
3
was dried using MgSO and filtered. The filtrate was evaporated to remove the DCM to give the
4
1
L-valine ethyl ester as a brown-yellow liquid. Yield: 7.63g (81.5%). H-NMR (500 MHz, CDCl , TMS,
3
ppm):
δ
= 4.20-4.00 (q, 2H, -CH2-O-), 3.17–3.10 (d, 1H, -CH-NH ), 2.00–1.85 (m, 1H, -CH-(CH ) ),
2 3 2
1
.35 (s, 2H, -NH ), 1.20–1.10 (t, 3H, -O-CH -CH ), 0.88–0.79 (dd, 6H, -CH-(CH ) ).
2
2
3
3 2
3
.4. Synthesis of N-(4-Ethynylbenzoyl)-L-Valine Ethyl Ester (PAA-Val)
The N-(4-ethynylbenzoyl)-L-valine ethyl ester (PAA-Val) was synthesized via the amidation
reaction between 4-ethynylbenzoic acid and L-valine ethyl ester. A typical procedure is described as
follows: To a solution of the 4-ethynylbenzoic acid (3.85 g, 26.3 mmol) and L-valine ethyl ester (3.82 g,
26.3 mmol) in MeOH (132 mL) was added the DMT-MM (7.99 g, 28.9 mmol). After stirring at room
temperature for 18 h, the reaction mixture was purified by column chromatography on silica gel with
1
hexane/ethyl acetate (3/1, v/v) to give PAA-Val as a white solid. Yield: 5.36 g (74.5%). H-NMR
(
500 MHz, CDCl , TMS, ppm):
δ
= 7.80–7.76 (d, 2H, Ar-
H
), 7.54–7.52 (d, 2H, Ar-
H
), 6.80–6.79 (d, 1H,
3
-
N
H
-), 4.78–4.70 (q, 1H, NH-C
H
-), 4.35–4.15 (q, 2H, -O-CH2-), 3.24 (s, 1H,
≡
CH), 2.29–2.20 (m, 1H,
13
-CH-(CH ) ), 1.40–1.20 (t, 3H, -O-CH -CH3), 1.02–0.85 (dd, 6H, -CH-(CH ) ). C-NMR (125 MHz,
3 2 2 3 2
CDCl , TMS, ppm):
1
δ
= 174.5 (-CO- (amido)), 155.0 (-CO-(ester)), 138.7 (aromatic), 132.5 (aromatic),
16.9 (aromatic), 113.0 (aromatic), 82.6 (-C CH), 78.2 (-C CH), 61.4(-CH -CH ), 52.6 (-CH-NH), 24.5
-CH), 22.3 (-CH ), 21.7 (-CH ), 14.2 (-CH2-CH ). IR (cm , KBr): 3342 (N-H), 3259 (
3
≡
≡
1
2
3
−
(
(
≡
C-H), 2108
3
3
3
C≡C), 1733 (C=O), 1644 (C=O).
.5. Polymerization
The polymerization of PAA-Val was carried out in a dry Schlenk flask under a dry nitrogen
3
atmosphere using Rh(nbd)BPh as the catalyst in similar way as previously reported [19 21 45].
A typical procedure is described as follows: PAA-Val (2.00 g, 7.32 mmol) was weighed into a Schlenk
flask and dissolved in dry THF (220 mL) before a solution of Rh(nbd)BPh (75.3 mg, 146 mol)
, ,
4
µ
4
in dry tetrahydrofuran (THF, 24.0 mL) was added. After stirring at room temperature for 24 h,
triphenylphosphine (75.3 mg, 0.292 mmol) was added to the reaction mixture. The solution was
concentrated, then poured into a large amount of hexane (2000 mL). The precipitates were purified by

Molecules 2016, 21, 1583
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reprecipitation using hexane, then dried under reduced pressure to give PPAA-Val as a yellow solid
5
1
(
(
4
(
1.54 g, 76.8%). M
d, 1H, -N -), 7.48 (d, 2H, Ar-
.05 (q, 2H, -O-CH2-), 2.11 (m, 1H, -C
n
= 1.10
×
10 ; M
), 6.68 (d, 2H, Ar-
-(CH ) ), 1.11 (t, 3H, -CH -CH3), 0.88 (dd, 6H, -CH-(CH
3 2
w
/M
n
= 2.23. H-NMR (500 MHz, DMSO-d , TMS, ppm):
δ
= 7.74
), 5.75 (s, 1H , main chain), 4.30 (q, 1H, NH-C -),
) ). IR
6
H
H
H
H
H
3 2
2
−
1
cm , KBr): 3341 (N-H), 1734 (C=O), 1644 (C=O).
3
.6. Heat Treatment of PPAA-Val
A typical procedure is described as follows: In a nitrogen atmosphere, PPAA-Val (0.3 g) was
weighed into a flask and partially dissolved in dry dimethyl sulfoxide (DMSO, 30 mL). After stirring
◦
at 150 C for 10 min, the mixture was cooled to room temperature, then the solution was poured into
a large amount of deionized water. The precipitate was purified by reprecipitation with diethyl ether,
then dried under reduced pressure to give PPAA-Val-H as a yellow solid.
3
.7. Preparation of Chiral Stationary Phases (CSPs)
The poly(phenylacetylene) derivatives (PPAA-Val and PPAA-Val-H) (0.2 g each) were first
dissolved in a coating solvent (5 mL), then coated on aminopropyl silanized silica gel (0.8 g) according
0
to a previous method [46]. The coating solvents were THF and N,N -dimethylformamide (DMF)
and the weight ratio of the polymers to silica gel was 1:4. The polymer-coated silica gels were then
packed in a stainless-steel tube (25 cm
packed columns were 1600–3500 for benzene using a hexane/2-propanol (90/10, v/v) mixture as the
eluent at the flow rate of 0.1 mL/min at 25 C. The dead time (t ) of the columns was estimated using
×
0.20 cm i.d.) by a slurry method. The plate numbers of the
◦
0
1
,3,5-tritert-butylbenzene as the non-retained compound [47].
3
.8. HPLC Measurement
The retention factor, k1 = ((t1
0
−
t )/t ), is the factor indicating the interaction strength between
0 0
a CSP and the corresponding enantiomer and can be obtained from its elution time t and the dead
1
0
time t . The k1 values varied with the coating solvents of the polymers. The separation factor
α
,
0
which is directly correlated to the chiral recognition ability of the CSPs, is an important factor for
evaluating the CSPs. If is equal to 1.00, this means no chiral recognition, and the higher the value,
the better the chiral recognition ability of the CSPs.
α
α
4
. Conclusions
A phenylacetylene linked with an L-valine ethyl ester group through an amide linkage was
synthesized and polymerized to produce a novel one-handed helical cis-poly(phenylacetylene)
derivative, PPAA-Val. PPAA-Val showed a unique temperature-triggered helix-helix inversion
in chloroform without the assistance of extraneous molecules, while it was not observed in
a highly polar solvent, such as DMF. This helix-helix inversion is reversible, even after six cycles
of a heating-cooling treatment, and the helix sense of the PPAA-Val backbone was still switchable
without any loss of the CD intensity. A novel CSP for HPLC prepared by coating PPAA-Val on silica
gel particles showed a high chiral recognition ability for racemic mandelonitrile (
α
= 2.18) and racemic
0
trans-N,N -diphenylcyclohexane-1,2-dicarboxamide (
α
= 2.60). The coating solvent clearly influenced
the chiral recognition ability of the CSP; the CSP, coated using DMF as the coating solvent, showed the
best chiral recognition. The one-handed helical cis-polyene backbone of PPAA-Val was destroyed by
◦
heat treatment at 150 C to afford PPAA-Val-H. Although PPAA-Val-H had the same L-valine ethyl
ester pendants as PPAA-Val, PPAA-Val-H did not show any chiral recognition. It was concluded that
the one-handed helical cis-polyene backbone of the polymer plays a key role in its chiral recognition.
In addition, the temperature triggered helix-helix inversion of PPAA-Val may be affected by the
molecular weight of the polymer. The research regarding the dependence of the molecular weight on
the helix-helix inversion of PPAA-Val will be studied in the near future.

Molecules 2016, 21, 1583
12 of 14
Acknowledgments: This work was financially supported by the National Nature Science Foundation of China
(
51373044, 21576060 and 21574033), National Natural Science Foundation of Heilongjiang province, China
LC201404 and E2016019), and the Fundamental Research Funds for the Central Universities, China (HEUCFQ1417,
(
HEUCF20161004). This work was also partially supported by Daicel Corporation (Tokyo, Japan). The authors are
grateful for the support.
Author Contributions: Chunhong Zhang, Lijia Liu and Yoshio Okamoto conceived and designed the experiments;
Taotao Yang, Yuan Qiu and Yanli Zhou performed the experiments; Yuan Qiu and Yanli Zhou analyzed the data;
Hongxing Dong and Toshifumi Satoh contributed analysis tools; Yanli Zhou and Yuan Qiu wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
©
2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).