Tyrosine-based poly(m-phenyleneethynylene-p-phenyleneethynylene)s. Helix folding and responsiveness to a base

Article abstract of DOI:10.1016/j.polymer.2010.03.048

The Sonogashira-Hagihara polymerization of 3',5'-diiodo-N-α-tert-butoxycarbonyl-l-tyrosine methyl ester (1) and 3',5'-diiodo-N-α-tert-butoxycarbonyl-O-methyl-l-tyrosine methyl ester (2) with para-diethynylbenzene (3) was carried out to obtain optically active poly(m-phenyleneethynylene-p-phenyleneethynylene)s [poly(1) and poly(2)] with M_n's ranging from 9900 to 15,000 in 80-87% yields. Poly(1) exhibited intense CD signals in DMSO and THF, but did not in CH₂Cl₂, indicating that it took a predominantly one-handed helical conformation in the former two solvents. On the other hand, there was no evidence for poly(2) to take a helical structure in these solvents. Poly(1) turned the CD sign at 390 nm from plus to minus in DMSO/H₂O = 9/1 (v/v) by the addition of NaOH. Alkaline hydrolysis of ester moieties of poly(1) and poly(2) gave the corresponding polymers having carboxy groups [poly(1a) and poly(2a)]. Poly(1a) and poly(2a) increased the CD intensity by the addition of NaOH.

Full text of DOI:10.1016/j.polymer.2010.03.048

Polymer 51 (2010) 2255e2263
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Tyrosine-based poly(m-phenyleneethynylene-p-phenyleneethynylene)s.
Helix folding and responsiveness to a base
,
Ruiyuan Liu a, Hiromitsu Sogawa a, Masashi Shiotsuki a, Toshio Masuda b, Fumio Sanda a
*
a Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura Campus, Nishikyo-ku, Kyoto 615-8510, Japan
b Faculty of Engineering, Department of Environmental and Biological Chemistry, Fukui University of Technology, 3-6-1 Gakuen, Fukui 910-8505, Japan
a r t i c l e i n f o
a b s t r a c t
The SonogashiraeHagihara polymerization of 30,50-diiodo-N-
a
-tert-butoxycarbonyl-L-tyrosine methyl
Article history:
ester (1) and 30,50-diiodo-N-
a-tert-butoxycarbonyl-O-methyl-L-tyrosine methyl ester (2) with para-
Received 8 January 2010
Received in revised form
25 March 2010
Accepted 25 March 2010
Available online 1 April 2010
diethynylbenzene (3) was carried out to obtain optically active poly(m-phenyleneethynylene-p-phe-
nyleneethynylene)s [poly(1) and poly(2)] with Mn’s ranging from 9900 to 15,000 in 80e87% yields. Poly
(1) exhibited intense CD signals in DMSO and THF, but did not in CH2Cl2, indicating that it took
a predominantly one-handed helical conformation in the former two solvents. On the other hand, there
was no evidence for poly(2) to take a helical structure in these solvents. Poly(1) turned the CD sign at
390 nm from plus to minus in DMSO/H2O ¼ 9/1 (v/v) by the addition of NaOH. Alkaline hydrolysis of
ester moieties of poly(1) and poly(2) gave the corresponding polymers having carboxy groups [poly(1a)
and poly(2a)]. Poly(1a) and poly(2a) increased the CD intensity by the addition of NaOH.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Amino acid
Helix
Poly(phenyleneethynylene)
1. Introduction
thermodynamically driven by solvophobic effects [3]. Inouye and
coworkers have synthesized poly(m-pyridinyleneethynylene)s
A precisely ordered stereostructure of biopolymers such as
peptide and DNA is a key importance for them to exhibit their
biological functions and properties. Helix is the most common and
fundamental secondary structure of synthetic polymers as well as
biopolymers. The study on helical conformation in nonbiological
polymers possibly enhances understanding of the mechanism for
helix formation in biomacromolecules. Synthetic helical polymers
are applicable to molecular recognition (separation, catalysis,
enantioselective sensors), circularly polarized photo- and electro-
luminescent materials, and liquid crystalline materials [1]. Poly
(phenyleneethynylene)s have attracted much attention owing to
the features such as nonlinear optics, asymmetric electrodes, light
polarization, photonic switching, chiral separation, and supramo-
lecular organization [2]. Poly(m-phenyleneethynylene)s having
polar side groups tend to fold into a helical structure in polar
solvents due to the bended linkage and amphiphilic character, i.e.,
hydrophobic phenyleneethynylene backbones and hydrophilic side
chains interacting with polar media. Moore and coworkers have
reported that oligo(m-phenyleneethynylene)s with hydrophilic
with hydrophilic oligo(ethylene glycol) chains, and induced
a helical conformation by protonation in protic media [4]. Poly(m-
phenyleneethynylene-p-phenyleneethynylene)s are polymers
consisting of conjugated m-phenyleneethynylene and p-phenyl-
eneethynylene linkages alternatingly. Tew and coworkers have
demonstrated that amphiphilic poly(m-phenyleneethynylene-p-
phenyleneethynylene)s exhibit spectroscopic changes due to
folding of the backbone into a helical conformation [5]. Schanze
and coworkers have synthesized poly(m-phenyleneethynylene-p-
phenyleneethynylene)s featuring sulfonate and carboxylate side
groups that take a helical structure in water [6]. Zhu and coworkers
have reported that poly(m-phenyleneethynylene-p-phenyl-
eneethynylene)s containing pyridine units undergo conformation
transition from an extended coil structure to a helical structure
driven by solvents, and stabilized by metal ions [7]. All of these
polymers require polar media to take helical forms because the
driving force of helix folding is solvation of hydrophilic exterior
(polar side groups) with polar solvents. A helical form is favorable
for shielding the hydrophobic interior (phenyleneethynylene main
chain) from polar media. On the other hand, we have recently
found new examples of poly(m-phenyleneethynylene-p-phenyl-
eneethynylene)s that form helices based on a mechanism different
from that of poly(phenyleneethynylene) derivatives reported so far.
Namely, hydroxyphenylglycine-based poly(m-phenyleneethynyl
oligo(ethylene glycol) chains adopt
a helical conformation
* Corresponding author. Tel.: þ81 75 383 2591; fax: þ81 75 383 2592.
E-mail address: sanda@adv.polym.kyoto-u.ac.jp (F. Sanda).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.03.048

2256
R. Liu et al. / Polymer 51 (2010) 2255e2263
point apparatus. Elemental analysis was performed at the Micro-
analytical Center of Kyoto University. Specific rotations ([ ]D) were
a
measured on a JASCO DIP-1000 digital polarimeter. CD and UVevis
spectra were recorded using a quartz cell (path length: 1 cm) on
a JASCO J-820 spectropolarimeter.
2.2. Materials
DMF, DMSO, and Et3N used for polymerization were distilled
over CaH2 prior to use. All other reagents were commercially
obtained, and used as received without purification.
2.3. Monomer synthesis
Chart 1. Hydroxyphenylglycine-based poly(m-phenyleneethynylene-p-phenylene-
ethynylene)s.
2.3.1. 30,50-Diiodo-N-
-Tyrosine (10.9 g, 60.1 mmol) was suspended in MeOH (200 mL),
a-tert-butoxycarbonyl-L-tyrosine methyl ester(1)
L
and thionyl chloride (8.00 mL, 110 mmol) was added to the
suspension dropwise at 0 ꢀC. After the resulting mixture was stirred
at room temperature overnight, the solvent was removed by rotary
evaporation, and the residue was washed twice with ether to yield L-
tyrosine methyl ester as a white solid (13.9 g, 60.0 mmol, 99%). A
ene-p-phenyleneethynylene)s having hydrophilic phenolic hyd
roxy group at the phenylene main chain and hydrophobic alkyl
groups at the side chain (Chart 1) form helices in low polar organic
solvents such as CHCl3 and CH2Cl2 [8]. Addition of water decreases
the helicity. It is considered that the key importance of this helix
formation is amphiphilicity (hydrophobic exterior and hydrophilic
interior), which is the reverse of poly(phenyleneethynylene)
derivatives reported so far. Regulated intramolecular hydrogen
bonding between the amide groups at the side chains also seems to
be effective to stabilize the helical structure.
In the present article, we report the synthesis of tyrosine-
based novel poly(m-phenyleneethynylene-p-phenyleneethynyl
ene)s (Scheme 1), and elucidation of the effect of protection of the
phenolic hydroxy and carboxy groups on the secondary structures,
i.e., effect of amphiphilic balance between the interior and exterior
of the polymers. We also report the effects of NaOH addition and
solvent on the secondary structures.
solution of di-tert-butyl dicarbonate (15.7 g, 71.5 mmol) in 1,4-
dioxane (75 mL) was added to a mixture of L-tyrosine methyl ester
(13.9 g, 60.0 mmol) and NaHCO3 (7.55 g, 90.0 mmol) in H2O (75 mL).
The mixture was stirred at 0 ꢀC for 1 h and then at room temperature
overnight. After the mixture was acidified with 2 M HCl to approx-
imately pH 2, it was extracted with three portions of AcOEt (100 mL).
The combined organic layer was washed with H2O (100 mL) and
brine (100 mL), and then dried over MgSO4. The solvent was
removed by rotary evaporation to yield N-
tyrosine methyl ester as a white solid (16.8 g, 56.8 mmol, 94.6%).
N- -tert-butoxycarbonyl- -tyrosine methyl ester (5.31 g
a-tert-butoxycarbonyl-L-
a
L
18.0 mmol), NaCl (4.20 g, 72.0 mmol) and NaIO4 (3.85 g, 18.0 mmol)
were dissolved in AcOH/H2O [9/1 (v/v), 60 mL]. The reaction mixture
was stirredfor15 min. Then, KI(8.96 g, 54.0 mmol)was added at0 ꢀC,
and the resulting mixture was stirred at room temperature over-
night. After H2O (200 mL) was added, the solution was extracted
with CHCl3. The organic layer was washed first with 1 M aqueous
Na2S2O3$5H2O and saturated aq. NaCl, dried over anhydrous MgSO4,
and concentrated on a rotaryevaporator. The residuewas purified by
silica gel column chromatography eluted with n-hexane/AcOEt [1/1
(v/v)] to obtain 1 as white powder in 77% yield (7.58 g, 13.8 mmol).
2. Experimental part
2.1. Measurements
1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded on
a JEOL EX-400 spectrometer. IR spectra were measured on a JASCO
FTIR-4100 spectrophotometer. The number- and weight-average
molecular weights (Mn and Mw) 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 or
DMF as an eluent calibrated by polystyrene standards at 40 ꢀC.
Melting points (mp) were measured on a Yanaco micro melting
Mp 126e127 ꢀC, [
a
]D ¼ þ18ꢀ (c ¼ 0.1 g/dL, CHCl3, room temperature).
1H NMR (400 MHz, CDCl3):
d
¼ 1.45 [s, 9H, (CH3)3], 2.91e3.00 (m, 2H,
ArCH2), 3.74(s, 3H, COOCH3), 4.49 (s,1H, NHCHCOO), 5.03(s,1H, OH),
5.73 (s, 1H, CONH), 7.44 (s, 2H, Ar). 13C NMR (100 MHz, CDCl3):
d
¼ 28.3, 36.3, 52.4, 54.3, 80.2, 82.1,132.2,140.0,152.7,155.6,171.9. IR
(KBr): 3362, 2979, 1758, 1729, 1686, 1524, 1458, 1405, 1367, 1254,
Scheme 1. SonogashiraeHagihara coupling polymerization of monomers 1 and 2 with 3 and alkaline hydrolysis of ester moieties of the obtained polymers.

R. Liu et al. / Polymer 51 (2010) 2255e2263
2257
Scheme 2. Synthesis of monomers 1 and 2.
1165, 1019, 930, 850, 761, 647 cmꢁ1. Anal. Calcd for C15H19I2NO5: C,
32.93; H, 3.50; N, 2.56. Found: C, 32.94; H, 3.39; N, 2.57.
2.6. Spectroscopic data of the polymers
Poly(1): 1H NMR (400 MHz, CDCl3):
d
¼ 1.43 (s, 9H, (CH3)3),
2.3.2. 30,50-Diiodo-N-
a
-tert-butoxycarbonyl-O-methyl-
L
-tyrosine
2.94e3.14 (m, 2H, ArCH2), 3.74 (s, 3H, COOCH3), 4.62 (s, 1H,
NHCHCO), 5.10 (s,1H, NHCOO), 7.26e7.71 (br, 6H, Ar). IR (KBr): 3427,
2975, 2207, 1714, 1600, 1408, 1365, 1278, 1166, 1013, 913, 840, 750,
methyl ester (2)
A
solution of 30,50-diiodo-N-
a-tert-butoxycarbonyl-L-tyrosine
methyl ester (5.47 g, 10.0 mmol) in DMF (50 mL) was cooled using an
ice bath. Tothe solution, freshlygroundK2CO3 (1.53 g,11.2mmol), and
then a cooled solution of iodomethane (0.70 mL,1.59 g,11.2 mmol) in
DMF (20 mL) were added dropwise at 0 ꢀC. The mixture was stirred at
room temperature overnight. It was poured into ice water, and
extracted with ethyl acetate. The organic layers were washed with
water, and saturated aq. NaCl, dried over anhydrous MgSO4, and
concentratedona rotaryevaporator. Theresiduewaspurified bysilica
gel column chromatography eluted with n-hexane/AcOEt [4/1 (v/v)]
to obtain 2 as white powder in 66% yield (3.7 g, 6.6 mmol). Mp
625 cmꢁ1. Poly(2): 1H NMR (400 MHz, CDCl3):
d
¼ 1.46 [s, 9H, (CH3)3],
2.94e3.11 (m, 2H, ArCH2), 3.77e4.16 (m, 6H, OCH3, COOCH3), 4.57 (s,
1H, NHCHCO), 5.09 (s, 1H, NHCOO), 7.28e7.54 (br, 6H, Ar). IR (KBr):
3338, 2972, 2204,1743,1714,1598,1416,1365,1242,1161,1057,1003,
836, 745, 634 cmꢁ1. Poly(1a): 1H NMR (400 MHz, DMSO):
d
¼ 1.28 [s,
9H, (CH3)3], 2.74e2.93 (m, 2H, ArCH2), 4.18 (s, 1H, NHCHCO), 5.14 (s,
1H, NHCOO), 7.18e7.61 (br, 6H, Ar), 12.69 (s, 1H, COOH). IR (KBr):
3437, 2982, 2204, 1704, 1612, 1415, 1385, 1272, 1146, 983, 903, 839,
737, 645 cmꢁ1. Poly(2a): 1H NMR (400 MHz, DMSO):
d
¼ 1.89 [s, 9H,
(CH3)3], 2.79e3.13 (m, 2H, ArCH2), 3.74e3.96 (m, 3H, OCH3), 4.83 (s,
1H, NHCHCO), 4.99 (s, 1H, NHCOO), 8.17e8.33 (br, 6H, Ar), 12.01 (s,
1H, COOH). IR (KBr): 3318, 2991, 2212, 1751, 1704, 1577, 1406, 1331,
74e75 ꢀC, [
NMR (400 MHz, CDCl3):
a
]D ¼ þ22ꢀ (c ¼ 0.1 g/dL, CHCl3, room temperature). 1H
d
¼ 1.45 [s, 9H, (CH3)3], 2.91e3.04 (m, 2H,
CH2Ar), 3.75e3.83 (m, 6H, CH3O, COOCH3), 4.52 (s, 1H, NHCHCOO),
1210, 1122, 1009, 957, 831, 771, 629 cmꢁ1
.
5.06 (s, 1H, CONH), 7.53 (s, 2H, Ar). 13C NMR (100 MHz, CDCl3):
d
¼ 28.3, 36.4, 52.4, 54.2, 60.6, 80.1, 90.3,136.1,140.5,154.8,157.8,171.7.
3. Results and discussion
IR (KBr): 3345, 2983, 2849, 1738, 1694, 1538, 1459, 1416, 1321, 1174,
1057, 911, 868, 727, 646 cmꢁ1. Anal. Calcd for C16H21I2NO5: C, 34.25; H,
3.77; N, 2.50. Found: C, 34.10; H, 3.65; N, 2.58.
3.1. Monomer synthesis and polymerization
Novel diiodo compounds 1 and 2 were synthesized from
L
-
2.4. Polymerization
tyrosine by the route illustrated in Scheme 2. First, N-
a
-tert-
-tyrosine methyl ester was synthesized by methyl
-tyrosine, followed by protection of amino group
butoxycarbonyl-
L
All the polymerizations were carried out in a glass tube equipped
with a three-way stopcock under nitrogen. A typical experimental
procedure for polymerization of 1 with para-diethynylbenzene (3) is
given below.
A solution of 1 (547 mg, 1.00 mmol), 3 (126 mg, 1.00 mmol),
PdCl2(PPh3)2 (35 mg, 50
esterification of
L
with tert-butoxycarbonyl (Boc) group. Then it was iodinated with
potassium iodide, sodium periodate, and sodium chloride in acetic
acid/water ¼ 9/1 (v/v) to afford compound 1. The hydroxy group of
1 was transformed into methyl ether with iodomethane in the
presence of K2CO3 to obtain 30,50-diiodo-N-
a-tert-butoxycarbonyl-
mmol), CuI (4.7 mg, 25
mmol), PPh3
(26.2 mg, 100
m
mol), and Et3N (2.00 mL, 14.3 mmol) in DMF (3 mL)
O-methyl- -tyrosine methyl ester. Both 1 and 2 were obtained as
L
was stirred at 80 ꢀC for 24 h. After that, the resulting mixture was
poured into MeOH/acetone [4/1 (v/v), 300 mL] to precipitate
a polymer. It was separated by filtration using a membrane filter
(ADVANTEC H100A047A) and dried under reduced pressure.
white powders and characterized by 1H, 13C NMR, and IR spec-
troscopies besides elemental analysis.
The polymers were synthesized by the SonogashiraeHagihara
polycondensation of 1 or 2 with 3 in Et3N/DMF or Et3N/DMSO at
80 ꢀC for 24 h [9]. The corresponding polymers [poly(1) and poly(2)]
2.5. Alkaline hydrolysis of poly(1) and poly(2) [synthesis of poly
(1a) and poly(2a)]
Table 1
Polycondensation of 1 and 2 with 3a.
The ester groups of poly(1) and poly(2) were hydrolyzed under
basic conditions. A typical experimental procedure for the hydro-
lysis of poly(1) to poly(1a) is given below.
Aqueous sodium hydroxide (10%, 20 mL) was added to a solution
of poly(1) (250 mg, 0.62 mmol) in DMF (20 mL) dropwise at 0 ꢀC,
and then the resulting mixture was stirred at 50 ꢀC for 3 h. The
reaction mixture was poured into 2 M HCl (300 mL) to precipitate
a polymer. It was separated by filtration using a membrane filter
(ADVANTEC A010A047A) and dried under reduced pressure.
Yieldb (%)
Mn
Mw/Mn
c
c
Run
Monomer
Solvent
1
2
3
4
1 þ 3
1 þ 3
2 þ 3
2 þ 3
DMF
DMSO
DMF
87
84
83
80
10,600
9900
15,000
13,200
1.53
2.26
1.44
1.72
DMSO
a
Conditions: [1]0 ¼ [2]0 ¼ [3]0 ¼ 0.2 M, [PdCl2(PPh3)2] ¼ 0.01 M, [CuI] ¼ 0.005 M,
[PPh3] ¼ 0.02 M, Et3N/solvent ¼ 2/3 (v/v), 80 ꢀC, 24 h.
b
MeOH/acetone ¼ 4/1 (v/v)-insoluble part.
c
Determined by GPC eluted with THF calibrated by polystyrene standards.

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R. Liu et al. / Polymer 51 (2010) 2255e2263
with Mn’s ranging from 9900 to 15,000 were obtained in 80e87%
yields as listed in Table 1. The structures of the polymers were
characterized spectroscopically. The 1H NMR and IR spectra of the
polymers exhibited signals reasonably assignable to the structures
illustrated in Scheme 1. The polymers were soluble in CH2Cl2, CHCl3,
THF, DMF, and DMSO, while insoluble in n-hexane and MeOH.
than those of the polymers. The similar CD and UVevis spectro-
scopic patterns are also reported regarding a poly(m-phenyl-
eneethynylene-p-phenyleneethynylene) featuring optically active
carboxylate side groups [6b]. Thus, it is concluded that poly(1) takes
a helical structure with predominantly one-handed sense in THF
and DMSO [2]. Hence it is clear that the conformation is susceptible
to solvent in a manner similar to poly(phenyleneethynylene)s [10].
We measured the 1H NMR spectra of poly(1) in CD2Cl2 and DMSO-
d6 to find no apparent difference between them. Judging from the
same lmax of poly(1) in the solvents, it is considered that the
conjugated length does not depend on the solvents. Consequently,
it seems that poly(1) does not adopt a random conformation but
a helical one in CH2Cl2 as well, wherein the ratio of right- and left-
handed helices is 1:1, resulting in no CD signal.
3.2. Secondary structure of poly(1) and poly(2)
We measured the CD and UVevis spectra of poly(1) in CH2Cl2,
THF, and DMSO at room temperature. As shown in Fig. 1, poly(1)
exhibited a very large positive CD signal around 390 nm in THF and
DMSO, while no signal in CH2Cl2. In every case, poly(1) exhibited
a UVevis absorption peak around 350 nm, and the relative intensity
agreed with that of the CD signals. The CD signals and UVevis
absorption peaks of poly(1) definitely come from the conjugated
Fig. 2 depicts the CD and UVevis spectra of poly(1) measured in
DMSO/H2O with various compositions at room temperature. When
the DMSO:H2O proportion was 5:5, the [q] of poly(1) became 63% of
phenyleneethynylene backbone because the lmax’s of the mono-
that in DMSO, while the 3max remained at 93%. When the main
mers locate at shorter wavelength regions (1: 285 and 3: 276 nm)
Fig. 1. CD and UVevis spectra of poly(1) measured in CH2Cl2, THF, and DMSO
(c ¼ 4.5 ꢂ 10ꢁ5 mol Lꢁ1) at room temperature. Sample: run 1 in Table 1.
Fig. 2. CD and UVevis spectra of poly(1) measured in DMSO/H2O with various
compositions (c ¼ 4.5 ꢂ 10ꢁ5 mol Lꢁ1) at room temperature. Sample: run 1 in Table 1.

R. Liu et al. / Polymer 51 (2010) 2255e2263
2259
Fig. 3. Top and side views of possible conformers of poly(1) (48-mer) optimized by MMFF94. A: Helix with 6 monomer units per turn. B: helix with 5.8 monomer units per turn.
chains of conjugated helical polymers become irregularly twisted,
the values decrease because the conjugation length becomes
short [11]. It is therefore assumed that poly(1) decreased one-
handedness of screw sense, almost keeping the total helix content
upon addition of H2O.
fashion, meta-linked poly(phenyleneethynylene)s having hydro-
philic side groups such as oligo(ethylene glycol) chains, amine,
sulfonate and carboxylate ionic side groups take a helical confor-
mation driven by amphiphilicity as described in the Introduction
[3,6]. The phenolic hydroxy groups of poly(1) are hydrophilic,
while the ester and Boc groups are hydrophobic compared to the
hydroxy groups. It is likely that poly(1) forms a helical structure due
to the amphiphilic character. It seems that the amphiphilic char-
acter of poly(2) having ether-protected hydroxy groups is not
enough to drive poly(2) to take a helical conformation.
3
We also measured the CD and UVevis spectra of poly(2) in
CH2Cl2, THF, and DMSO to find no CD signal. The phenolic hydroxy
groups play an important role for poly(1) to take a helical structure
with biased screw sense. It is not clear which of a random confor-
mation and a helical one with the same amount of right- and left-
handed screw senses poly(2) adopts. Poly(2) exhibited a UVevis
absorption peak around 330 nm, which is 20 nm shorter than that of
poly(1). Since both 1 and 2 showthe lmax at 285 nm, poly(2) seems to
have a conjugation length shorter than that of poly(1), suggesting
the existence of a random conformation. However, the coexistence
of right- and left-handed helices with a 1:1 ratio is not deniable.
Intramolecular hydrogen bonding is used to stabilize helical
structures not only of biopolymers such as protein and DNA but also
3.3. Conformational analysis of poly(1)
Thus far, the conformations of poly(m-phenyleneethynylene)s
have been analyzed in detail. Amine- [16] and ester-functionalized
[17] poly(m-phenyleneethynylene)s are suggested to prefer helical
conformations to coiled and extended ones in water, wherein
surrounding water molecules play an important role to fold the
polymer chains. This is understood from the fact that the helical
structure maximizes interactions between the polar side chains
of some artificial
p-conjugated polymers, which include poly
(phenylacetylene)s substituted with hydroxy groups [12], poly(N-
propargylamide)s [13], and poly(N-propargylcarbamate)s [14]. In
a similar fashion, poly(1) and poly(2) may form intramolecular
hydrogen bonding between the carbamate groups. We measured
the solution-state IR spectra of the polymers to check this possi-
bility. As a result, no difference was found in the VC]O values of
carbamate and ester groups from those of 1 and 2 in CHCl3, THF, and
DMSO (See supplementary data). Thus it is concluded that the
carbamate and ester groups of the present polymers do not form
intramolecular hydrogen bonding, which is different from the
aforementioned polymers.
and solvents, and also
p-stacking interaction. Namely, the helical
structure minimizes unfavorable contacts between the hydro-
carbon backbone and polar solvents as well [1b].
Fig. 3 shows possible conformers of poly(1) (48-mer), whose
geometries were optimized by the molecular mechanics method
(MMFF94) [18]. It seems that poly(1) takes a helical conformer with
6 monomer units per turn (A in Fig. 3), because a folded m-PE-p-PE
linkage theoretically forms a regular hexagon. However, this helical
structure is as large as 21.7 kJ/(mol unit) unfavorable than a helix
with 5.8 monomer units per turn (B in Fig. 3) due to the steric
repulsion between the eclipsed substituents on the phenylene
rings at i th and (i þ 6)th monomer units. Poly(1) is likely to take an
Amphiphilicity is an important driving force in guiding
secondary and tertiary structures of proteins [15]. In a similar

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R. Liu et al. / Polymer 51 (2010) 2255e2263
Chart 2. Possible DMSO-assisted hydrogen bonding between the hydroxy groups at i
th and (i þ 6)th units of poly(1).
incompletely regular hexagonal helix structure like B, in which the
substituents on the phenylene rings at i th and (i þ 6)th monomer
units are not eclipsed. The diameter of helix B is 24.0 Å, which is
much larger than that of poly(m-phenyleneethynylene)s
(12.0e14.4 Å) [17] due to the presence of p-phenylene linkages. The
helical pitch is 4.1 Å, which is consistent with the turns of the helix
being near van der Waals contact and comparable to
between aromatic rings [17].
p-stacking
The distances between the hydroxy oxygen at i th unit and
hydroxy hydrogen at (i þ 6)th units of helices A and B are 3.1 and
3.5 Å, respectively, both of which are too long to form hydrogen
bonds. As described above, poly(1) forms a helical structure effi-
ciently in DMSO that possibly participates in hydrogen bonding,
while it does not in CH2Cl2 that has no hydrogen bonding ability. It
should be noted that poly(2) having methyl ether-protected
hydroxy groups does not form a helix. These results can be
explained by considering the formation of hydrogen bonds
between hydroxy groups assisted by DMSO as illustrated in Chart 2.
In the case of helix B, the distance between hydrogen atoms of
hydroxy groups and oxygen atom of DMSO is 1.8 Å, when they form
hydrogen bonds as shown in Chart 2. This is acceptable from the
literature regarding DMSO-hydrogen bonds [19]. It is likely that
the DMSO-assisted hydrogen bonding contributes to stabilize the
helical structure of poly(1), and the lack of this contribution causes
no helix formation of poly(2).
It is considered that the bisignated CD signals at 390 and 330 nm
of the polymer in DMSO (Fig. 1) are caused by exciton chirality
based on the cooperative interaction between the main chain
chromophore [20], because the lmax of the UVevis absorption is
positioned at the inflection point (350 nm) of the plus and minus
CD signals. Judging from the positive first and negative second
Cotton effects together with the molecular modeling, the helical
sense is considered to be right-handed as illustrated in Fig. 3.
Fig. 4. CD and UVevis spectra of poly(1) upon addition of NaOH measured in DMSO/
H2O ¼ 9/1 (v/v, c ¼ 4.5 ꢂ 10ꢁ5 mol Lꢁ1) at room temperature. Sample: run 1 in Table 1.
DMSO/H2O ¼ 9/1 (v/v). The CD spectroscopic pattern was very
sensitive to NaOH as was predicted. The plus-signed CD signal
around 390 nm gradually decreased the intensity by raising the
amount of NaOH, and almost disappeared at 200 equiv. Further
addition of NaOH brought about appearance of a minus-signed CD
signal at 390 nm, and increase of intensity. These results suggest
3.4. Conformational change of poly(1) upon addition of base
The helical conformation of hydroxyphenylglycine-based poly
(m-phenyleneethynylene-p-phenyleneethynylene)s (Chart 1) is
stable to addition of alkali metal hydroxides, although the polymers
have phenolic hydroxy groups that are susceptible to base [8]. This
is presumably due to the presence of intramolecular hydrogen
bonding between the amide groups, which effectively stabilizes the
helical structure. Since poly(1) in the present study takes a helical
conformation without the assistance of such intramolecular
hydrogen bonding, it is forecasted that poly(1) changes the
conformation according to the addition of base more largely than
the polymers illustrated in Chart 1. Fig. 4 depicts the effect of NaOH
addition on the CD and UVevis spectra of poly(1) measured in
Table 2
Alkaline hydrolysis of poly(1) and poly(2)a.
Yieldd (%)
Mn
Mw/Mn
e
e
Polymer
Poly(1)b
62
64
6600
7500
3.73
3.81
Poly(2)c
a
Conditions: in DMF/H2O ¼ 1/1 (v/v), 50 ꢀC, 3 h.
Sample: run 1 in Table 1.
Sample: run 3 in Table 1.
b
c
d
e
H2O-insoluble part.
Determined by GPC eluted with LiBr solution in DMF (10 mM) calibrated by
polystyrene standards.

R. Liu et al. / Polymer 51 (2010) 2255e2263
2261
Fig. 5. CD and UVevis spectra of poly(1a) and poly(2a) upon addition of NaOH measured in MeOH/H2O ¼ 1/1 (v/v, c ¼ 5.0 ꢂ 10ꢁ5 mol Lꢁ1) at room temperature.
that poly(1) transformed the predominant screw sense upon NaOH
addition [21]. Since the spectra are close to mirror-images before
and after 1000 equiv of NaOH addition, it is considered that the
increasing amount of NaOH yields an excess of the opposite-
handed helix, which is very similar in structure to and in equilib-
rium with the original one. An isodichroic point is observed at the
maximum UV absorption. So evidently the base plays a role in the
transfer of chirality from the tyrosine side groups to the main chain
secondary structure. It is likely that other alkali metal hydroxides
also interact with poly(1) to affect the conformation. We also
measured the CD and UVevis spectra of poly(1) in the presence of
LiOH, KOH, CsOH, and RbOH as well as NaOH to find no clear
difference and tendency among these metal hydroxides (See
supplementary data).
3.6. Chiroptical properties of poly(1a) and poly(2a)
Fig. 5 depicts the effect of NaOH addition on the CD and UVevis
spectra of poly(1a) and poly(2a) in MeOH/H2O ¼ 1/1 (v/v). It should
be noted that poly(1) showed a plus-signed first Cotton signal, while
poly(1a) showed a minus-signed one. This indicates that the poly-
mer kept a helical form after hydrolysis but changed the predomi-
nant screw sense. On the contrary, poly(2a), obtained by the
hydrolysis of poly(2), exhibited no CD signal similar to the prepol-
ymer [poly(2)], indicating that neither of them forms a predomi-
nantly one-handed helical conformation irrespective of the
substituents (carboxy and ester groups). It seems that the phenolic
hydroxy groups play a more important role on formation helix than
carboxy groups. Addition of NaOH resulted in increase of CD inten-
sity of poly(1a), and induction of CD signals in the case of poly(2a). It
is considered that NaOH ionized the carboxy groups to increase the
polarity, leading to folding into a predominantly one-handed helical
structure. The helical structure of poly(2a) was more sensitive to
base than that of poly(1a). Since the phenolic hydroxygroups of poly
(1a) are effective to induce a predominantly one-handed helix as
described above, it can form a helixeven in the absence of NaOH, and
therefore the responsiveness seems to be small. On the other hand,
poly(2a) cannot form a predominantly one-handed helix in the
absence of NaOH, probably because the phenolic hydroxygroups are
protected. By the addition of base, poly(2a) acquires highly polar
carboxylate groups, which seem to be effective to induce
a predominantly one-handed helical structure.
3.5. Hydrolysis of the ester groups of the polymers
We further examined the hydrolysis of methyl ester moieties
of poly(1) and poly(2) using NaOH. The polymers satisfactorily
underwent alkaline hydrolysis to give the corresponding poly-
mers [poly(1a) and poly(2a)] having carboxy groups in good
yields as listed in Table 2. The GPC-determined Mn’s of the
hydrolyzed polymers were lower than those of the polymers
before hydrolysis. This is partly because of removal of methyl
groups from the side chains, and probably the large interaction
between the pendent carboxy groups and polystyrene gels as
well, resulting in the long elution times. The polymers were
soluble in polar solvents such as DMF, DMSO, MeOH, and basic
water.
Fig. 6 depicts the dependence of CD and UVevis spectra of poly
(1a) and poly(2a) on the composition of MeOH/H2O. Interestingly,

2262
R. Liu et al. / Polymer 51 (2010) 2255e2263
Fig. 6. CD and UVevis spectra of poly(1a) and poly(2a) measured in MeOH/H2O with various compositions (c ¼ 5.0 ꢂ 10ꢁ5 mol Lꢁ1, cNaOH ¼ 0.05 M) at room temperature.
the amplitude of CD signal of the polymers varied with the solvent
composition. Poly(1a) and poly(2a) increased the CD intensity with
increasing water content to reach the maxima at 50 and 60 volume
% water, respectively. This result indicates that the hydrophobic
effect is important for the polymers to adopt a helical conformation
to a greater extent [6b]. Further addition of H2O resulted in
a decrease in the amplitude of the CD signal.
design of helical poly(phenyleneethynylene) derivatives having
phenolic hydroxy groups.
Acknowledgments
The authors are grateful for the financial support from the
Global COE Program, “International Center for Integrated Research
and Advanced Education in Materials Science”. R. L. acknowledges
financial support from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan.
4. Conclusion
In this paper, we have demonstrated the SonogashiraeHagihara
polymerization of 30,50-diiodo-N-
a
-tert-butoxycarbonyl-
methyl ester (1) and 30,50-diiodo-N-
-tert-butoxycarbonyl-O-
methyl- -tyrosine methyl ester (2) with para-diethynylbenzene (3).
L-tyrosine
Appendix. Supplementary data
a
L
Supplementary data associated with this article can be found in
the on-line version, at doi:10.1016/j.polymer.2010.03.048.
The polymerization satisfactorily proceeded to give the corre-
sponding poly(m-phenyleneethynylene-p-phenyleneethynylene)s
[poly(1) and poly(2)]. Poly(1) took a predominantly one-handed
helical conformation in DMSO, but poly(2) did not. It is considered
that the phenolic hydroxy groups play an important role to induce
the regulated structure of poly(1); the helical conformation of poly
(1) is induced by the amphiphilic character. Poly(1) inverted the
predominant screw sense in DMSO/H2O ¼ 9/1 (v/v) by the addition
NaOH. Poly(1) and poly(2) were satisfactorily converted into the
corresponding polymers having carboxy groups [poly(1a) and poly
(2a)] by alkaline hydrolysis of the ester groups. The polymers
changed the helical structures responding to base and MeOH/H2O
compositions, especially the case of poly(2a) having protected
phenolic hydroxy groups. The results obtained in the present
research together with those of hydroxyphenylglycine-based
polymers (Chart 1) [8] have developed a new way of molecular
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