Transformation of helical sense of poly(N-propargylamides) controlled by competition between structurally different enantiomeric amino acids

Article abstract of DOI:10.1021/ma048834f

Copolymerizations of structurally different chiral amino acid-based N-propargylamides, N-(tert-butoxycarbonyl)-D-alanine-N-propargylamide (DA) with either N-(tert-butoxycarbonyl)-L-valine-N-propargylamide (LV) or N-(tert-butoxycarbonyl)-L-phenylalanine-N-

Full text of DOI:10.1021/ma048834f

8888
Macromolecules 2004, 37, 8888-8892
Transformation of Helical Sense of Poly(N-propargylamides) Controlled
by Competition between Structurally Different Enantiomeric Amino
Acids
Haichao Zhao, Fumio Sanda,* and Toshio Masuda*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University,
Katsura Campus, Kyoto 615-8510, Japan
Received June 13, 2004; Revised Manuscript Received September 13, 2004
ABSTRACT: Copolymerizations of structurally different chiral amino acid-based N-propargylamides,
N-(tert-butoxycarbonyl)-D-alanine-N-propargylamide (DA) with either N-(tert-butoxycarbonyl)-L-valine-
N-propargylamide (LV) or N-(tert-butoxycarbonyl)-L-phenylalanine-N-propargylamide (LF), were conducted
+
6
-
6 5 6 5 3
with (nbd)Rh [η -C H B (C H ) ] as a catalyst to obtain the corresponding copolymers with moderate
molecular weights in good yields. The specific rotation, CD, and UV-vis spectra showed that some of the
copolymers underwent a helix-helix transition driven by temperature change. The transition of helix of
poly(LV-co-DA) was much more obvious than that of poly(LF-co-DA). Poly(LV50-co-DA50) underwent a
solvent-induced helix-helix transition via random coil.
Introduction
n w
weight-average molecular weights (M and M ) of polymers
were determined by gel permeation chromatography (GPC) on
a Jasco Gulliver system (PU-980, CO-965, RI-930, and UV-
1570) equipped with polystyrene gel columns (Shodex columns
K804, K805, and J806), using THF as an eluent at a flow rate
of 1.0 mL/min, calibrated by polystyrene standards at 40 °C.
Melting points (mp) were measured on a Yanaco micro melting
Helix is a common higher order structure in many
biopolymers and stereoregular synthetic polymers, which
prefers one-handed screw sense when chiral moieties
are incorporated into the main or side chain.1 In
accordance with advance in well-ordered polymer syn-
thesis, several helical polymers have been successfully
D
point apparatus. Specific rotations ([R] ) were measured on a
2
Jasco DIP-1000 digital polarimeter with a sodium lamp as a
light source. CD and UV spectra were recorded in a quartz
cell (thickness: 1 cm) using a Jasco J-820 spectropolarimeter.
synthesized, which include polyacetylenes, poly(alkyl
3
4
5
methacrylates), polychloral, polyisocyanates, polyiso-
6
7
8
cyanides, polysilanes, and so forth. There are several
synthetic polymers capable of undergoing transitions of
screw sense by external stimuli, and they attract much
attention because materials with a two-state “on-off”
function can be potentially applied to the devices of
switches or data storage. The examples of helix inver-
Materials. CH
-[4,6-Dimethoxy-1,3,5-triazine-2-yl]-4-methylmorpholinium
chloride (TRIAZIMOCH) was supplied by Tokuyama Co. (nbd)-
2 2 2
Cl was distilled over CaH prior to use.
4
+
6
-
16
Rh [η -C H B (C H ) ] was prepared as reported. All other
6
5
6
5 3
regents were used as received without purification.
Monomer Synthesis. N-(tert-Butoxycarbonyl)-L-valine-
N-propargylamide (LV). N-(tert-Butoxycarbonyl)-L-valine
(10 g, 46 mmol) and propargylamine (2.57 g, 46 mmol) were
dissolved in AcOEt (250 mL), and the resulting solution was
stirred at room temperature for 10 min. TRIAZIMOCH (11.9
g, 46 mmol) was added to the solution, and the resulting
mixture was stirred at room temperature for 3 h. The mixture
9
sion include poly(L-aspartate â-esters), poly(aryl iso-
1
0
11
12
cyanates), polysilanes, polyacetylenes, and isocy-
anate copolymers consisting of competing chiral mono-
1
3
meric units. On the other hand, synthesis of amino
acid and peptide-containing polymers is a topic of much
interest due to their potential properties such as bio-
compatibility, biodegradability, and unique optical prop-
was subsequently washed with 1 N HCl(aq), saturated NaHCO
3
-
(
aq), and saturated NaCl(aq), then dried over anhydrous
1
4
erties based on their higher order structures. We have
recently reported that poly(N-propargylamides) bearing
alanine in the side chain form a helical structure, which
is stabilized by intramolecular hydrogen bonding be-
tween the amide groups. In this article, we report the
synthesis of copolymers of N-propargylamides composed
of structurally different enantiomeric amino acids. We
demonstrate that the copolymers can undergo transfor-
mation of helical sense upon external stimuli.
MgSO
4
, and concentrated by rotary evaporation. The residue
was purified by recrystallization from n-hexane and AcOEt to
D
obtain solid LV in 79% yield; mp 119 °C, [R] -9.5° (c ) 0.1
1
g/dL, in THF at room temperature). H NMR (400 MHz,
1
5
CDCl ): δ 0.82-0.92 (m, 6H, (CH ) ), 1.45 [s, 9H, (CH ) ], 2.13
3
3 2
3 3
[
3 2 2
s, 1 H, CH(CH ) ),], 2.21 (s, 1 H, CtCH), 4.06 (s, 2H, CH ),
4
.15 (s, 1 H, CHNH), 5.10 (s, 1H, NHCOO), 6.46 (s, 1 H,
1
3
NHCO). C NMR (100 MHz, CDCl
3
): δ 19.23 (CH
(CH ], 30.83 (CH NH), 49.85 (H
3 3
1.56 [C(CH ) ], 79.22 (HCt), 80.24 (HCtC), 155.83 (NH-
3 2
)
, 28.30
3
[
7
3
)
3
], 29.02 [(C(CH
)
3 2
2
CCH),
-
1
COO), 171.35 (CONH). IR (cm , KBr): 3067 (H-Ct), 2974,
2934, 2878, 1670 (CdO), 1541 (N-H, C-H), 1421, 1390, 1365,
Experimental Section
Measurements. H and 13C NMR spectra were recorded
3
in chloroform-d (CDCl ) on a JEOL EX-400 spectrometer. IR
spectra were measured on a Shimadzu FTIR-8100 spectro-
photometer. Elemental analysis was carried out at the Kyoto
University Elemental Analysis Center. The number- and
1
7
340, 1296, 1175, 1134, 1061, 1045, 1022, 928, 907, 871, 850,
1
20, 601, 538. Anal. Calcd for C13
22 2 3
H N O : C, 61.39; H, 8.72;
N, 11.01. Found: C, 61.26; H, 8.61; N, 11.00.
N-(tert-Butoxycarbonyl)-L-phenylalanine-N-propar-
gylamide (LF). The title compound was synthesized from
N-(tert-butoxycarbonyl)-L-phenylalanine instead of N-(tert-
butoxycarbonyl)-L-valine in a manner similar to LV in 44%
yield; mp 105 °C, [R]
at room temperature). H NMR (400 MHz, CDCl
9H, (CH ], 2.10 (s, 1H, CtCH), 3.09 (s, 2H, CHCH
2
D
-3.5° (c ) 0.1 g/dL measured in THF
*
Corresponding authors: Tel +81-75-383-2589; Fax +81-75-
83-2590; e-mail: sanda@ adv.polym.kyoto-u.ac.jp and masuda@
adv.polym.kyoto-u.ac.jp.
1
3
3
): δ 1.46 [s,
), 4.01 (s,
3
)
3
1
0.1021/ma048834f CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/27/2004

Macromolecules, Vol. 37, No. 24, 2004
Poly(N-propargylamides) 8889
Scheme 1
Scheme 2
Table 1. Copolymerization of LV with DAa
monomer feed yieldb
run
ratio LV:DA
(%)
Mnc
Mw/Mnc
[R]Dd (deg)
2
H, CH
2 3
NH), 4.37 (s, 1H, H CCHNH), 5.08 (s, 1H, NHCOO),
6
.23, (1H, NHCO), 7.06 (s, 1H, aromatic proton, para to CH
2
),
1
2
3
4
5
6
7
8
9
100:0
87.5:12.5
75:25
62.5:37.5
55:45
50:50
25:75
37.5:62.5
12.5:88
0:100
83
64
87
81
92
59
90
82
81
94
6 374
24 400
24 000
20 500
10 300
35 500
13 300
28 800
37 100
24 000
1.26
2.73
3.52
2.59
2.35
1.71
3.51
2.51
1.27
2.62
e
-1175
-1132
-744
7
.24 (m, 2H, aromatic protons, ortho to CH
2
), 7.31 (m, 2H,
1
3
aromatic protons, meta to CH
2
). C NMR (100 MHz, CDCl
NH), 38.47 [CH(CH
], 78.99 (HCt), 80.01 (HCtC), 126.93
aromatic carbon para to CH ), 128.63 (aromatic carbons, ortho
), 129.27 (aromatic carbons, meta to CH ), 136.61
aromatic carbon, attached to CH ), 156.13 (NHCOO), 170.93
CONH). IR (cm , KBr): 3335 (H-Ct), 1687 (CdO), 1655,
3
):
δ 28.25 [(CH
3
)
3
], 29.07 (CH
2
2
)], 55.36
(
CHCH
2
), 71.61 [C(CH
3
)
3
-166
(
2
+163
+655
+1070
+1213
+1230
to CH
2
2
(
(
2
-
1
10
1
5
6
445, 1389, 1366, 1269, 1089, 1048, 1026, 855, 758, 702, 669,
74, 515. Anal. Calcd: C, 67.53; H, 7.33; N, 9.26. Found: C,
7.47; H, 7.33; N, 9.37.
a
Conditions: [M]0 total ) 1.0 M in CH2Cl2, [M]0,total/[Cat] ) 50,
+
6
-
catalyst: (nbd)Rh [η -C6H5B (C6H5)3], nbd ) norbornadiene, 30
N-(tert-Butoxycarbonyl)-D-alanine-N-propargyl-
°C, 1 h. b n-Hexane-insoluble part. c Estimated by GPC (THF, PSt
d
e
amide (DA). The title compound was synthesized from N-(tert-
butoxycarbonyl)-L-alanine instead of N-(tert-butoxycarbonyl)-
L-valine in a manner similar to LV in 74% yield; mp 137 °C,
standards). c ) 0.10-0.11 g/dL in THF. Not determined because
of poor solubility.
1
Table 2. Copolymerization of LF with DAa
[
R]
D
) +8.3° (c ) 0.10 g/dL in THF at room temperature). H
NMR (400 MHz, CDCl
3
): δ 1.37-1.39 (m, 3H, CH
], 2.23 (s, 1H, CtCH), 4.05 (s, 2H, CH
CCHNH), 4.92 (s, 1H, NHCOO), 6.43, (1H, NHCO).
): δ 18.16 (CH ), 28.32 [(CH ], 29.14
CCH), 71.50 [C(CH ], 79.29 (HCt), 80.31
HCtC), 155.57 (NHCOO), 172.29 (CONH). IR (cm , KBr):
3
), 1.45 [s,
yieldb
(%)
monomer feed
ratio LF:DA
9
1
H, (CH
H, H
3
)
3
2
), 4.15 (s,
Mnc
Mw/Mnc
e
run
[R]D (deg)
13
3
C
1
2
3
4
5
6
7
8
9
100:0
87.5:12.5
75:25
62.5:37.5
50:50
45:55
37.5:62.5
25:75
12.5:87.5
91
75
76
86
88
82
75
74
72
d
d
d
-220
-43
NMR (100 MHz, CDCl
(
(
3
3
3 3
)
28 800
16 000
22 700
21 100
30 100
26 400
22 400
23 400
2.20
2.47
1.81
1.91
1.71
2.04
2.51
2.01
CH
2
), 49.85 (H
3
3 3
)
-
1
-22
3340 (N-H), 3297 (H-Ct), 3063, 3046, 2973, 1675 (CdO),
1453, 1389, 1368, 1302, 1266, 1229, 1173, 1123, 1080, 1040,
1024, 1003, 931, 874, 787, 756, 743, 691, 586, 522. Anal. Calcd
-7
+32
+67
for C11
H, 8.18; N, 12.39.
Co)polymerization Procedures. All the polymerizations
were carried out in a glass tube equipped with a three-way
stopcock under nitrogen. (nbd)Rh [η -C
added to a CH Cl solution of monomers under dry nitrogen,
and the resulting solution ([M]total ) 1.0 M, [M]0,total/[M]cat
18 2 3
H N O : C, 58.37; H, 8.02; N, 12.38. Found: C, 58.32;
+610
+1033
(
a
Conditions: [M]0,total ) 1.0 M in CH2Cl2, [M]0 total/[Cat] ) 50,
+
6
-
+
6
-
catalyst: (nbd)Rh [η -C6H5B (C6H5)3], nbd ) norbornadiene, 30
C, 1 h. n-Hexane-insoluble part. Estimated by GPC (THF, PSt
standards). Not determined because of poor solubility. c ) 0.10-
.11 g/dL in THF.
6 5 6 5 3
H B (C H ) ] was
b
c
°
2
2
d
e
)
0
5
0) was kept at 30 °C for 1 h. The resulting solution was
poured into a large amount of n-hexane to precipitate a
polymer. It was separated by filtration using a membrane filter
the same as the monomer feed ratios. The copolymers
were soluble in CH2Cl2, CHCl3, and THF, while the
homopolymers of LV and LF were partly soluble in these
solvents.
(ADVANTEC H100A047A) and dried under reduced pressure.
Results and Discussion
Monomer Synthesis. Scheme 1 illustrates the syn-
thetic routes for the monomers used, LV, LF, and DA.
They were prepared by the reaction of propargylamine
with the corresponding BOC-protected amino acids
using TRIAZIMOCH as a condensing agent, as shown
The secondary structure of the copolymers in THF
solution was examined by polarimetry and CD spec-
troscopy. The specific rotations of poly(LV-co-DA) ranged
from -1175° to +1213°, and those of poly(LF-co-DA)
ranged from -220° to 1033° in THF at room tempera-
ture, as shown in Tables 1 and 2. They exhibited
nonlinear relationships against the unit excess of the
monomer in the copolymers (Figure 1), indicating that
chiral amplification occurred. The large specific rota-
tions of the copolymers indicate that they take predomi-
nantly one-handed helical conformation, which is also
supported by intense CD effects in THF (Figure 2).
Temperature Dependence of the Competition
between the Chiral Amino Acids for Control of
Helicel Sense. Because the homopolymers of LV and
LF were not completely soluble in common organic
solvents, it was difficult to compare the stability of
helical conformation of the homopolymers of LV, LF,
and DA. According to the Ising model18 describing the
effect of conflicting enantiomeric chiral information on
1
7
in Scheme 1. The crude product was obtained quan-
titatively in every case. The monomers were purified
by recrystallization, and the structures were confirmed
1
13
by H, C NMR, and IR spectroscopies besides elemen-
tal analysis.
Synthesis and Secondary Structure of the Co-
polymers. Scheme 2 and Tables 1 and 2 summarize
the conditions and results of the copolymerization of LV
+
6
with DA and LF with DA catalyzed by (nbd)Rh [η -
-
C6H5B (C6H5)3] in CH2Cl2 at 30 °C for 1 h. The
monomers satisfactorily underwent copolymerization
and quantitatively transformed into the corresponding
copolymers with moderate molecular weights (Mn )
1
0 300-35 500). The copolymer compositions, which
1
were determined by H NMR spectroscopy were almost

8890 Zhao et al.
Macromolecules, Vol. 37, No. 24, 2004
Figure 1. Relationship between unit excess of LV and LF
over DA in poly(LV-co-DA) and poly(LF-co-DA) and [R]
measured in THF (c ) 0.10-0.11 g/dL) at room temperature.
D
Figure 3. Specific rotation of poly(LV50-co-DA50) measured
in THF and CHCl (c ) 0.10-0.11 g/dL) at -30 to 50 °C.
3
Figure 2. CD spectra of poly(LV-co-DA) (c ) (1.60-1.70) ×
-
4
-4
10
mol/L) and poly(LF-co-DA) (c ) 2.0 × 10
mol/L)
measured in THF at room temperature.
choosing helical sense, a polymer with a helical confor-
mation that is interrupted infrequently by mobile helical
reversal effects allows the chains to interconvert be-
tween left- and right-handed conformations dynami-
cally. On the other hand, when the polymer consists of
structurally different enantiomers, the chiral units
might have different dependence on external stimuli
and can reversibly control the helical sense without
changing the composition. On the basis of this idea,
Green and co-workers have succeeded in the synthesis
of polyisocyanates with various compositions of struc-
turally different chiral enantiomers.13 We have previ-
ously reported that the copolymers of chiral N-pro-
pargylamides, (S)-N-propargyl-2-methyldecanamide,
and either (R)-N-propargyl-3,7-dimethyloctanamide or
Figure 4. Temperature-variable CD and UV-vis spectra of
-
4
poly(LV62.5-co-DA37.5) measure in THF (c ) 1.6 × 10 mol/L).
the cases of poly(LV -co-DA ) and poly(LV62.5-co-
5
5
45
DA37.5), switching points of sign of specific rotation were
observed in THF, which suggests that the copolymer
underwent a helix-helix transition according to tem-
perature. The copolymers with other compositions also
exhibited large changes of specific rotation in THF, but
the sign did not change. On the other hand, the effect
of temperature on specific rotation was small in CHCl3
compared to the case of THF. Figure 4 shows the CD
and UV-vis spectra of poly(LV62.5-co-DA37.5) measured
in THF at various temperatures. The CD signal of the
copolymer at 400 nm changed from a positive peak to a
negative one with increasing temperature, which is
(1S)-N-propargyl-3,2-oxo-4,7,7-trimethyl-2-bicyclo[2.2.1]-
heptan-1-acetamide with a given composition under-
went a helix-helix transition driven by temperature.1
We therefore examined the conformational change of the
copolymers at various temperatures in the current
study.
2d
Figure 3 shows the temperature dependences of
specific rotation of poly(VA-co-DA)s with several com-
positions measured in THF and CHCl3, respectively. In

Macromolecules, Vol. 37, No. 24, 2004
Poly(N-propargylamides) 8891
Figure 5. Specific rotation of poly(LF-co-DA) as a function
of temperature measured in THF (c ) 0.10-0.11 g/dL) at -30
to 50 °C.
consistent with the helical inversion of the copolymer.
During the course of CD inversion phenomenon, only a
slight change was observed in the UV-vis absorption.
We have previously demonstrated that alanine-based
poly(N-propargylamides) exhibit a UV-vis absorption
peak centered at 400 nm when they take a helical
conformation, whereas they show an absorption maxi-
1
5
mum at 320 nm in the random coil form. Conse-
quently, we can conclude that poly(LV62.5-co-DA37.5)
transformed from one helical motif to another one
directly without either taking a random coil conforma-
tion or being accompanied by the change of conformation
such as helix pitch.
Figure 6. Solvent dependence of CD and UV-vis spectra of
-
4
poly(LV50-co-DA50) measured in THF/CHCl
3
(c ) 1.20 × 10
mol/L) at 20 °C.
Figure 5 shows the change of specific rotation of poly-
(LF-co-DA) at various temperatures measured in THF.
Poly(LF50-co-DA50) and poly(LF55-co-DA45) also dis-
played switching points of specific rotation, while the
absolute values of specific rotation were small (below
1
20°), which suggests that the copolymers take a helical
structure but the content is small. In fact, poly(LF502-
co-DA50) showed weak CD signals (|[θ]| < 3000 deg cm
-
1
dmol ), and the transition could not be clearly ob-
served.
Solvent Effect on the Helical Sense. Since the
helical structure of poly(N-propargylamides) is stabi-
lized by intramolecular hydrogen bonding, it is likely
that the solvents influence the formation of hydrogen
bonding. Can the helix sense be manipulated continu-
ously and reversibly by changing solvents? As shown
in Figure 3, the sign of specific rotation of poly(LV50-
co-DA50) was positive in THF but negative in CHCl3
irrespective of temperature. The solvent effect on the
helical sense was estimated by CD and UV-vis spectra
at 20 and 0 °C (Figures 6 and 7). At 20 °C in THF, poly-
(
LV50-co-DA50) exhibited a positive CD signal and two
UV-vis absorption peaks at 320 and 400 nm (Figure
). When a small amount of CHCl3 (10%) was added to
6
Figure 7. Solvent dependence of CD and UV-vis spectra of
-4
THF, the CD signal almost disappeared. Meanwhile, the
UV-vis absorption peak at 400 nm almost disappeared
and that at 320 nm became stronger. This phenomenon
indicates that the polymer transformed into a random
coil by adding 10% CHCl3 to THF. Reformation of a helix
with the opposite screw sense was observed by continu-
ously enlarging the proportion of CHCl3. When the
CHCl3 content was 100%, the CD signal showed a large
negative peak, and the UV-vis absorption at 400 nm
reached a maxima. At 0 °C (Figure 7), the transforma-
tion behavior was more clearly observed compared to
that at 20 °C. It required 40% CHCl3 to transform the
polymer structure from a helix to a random coil.
Continuous addition of CHCl3 resulted in re-formation
of a helix with the opposite screw sense.
poly(LV50-co-DA50) measured in THF/CHCl
3
(c ) 1.20 × 10
mol/L) at 0 °C.
Summary
In this article, we have demonstrated the synthesis
and transition of higher order structure of chiral co-
polymers of N-propargylamides bearing structurally
different amino acids, poly(LV-co-DA) and poly(LF-co-
DA). The helix-helix transition driven by temperature
change was observed in the copolymers with certain
compositions. Poly(LV50-co-DA50) underwent a transi-
tion from a helix with one-handed screw sense into a
helix with the opposite sense through a random coil by
varying the composition of the solvents. Considering the
ease of obtaining L- and D-amino acids, we believe that

8892 Zhao et al.
Macromolecules, Vol. 37, No. 24, 2004
the copolymers in the current study can be promising
candidates of stimuli-responsive switching materials.
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Acknowledgment. The authors are grateful to Prof.
Mark M. Green at Polytechnic University for his helpful
suggestions. The authors are also grateful to Dr. Fumi-
aki Iwasaki and Dr. Masao Yamaguchi at Tokuyama
Co. for their assistance with monomer synthesis.
(
9) (a) Bradbury, E. M.; Carpenter, B. G.; Goldman, H. Biopoly-
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(
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