Poly(phenylacetylene)s bearing a peptide pendant: Helical conformational changes of the polymer backbone stimulated by the pendant conformational change

Article abstract of DOI:10.1002/chem.200400315

Optically active, cis-transoid poly(phenylacetylene) derivatives bearing a poly(γ-benzyl-L-glutamate) [poly-(PBGA_m)] or poly(L-glutamic acid) [poly(PGA_m)] chain as the pendant were prepared by polymerisation of the corresponding macromonomer with a rhodium catalyst followed by hydrolysis of the pendant ester groups. Their conformational changes in solution, induced by a helix-coil transition of the pendant polypeptides, were investigated using circular dichroism (CD) and absorption spectroscopies. A series of macromonomers with a different peptide chain lengths was synthesised by the polymerisation of the N-carboxyanhydride of γ-benzyl-L-glutamate with a phenylacetylene bearing an alanine residue as the initiator. The obtained macromonomers (PBGA_m) were further polymerised with a rhodium catalyst in N,N-dimethylformamide (DMF) to yield novel poly(phenylacetylene)s [poly(PBGA_m)] with a poly(γ-benzyl-L-glutamate) pendant. The poly(PBGA_m) exhibited an induced circular dichroism (ICD) in the UV/Vis region of the polymer backbone in dimethyl sulfoxide (DMSO), probably due to the prevailing one-handed helix formation. The Cotton effect signs of a DMSO solution of the poly(PBGA_m) were inverted and accompanied by a visible colour change in the presence of an increasing amount of chloroform or DMF containing lithium chloride. The results suggest that poly(PBGA_m) may undergo a conformational change such as a helix-helix transition with a different helical pitch responding to a change in the α-helix content of the poly(γ-benzyl-L-glutamate) pendant. Moreover, a water-soluble poly(PGA_m) also showed a similar, but dramatic change in its helical conformation with a visible colour change stimulated by a helix-coil transition of the pendant poly(L-glutamic acid) chains by changing the pH in water.

Full text of DOI:10.1002/chem.200400315

FULL PAPER
Poly(phenylacetylene)s Bearing a Peptide Pendant:
Helical Conformational Changes of the Polymer Backbone
Stimulated by the Pendant Conformational Change
Katsuhiro Maeda, Nobuto Kamiya, and Eiji Yashima*[a]
Abstract: Optically active, cis-transoid
poly(phenylacetylene) derivatives bear-
ing a poly(g-benzyl-l-glutamate) [poly-
(PBGAm)] or poly(l-glutamic acid)
[poly(PGAm)] chain as the pendant
were prepared by polymerisation of
the corresponding macromonomer with
a rhodium catalyst followed by hydrol-
ysis of the pendant ester groups. Their
conformational changes in solution, in-
duced by a helix coil transition of the
pendant polypeptides, were investigat-
ed using circular dichroism (CD) and
absorption spectroscopies. A series of
macromonomers with a different pep-
tide chain lengths was synthesised by
the polymerisation of the N-carboxyan-
hydride of g-benzyl-l-glutamate with a
phenylacetylene bearing an alanine res-
idue as the initiator. The obtained mac-
romonomers (PBGAm) were further
polymerised with a rhodium catalyst in
N,N-dimethylformamide (DMF) to
yield novel poly(phenylacetylene)s
[poly(PBGAm)] with a poly(g-benzyl-l-
glutamate) pendant. The poly(PBGAm)
exhibited an induced circular dichroism
(ICD) in the UV/Vis region of the
polymer backbone in dimethyl sulfoxide
(DMSO), probably due to the prevail-
ing one-handed helix formation. The
Cotton effect signs of a DMSO solu-
tion of the poly(PBGAm) were inverted
and accompanied by a visible colour
change in the presence of an increasing
amount of chloroform or DMF con-
taining lithium chloride. The results
suggest that poly(PBGAm) may under-
go a conformational change such as a
helix helix transition with a different
helical pitch responding to a change in
the a-helix content of the poly(g-
benzyl-l-glutamate) pendant. More-
over, a water-soluble poly(PGAm) also
showed a similar, but dramatic change
in its helical conformation with a visi-
ble colour change stimulated by
a
helix coil transition of the pendant
poly(l-glutamic acid) chains by chang-
ing the pH in water.
Keywords: circular dichroism ¥ heli-
cal structures ¥ peptides ¥ poly-
(phenylacetylene) ¥ polymerisation
Introduction
mers are known to exhibit macromolecular helicity inversion
regulated by external, achiral stimuli, such as a change in
pH, temperature, solvent, salt concentration, or by irradia-
tion. These switchable helical polymeric materials have po-
tential applications in data storage, optical devices, and
liquid crystals for displays. On the other hand, switching of
the macromolecular helicity by chiral stimuli still remains
rare, but can be used to sense the chirality of chiral
guests.[1o]
In earlier studies, we reported that macromolecular helici-
ty with an excess of helical sense could be induced in opti-
cally inactive poly(phenylacetylene)s bearing various func-
tional groups such as carboxy,[4] amino,[5] boronate,[6] and
phosphonate[7] groups or the bulky crown ether as the pen-
dant.[8] Upon noncovalent complexation with optically active
compounds capable of interacting with the functional
groups, the polymers formed a predominantly dynamic one-
handed helix and the complexes exhibited a characteristic
induced circular dichroism (ICD) in the polymer backbone
region. Moreover, the macromolecular helicity of the poly-
(phenylacetylene) bearing a carboxy group induced by opti-
Constructing a helical structure with a controlled helix sense
has become one of the most attractive issues in the fields of
polymer and supramolecular chemistries. Optically active
helical polymers and helical assemblies with an excess heli-
cal sense possess interesting properties and functions based
on their helical chirality, such as chiral and chirality recogni-
tion and enantioselective catalysis.[1] Another unique feature
of helical polymers is a helix inversion (helix helix transi-
tion) between the right- and left-handed helical conforma-
tions. Up to now, several synthetic[2] and biological[3] poly-
[a] Dr. K. Maeda, N. Kamiya, Prof. Dr. E. Yashima
Department of Molecular Design and Engineering
Graduate School of Engineering, Nagoya University
Chikusa-ku, Nagoya 464-8603 (Japan)
Fax : (+81)52-789-3185
E-mail: yashima@apchem.nagoya-u.ac.jp
Supporting information for this article (1H NMR spectra of poly-
(PBGA20) and poly(PGA20)) is available on the WWW under http://
www.chemeurj.org/or from the author.
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¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/chem.200400315
Chem. Eur. J. 2004, 10, 4000 4010

4000 4010
cally active amines can be memorised when the optically
active amines are replaced by achiral amines in DMSO.[9]
Similar helical poly(phenylacetylene)s with optical activity
can also be prepared by the polymerisation of phenylacety-
lenes bearing an optically active pendant through covalent
bonding.[10] The helical structures of the poly(phenylacety-
lene)s are dynamic in nature,[4d,11] that is, an equilibrium
exists between the helices separated by the helix reversal
points.[1c,o] Thus the polymers have a chance to exhibit helix
inversion by changing the external chiral and achiral condi-
tions, because the right- and left-handed helices of the opti-
cally active poly(phenylacetylene)s are not enantiomers, but
diastereomers. Either of the helices may therefore have a
helical structure with a different helical pitch and can be
predominant under a particular condition. In fact, we found
that poly(phenylacetylene)s bearing an optically active
(1R,2S)-norephedrine,[12] b-cyclodextrin,[13] or (1-(1-naphthyl)-
ethyl)carbamoyl group[14] as the substituent underwent a
helix helix transition induced by diastereomeric complexa-
tion with small chiral molecules as well as by achiral stimuli.
These results demonstrate that the helical structures of opti-
cally active poly(phenylacetylene)s are dynamic and the
helix sense can be controlled not only by the chirality of the
pendant, but also by delicate intermolecular and/or intramo-
lecular interactions at the side chains.[11]
pendant polypeptide because PBLG has been found to un-
dergo a helix random coil conformational change depending
on the solvent composition,[16] and the poly(l-glutamic acid)
derived from the PBLG also exhibits a similar helix random
coil transition in water by changing the solution pH.[17] The
target phenylacetylene bearing a primary amino group (1)
was prepared starting from 4-iodoaniline and N-(tert-butoxy-
carbonyl)-l-alanine in four steps as outlined in Scheme 1.
Scheme 1. Synthesis of 1.
On the basis of these observations, we have designed and
synthesised a series of optically active, stereoregular poly-
(phenylacetylene)s bearing a polypeptide chain as the pen-
dant. Some polypeptides are known to undergo conforma-
tional changes regulated by a change in pH and solvent.[3]
Therefore, we anticipated that the ordered structural change
in the polypeptide pendants is transmitted to the polyacety-
lene backbone, resulting in a further conformational change
in the main chain of the poly(phenylacetylene)s. Here we
show the detailed results of our investigations of such a dy-
namic conformational change of the poly(phenylacetylene)
backbone. This occurs as a response to conformational
change in the pendant polypeptides induced by the external
stimuli, such as a change in pH and solvent composition.
The polymerisation of BLG-NCA with 1 as the initiator
was carried out at different molar ratios of [BLG-NCA]/[1]
in chloroform at 308C to yield a macromonomer with a
narrow molecular weight distribution (Scheme 2). The poly-
merisation results are summarised in Table 1. After the pol-
Scheme 2. Synthesis of macromonomer (PBGAm).
Results and Discussion
Synthesis and polymerisation of poly(g-benzyl-l-glutamate)-
based macromonomers bearing an acetylene residue
(PBGA) at the C-terminal end: Ring-opening polymerisa-
tion of the N-carboxyanhydride of amino acids (NCA) with
a primary amine as the initiator is the most common and
straightforward method for the preparation of polypeptides
with a controlled molecular weight in which the initiator res-
idue is introduced at the C-terminal end.[15] In order to syn-
thesise macromonomers bearing a polymerisable phenylace-
tylene residue at the chain end composed of polypeptide (or
oligopeptide) chains with various degrees of polymerisations
(DPs), we prepared a novel phenylacetylene bearing a pri-
mary amino group starting from l-alanine (1) (Scheme 1).
This was used as the initiator for the polymerisation of the
N-carboxyanhydride of g-benzyl-l-glutamate (BLG-NCA).
We selected poly(g-benzyl-l-glutamate) (PBLG) as the
ymerisation, the products (PBGAm: m represents DP) were
separated into a high molecular weight, methanol-insoluble
fraction and a low molecular weight, methanol-soluble frac-
tion that was recovered by precipitation in diethyl ether.
1
The DPs of the obtained PBGAm were estimated by H
NMR analysis. The 1H NMR spectrum of PBGA57 in
[D7]N,N-dimethylformamide (DMF) (methanol-insoluble
part of run 6 in Table 1) is shown in Figure 1. The peak in-
tensity of the methylene proton resonances of the benzyl
group of PBGAm segment relative to that of the methyl
proton resonances of the initiator 1 attached to the chain
end was used to determine the DPs of the PBGAm macro-
monomers. In the aromatic region, the two doublet peaks of
the initiator residue (HD and HE in Figure 1) were separate-
ly observed from the aromatic peaks of the PBGAm pend-
ants. The DPs of the PBGAm were estimated from their rel-
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E. Yashima et al.
FULL PAPER
Table 1. Polymerisation of BLG-NCA with 1 in chloroform at 308C.[a]
MeOH-insoluble part
MeOH-soluble and Et2O-insoluble part
Run [BLG-NCA]/
t
Yield
DP[b] Helix content [%]
Mn
Mw/
Mn
Yield
[%]
DP[b] Mn
(DP)[d]
Helix content
[%][b]
Mw/
Mn
[d]
[d]
[1]
[h] [%]
(DP)[d]
in
in
DMSO[b]
DMF[c]
1
2
3
4
5
6
5
17
17
40
40
45
45
41
65
75
82
80
79
11
16
20
26
34
57
42
58
67
74
82
90
53
79
81
87
90
96
5700 (25) 1.24
7000 (31) 1.12
7600 (34) 1.09
7900 (35) 1.07
8500 (38) 1.06
9800 (44) 1.05
41
23
17
12
12
18
3
5
5
7
10
21
2800 (12)
3500 (15)
3900 (17)
4400 (20)
5900 (26) 45
7900 (36) 72
1.53
1.16
1.15
1.13
1.15
1.14
10
15
20
30
50
[a] Polymerised under nitrogen, [BLG-NCA]=0.25m. [b] Degree of polymerisation (DP) determined by 1H NMR in [D6]DMSO. [c] Determined by 1H
NMR in [D7]DMF. [d] Determined by SEC using DMF containing 10 mm LiCl as the eluent.
11 and 26 in [D6]DMSO and
[D7]DMF
are
shown
in
Figure 2. The PBGAm exhibited
two separate peaks assigned to
the a-helix and random coil
forms at around d=3.92 and
4.26 in [D6]DMSO[19a] and at
d=4.12 and 4.42 in [D7]DMF,
respectively.[18b] The up-field
peak assigned to the a-helix
partly overlapped the peaks
from the ethynyl proton (HC)
and the a-CH (HB) proton res-
onances derived from the initia-
tor. As shown in Table 1, the
helix contents of the methanol-
insoluble PBGAm in DMSO
were smaller than those in
Figure 1. 1H NMR (500 MHz) spectrum of PBGA57 (run 6 in Table 1) in [D7]DMF at room temperature.
ative peak intensities and agreed well with the above-men-
tioned DP values. However, the DPs estimated by size-ex-
clusion chromatography (SEC) were different from those es-
DMF, but increased with an increase in the DP in both sol-
vents. These results agreed with those reported in the litera-
ture.[19,20]
The macromonomers with a different peptide length
(PBGAm) were then polymerised with a rhodium complex
1
timated by H NMR and the difference was remarkable for
the PBGAm with a low DP. The PBGAm obtained has a
narrow molecular weight distri-
bution (Mw/Mn <1.1) and the
DPs determined experimentally
by 1H NMR analysis are well
correlated with those expected
from the macromonomer/initia-
tor ratio ([BLG-NCA]/[1]) in
the feed except for the PBGAm
obtained at low [BLG-NCA]/
[1] feed ratios (runs 1 and 2 in
Table 1).
The helix content of the
1
PBGAm was determined by H
NMR analysis. It has been re-
ported that PBLG exhibits two
separate peaks due to the a-
helix and random coil forms in
the a-CH resonance region and
the up-field peak was assigned
1
to the a-helix.[18] The H NMR
Figure 2. 1H NMR (500 MHz) spectra of selected region of PBGA11 and PBGA26 in [D6]DMSO (A and B)
spectra of the a-CH (HA) re-
gions of the PBGAm with m= and [D7]DMF (C and D) at room temperature.
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Poly(phenylacetylene)s Bearing a Peptide Pendant
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1
([Rh(nbd)Cl]2 (nbd = norbornadiene)) in DMF at 308C
(Scheme 3). The polymerisation results are summarised in
Table 2. The concentrations of PBGAm in the feed were set
to a maximum. The polymerisation proceeded homogene-
ously, the solution became viscous with time (runs 1 6 in
Table 2), and then gelated within a few minutes (runs 1 3 in
Table 2). Conversely, an apparent increase in the viscosity
was not observed during the polymerisation of PBGAm with
m>20 (runs 7 and 8 in Table 2). After the polymerisation
reaction, the polymeric products were recovered as the
methanol-insoluble fractions. The methanol-insoluble frac-
tions obtained from the polymerisation of the PBGAm with
m=3, 5, and 7 were insoluble in common organic solvents,
such as THF, chloroform, DMSO and DMF, while those of
the PBGAm with m>7 were soluble in DMSO and DMF-
containing LiCl (10 mm). The SEC measurements of the
methanol-insoluble fractions of the PBGAm with m>10
showed trimodal SEC traces, suggesting that the methanol-
insoluble parts consisted of three fractions: high and low
molecular weight polymeric fractions (poly(PBGAm)) and
the unreacted macromonomer
(PBGAm) were determined by H NMR analysis in the same
way as mentioned above for the macromonomers. Figure 3
shows the 1H NMR spectra of the poly(PBGAm) (m=11,
26: runs 4 and 7 in Table 2) in [D6]DMSO and [D7]DMF. In-
terestingly, the helix content of the PBGA pendant in the
poly(PBGAm) significantly increased in DMF as compared
with those of the corresponding macromonomers; for in-
stance, from 53 to 89% (run 4 in Table 2). This increase in
the a-helix content of the pendant peptide after polymerisa-
tion of the macromonomer may be ascribed to the forma-
tion of a helix-bundle structure in DMF between the pend-
ant PBGAm chains. A similar enhancement of the a-helix
content has been observed for the designed artificial pepti-
des linked to the dendrimer surface, porphyrins, and bipyrid-
yl ligands forming the metal chelate complexes, when the
peptide chains self-assemble to form the helix-bundle struc-
tures.[21] These results indicate that the helical polypeptide
chains arrange in a one-handed helical array along the heli-
cal polyacetylene backbone whose helicity and conformation
may be significantly influenced by the pendant polypeptide
PBGAm. The low molecular
weight poly(PBGAm) and the
unreacted macromonomer were
removed by fractional precipi-
tation using DMF containing
10 mm LiCl as the solvent and
methanol as the precipitant to
give orange-coloured poly-
(PBGAm). This showed a unim-
odal SEC trace. The yield of
the high molecular weight poly-
mers significantly decreased
with an increase in the DP of
the macromonomers. This is
probably due to the decrease in
the concentration of the macro-
monomer ([PBGAm]) in the
feed because of the solubility
limit of the macromonomers.
Figure 3. 1H NMR (500 MHz) spectra of selected region of poly(PBGA11) and poly(PBGA26) in [D6]DMSO (a
The helix contents of the
pendant PBGAm in the poly- and b) and [D7]DMF (c and d) at room temperature.
Table 2. Polymerisation of macromonomer (PBGAm) with [Rh(nbd)Cl]2 in DMF at 308C.[a]
PBGAm
Helix content [%]
in DMSO[e] in DMF[f]
[d]
Run
m[b]
Helix content [%][b]
[PBGAm] [m]
t [h]
Yield[c] [%]
Mn î104[d]
Mw/Mn
in DMSO
in DMF
[h]
[h]
[h]
[h]
[h]
[h]
1
2
3
4
5
6
7
8
3
5
7
11
16
20
26
34
0.25
0.25
0.25
0.20
0.15
0.15
0.13
0.10
15
15
15
17
17
17
40
40
80
90
92
20
19
20
7
37.1[g]
1.91[g]
[h]
[h]
[h]
[h]
42
58
67
74
82
53
79
81
87
90
12.1
25.1
23.2
23.0
14.4[j]
1.73
2.00
1.97
1.67
53
55
59
55
89
92
97
95
4[i]
[a] Polymerised under nitrogen, [PBGAm]/[Rh]=25 and [Et3N]/[Rh]=50. [b] Degree of polymerisation (m) was determined by 1H NMR (see Table 1).
[c] After removal of oligomers and unreacted PBGAm by reprecipitation using DMF-LiCl as the solvent and MeOH as the precipitant. [d] Determined
by SEC using DMF containing 10 mm LiCl as the eluent. [e] Determined by 1H NMR in [D6]DMSO. [f] Determined by 1H NMR in [D7]DMF. [g] The
polymerisation solution was directly injected into the SEC system after dilution with the eluent. [h] Insoluble in common organic solvents. [i] Containing
oligomers. [j] Based on the peak top in the SEC chromatogram.
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E. Yashima et al.
FULL PAPER
structure. The helical structure and conformational dynamics
of the polyacetylene backbone stimulated by a change in the
pendant peptide structure will be discussed later in detail.
On the other hand, the helix contents of the pendant of poly-
(PBGAm) in DMSO were almost constant regardless of the
DP of the polypeptides, and rather lower than those of the
corresponding macromonomers except for the poly-
(PBGAm) with m=11. As reported in the literature,[16,19]
DMF is one of the favourable helix-supporting solvents for
PBLG, while DMSO acts as a weak helix supporter. PBLG
with low DPs possesses a random coil form in part, but
PBLG with high DPs can maintain a full helical struc-
ture.[16,19] In DMSO, a-helix formation of the pendant pepti-
des through intramolecular hydrogen bonding is not favour-
able such that further helix induction in the pendant PBLG
chains through the helix bundle formation may not be at-
tained.
Figure 4. Laser Raman spectrum of poly(PBGA20) (run 6 in Table 2) in
the solid state at room temperature (ca. 238C).
In order to get information on the stereoregularity of poly-
(PBGAm), the H NMR measurements were performed for
solvents. The macromonomers (PBGAm) showed no CD
bands at wavelengths greater than 300 nm. These results in-
dicate that the poly(PBGAm) possesses a predominantly
one-handed helical conformation induced by the chirality of
the covalently bonded pendant polypeptides. However, the
helix sense excess of the poly(PBGAm) backbone in DMSO
and DMF-LiCl may not be high judging from their relatively
small [q]max values compared with those of the previously re-
1
the polymers soluble in organic solvents. However, it was
difficult to evaluate the stereoregularity of the polymers by
1
their H NMR spectra. The peak due to the main chain pro-
tons, which are highly useful for assigning the conformation
and configuration of the polyacetylene backbone,[22] were
broad and their intensities were small relative to those of
the protons of the pendant peptides. We then tried to evalu-
ate the stereoregularity of poly(PBGAm) by laser Raman
spectroscopy, which has also been utilised to assign the ster-
eoregularity of polyacetylenes.[23] Figure 4 shows the laser
Raman spectrum of poly(PBGA20) measured in the solid
state. The Raman spectrum exhibited intense peaks at 1569,
1332, and 962 cmÀ1, which are characteristic peaks due to
14]
ported helical poly(phenylacetylene)s.[4 In DMF-LiCl, the
CD and absorption maxima of poly(PBGAm) shifted to a
longer wavelength with increasing DPs of the pendant and
the solution colour changed from yellow to red (Figure 5B).
By contrast, in DMSO such CD and absorption spectral
changes depending on the DPs of the pendant were hardly
observed. The colour changes of poly(PBGAm) can be ascri-
bed to a change in the twist angle of the conjugated double
bonds of the main chain. The bathochromic shift of the ab-
sorption spectra of poly(PBGAm) with the increasing DPs of
the pendants suggests that the poly(PBGAm) with high DPs
have a rather relaxed helical conformation with an extended
p-conjugation in DMF, while the yellow coloured poly-
(PBGAm) with low DPs has a more tightly twisted helical
conformation.[13] Percec et al. recently reported a similar ob-
servation that the increase in the steric repulsion between
the pendants may result in extending the polyacetylene
À
the cis polyacetylenes and can be assigned to the C=C, C C
À
and C H bond vibrations, respectively, while those of the
trans polyacetylenes were not observed.[24] This indicates
that the obtained poly(PBGA20) possesses a highly cis-trans-
oid structure. Similar Raman spectra were also obtained for
the other poly(PBGAm)s, indicating that the poly(PBGAm)
is highly cis-transoid.
Chiroptical properties of poly(PBGAm) and their conforma-
tional changes: The CD and absorption spectra of poly-
(PBGAm) were measured in
order to characterise the chi-
roptical properties of the opti-
cally active poly(phenylacety-
lene)s bearing the PBLG pend-
ant. Figure 5 shows the CD and
absorption spectra of poly-
(PBGAm) (runs 4 7 in Table 2)
in DMSO and DMF containing
10 mm LiCl, in which the PBLG
pendants favourably form
a
random coil and a-helix, re-
spectively. These polymers ex-
hibited a characteristic CD in
the UV/Vis region of the p-con-
jugated double bonds of the
polyacetylene backbone in both
Figure 5. CD and absorption spectra and the visible difference (inset) of poly(PBGAm) with various DPs in
A) DMSO and B) DMF containing 10 mm LiCl at room temperature (ca. 238C).
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Poly(phenylacetylene)s Bearing a Peptide Pendant
4000 4010
backbone.[25] Therefore, it can be presumed that in DMF-
LiCl, the steric repulsion between the pendants would in-
crease with an increase in the DPs of the a-helical peptides
at the pendant. This, in turn, results in the bathochromic
shift of the absorption spectra of poly(PBGAm). On the
other hand, in DMSO, such a steric repulsion between the
pendants seems to be nearly constant regardless of the DPs
of the pendant because the helix contents of the pendant
PBGAm chains are low.
the solvent composition, since the helix content of the pen-
dant peptides of poly(PBGAm) in DMSO was smaller than
those in DMF-LiCl by approximately 30%. As shown in
Figure 6, the CD intensity significantly decreased with the
increasing amount of DMSO. The sign became inverted
from the positive to negative direction at DMSO/DMF-LiCl
8:2 (v/v) with a clear isosbestic-like point at 376 nm, and fur-
ther increased in the negative direction. These CD spectral
changes were accompanied by remarkable changes in the
absorption spectra; the absorbance maxima (lmax) at 448 nm
shifted to a longer wavelength by 43 nm with a clear isosbes-
tic point at 435 nm, and the solution colour changed from
yellow to reddish orange (Figure 6).[27] The helix contents of
the pendant peptides in the DMSO/DMF-LiCl mixtures es-
More interestingly, the signs of the first Cotton effect of
the poly(PBGAm) in DMSO were opposite when compared
with those in DMF-LiCl. This implies that the predominant
helix sense of the polymers in DMSO may be opposite to
that in DMF-LiCl. The right- and left-handed helices of the
poly(PBGAm) are not exactly enantiomers, but diastereo-
mers because of the presence of chiral peptide pendants.
Their CD spectra, therefore, differ from one another. How-
ever, there may be other possibilities to explain the changes
in the CD patterns accompanied by the Cotton effect inver-
sion depending on the solvent; one is a change in the helical
pitch of the poly(PBGAm) with the same handedness rather
than the helix inversion, and the other is aggregations of the
poly(PBGAm) chains in specific solvent mixtures.[26] It is
well known that aggregations are highly sensitive to the poly-
mer concentrations. However, the magnitudes of the ICDs
of poly(PBGAm) in DMSO- and DMF-LiCl (10 mm) were
hardly changed over the concentration range of poly-
(PBGAm) 2 0.08 mgmLÀ1; this indicates that the formation
of aggregates could be excluded. The former possibility
could not be ruled out because the second Cotton sign was
negative in both solvents. We also measured the dynamic
light scattering (DLS) of poly(PBGA16) (run 5 in Table 2) in
DMSO and DMF containing 10 mm LiCl; the estimated hy-
drodynamic radius (Rh) values of the polymer were 67 and
88 nm, respectively. Therefore, these DLS results also sup-
port this conclusion. The CD spectral changes of poly-
(PBGA16) were then measured in DMSO with increasing
volumes of DMF (10 mm LiCl) (Figure 6).
1
timated by H NMR analyses decreased with an increasing
amount of DMSO, and this change in the helix content sig-
nificantly occurred in DMF-LiCl/DMSO (8:2 v/v), leading
to a possible helicity inversion of the polymer backbone as
evidenced by the CD inversion in the long wavelength
region. A similar solvent-driven CD spectral inversion ac-
companied by the visible colour change was also observed
in the DMSO-chloroform mixtures. As well as DMF, chloro-
form is one of the most effective helix supporting solvents
for PBLG.[16,18,19]
Figure 7 shows the CD and absorption spectral change of
poly(PBGA16) in the DMSO-chloroform mixtures. The CD
pattern dramatically changed with the increasing volumes of
We expected that the Cotton effect inversion of poly-
(PBGAm) might be induced by the change in the a-helix
content of the pendant PBGA chains during the change in
Figure 7. CD and absorption spectral changes of poly(PBGA16) (run 5 in
Table 2) in DMSO-chloroform mixtures at room temperature (ca. 238C).
chloroform, and the first Cotton sign became inverted at
DMSO/chloroform 6:4 (v/v) accompanied by a large blue
shift in the absorption spectra with isosbestic points at 352
and 420 nm. These changes in the CD and absorption spec-
tra caused by the change in the helix content of the pendant
peptides lead to the solvatochromism from reddish orange
to yellow. These results suggest that the poly(PBGA16) back-
bone may undergo a conformational transition such as a
helix helix transition by responding to the change in the
conformation of the pendant polypeptides (or oligopeptides)
(a-helix to random coil) stimulated by the external stimuli
(Figure 8).[28] Although the helix helix transition regulated
by achiral and chiral external stimuli such as solvent, tem-
Figure 6. CD and absorption spectral changes of poly(PBGA16) (run 5 in
Table 2) in DMSO-DMF mixtures containing 10 mm LiCl at room tem-
perature (ca. 238C). Inset photographs show the visible difference of poly-
(PBGA16) in DMSO and DMF-LiCl.
Chem. Eur. J. 2004, 10, 4000 4010
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E. Yashima et al.
FULL PAPER
the p-conjugated main-chain
region (330 600 nm) probably
due to a slight excess of the
single-handed helical formation
of the polyene backbone in
water at pH 10.7. However,
lowering the pH of the solution
caused
changes in the CD pattern from
nonsplit, broad positive
a series of dramatic
Figure 8. Illustration of interconvertible right- and left-handed helices of poly(PBGAm) stimulated by the
helix coil transition of the pendant peptide chains. The helix sense of the main chain is tentative.
a
perature, pH, and interaction with chiral or achiral guest
molecules has been reported for other synthetic helical poly-
mers[2] and biopolymers,[3] the present system is significantly
different from those previously reported; that is the inver-
sion of the macromolecular helicity regulated by the change
in the ordered structure of the peptides that occurred at the
remote side chain.[13]
Synthesis of water-soluble poly(PGAm)s and their chiropti-
cal properties: Poly(l-glutamic acid) (PGA) undergoes a
helix coil transition by changing the pH of the solution; it
forms an a-helix under acidic conditions (low pH) and a
random coil under basic conditions (high pH).[17] We then
hydrolysed the PBLG side chains of poly(PBGAm) in aque-
ous alkaline solution to obtain a water-soluble poly(PGAm)
bearing a poly(l-glutamic acid) pendant (Scheme 3), and
the effect of the pH on the main chain conformational
change induced by the change in the ordered conformation
Figure 9. CD and absorption spectral changes of poly(PGA20) in water at
room temperature (ca. 238C) with pH. The molar ellipticity ([q]) and
molar absorption coefficient (e) were calculated using the concentration
of the gultamic acid residue (200 330 nm) and the monomer units of poly-
(PGA20) (330 650 nm). Inset photographs show the visible difference of
poly(PGA20) in water at different pH.
(a-helix random coil) of the PGA pendants in water was in-
vestigated. Poly(PBGA20) was selected as the typical sample
and it was converted to poly(PGA20) by hydrolysis of the
benzyl ester groups of the pendant PBLG in DMF-aqueous
NaOH (1n) (1:6 v/v) (Scheme 3). The completion of the hy-
drolysis of the esters was confirmed by IR and 1H NMR
spectroscopies. The poly(PGA20) obtained was soluble in
water over pH 3.8. The stereoregularity of the poly(PGA20)
was evaluated by using laser Raman spectroscopy. Poly-
(PGA20À)1 showed characteristic peaks at 1551, 1342, and
9]
Cotton effect to a split-type one.[4 These CD changes were
accompanied by a gradual blue shift in the absorption spec-
tra with a clear isosbestic point at 387 nm and the solution
colour changed from deep yellow to light yellow. These CD
and absorption spectral changes were reversible. Simultane-
ously, the CD pattern corresponding to the pendant PGA
chains at wavelengths below 250 nm also significantly
changed. Poly(PGA20) exhibited a characteristic CD due to
a random coil conformation at pH 10.7, whereas the CD
pattern in that region dramatically changed by lowering the
pH and exhibited the two negative bands at 210 and
225 nm, a typical right-handed a-helix below pH 4.8,[17,21a,29]
À
À
962 cm , due to the C=C, C C, and C H bond vibrations
in the cis polyacetylenes, respectively, indicating that the
poly(PGA20) derived from the poly(PBGA20) maintained a
highly cis-transoid structure after hydrolysis of the ester
groups.
Figure 9 depicts the changes in the CD and absorption
spectra of poly(PGA20) in water at various pHs. Poly-
(PGA20) also exhibited a rather weak, but apparent ICD in
Scheme 3. Synthesis of poly(PBGAm) and poly(PGAm).
4006
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Chem. Eur. J. 2004, 10, 4000 4010

Poly(phenylacetylene)s Bearing a Peptide Pendant
4000 4010
Materials: THF and hexane were dried over sodium/benzophenone and
distilled under nitrogen. Chloroform was dried over calcium hydride and
distilled under nitrogen. These solvents were stored under nitrogen over
molecular sieves 4 ä (Nacalai Tesque, Kyoto, Japan). Triethylamine was
dried over KOH pellets and distilled onto KOH under nitrogen, which
was distilled under high vacuum just before use. DMF was dried over cal-
cium hydride and distilled under reduced pressure. Triphosgene, 4-iodo-
aniline, and Boc-Ala were purchased from Tokyo Kasei (TCI, Tokyo,
Japan). N,N’-Dicyclohexylcarbodiimide (DCC) and bis(triphenylphosphi-
ne)palladium dichloride were purchased from Nacalai Tesque and Wako
(Osaka, Japan), respectively. Triphenylphosphine, copper(i) iodide, l-glu-
tamic acid, and benzyl alcohol were obtained from Kishida (Osaka,
Japan). Bis[(norbornadiene)rhodium(i) chloride] {[Rh(nbd)Cl]2} and 1-
hydroxybenzotriazole monohydrate (HOBt) were purchased from Al-
drich. (Trimethylsilyl)acetylene (TMSA) was kindly supplied from Shi-
netsu Chemical (Tokyo, Japan). g-Benzyl l-glutamate was prepared by
the reaction of l-glutamic acid and benzyl alcohol.[32]
indicating that the pendant PGA chains underwent a
random coil to a-helix transition by changing the solution
pH. The a-helix content of the pendant PGA chains in the
poly(PGA20) in water at pH 3.6 was estimated to be about
43% on the basis of the reported value of molar ellipticity
of a 100% helical PGA at 222 nm ([q]222 =À3.4î104).[30]
The reason for this rather low a-helicity of the poly(PGA20)
may be due to the low DP of the PGA pendant,[31] and the
hydrogen-bonding formation between the pendant carboxy
acids may prevent the a-helix formation through the intra-
molecular hydrogen bonding interaction. However, we be-
lieve that the a-helix content may be underestimated and
thus be higher, because CDs from the poly(phenylacetylene)
overlapped in the region below 250 nm. Consequently, a
water-soluble poly(PGA20) also underwent a conformational
change from one helix to another with a different helical
pitch of the same or opposite helix sense, and this conforma-
tional change can be regulated by the change in the ordered
structure of the pendant peptides in water.
4-{N-[tert-Butoxycarbonyl]-l-alanylamino}iodobenzene: HOBt (20.9 g,
155 mmol) was added to a solution of Boc-Ala (12.3 g, 86.0 mmol) in dry
THF (50 mL) at 08C. After the solution was stirred at 08C for 1 h under
nitrogen, DCC (50.0 g, 242 mmol) in THF (50 mL) was added and the
mixture was stirred at 08C for 1 h. 4-Iodoaniline (20.1 g, 92.1 mmol) in
THF (50 mL) was then added to the reaction mixture at 08C and the so-
lution was stirred at room temperature for 17 h. After filtration, the fil-
trate was evaporated under reduced pressure. The residue was dissolved
in ethyl acetate and washed with saturated NaHCO3 aqueous solution,
aqueous citric acid (pH 3), and water, and the organic layer was dried
over Na2SO4. After filtration, the solvent was evaporated and the crude
product was purified by silica gel chromatography with ethyl acetate/
hexane (1:4 v/v) as the eluent, yielding a yellowish white solid (19.4 g,
57.7%).[33] M.p. 169.1 170.08C; [a]2D5 =À458 (c=0.5 in chloroform); IR
(KBr): n˜ =3297 (amide NH), 1687 (amide I), 1517 cmÀ1 (amide II); 1H
NMR (300 MHz, CDCl3): d=1.42 (d, 3H; CH3), 1.45 (s, 9H; C(CH3)3),
4.31 (q, 1H; CH), 5.18 (d, 1H; NH), 7.26 (d, 2H; aromatic), 7.57 (d, 2H;
aromatic), 8.64 (s, 1H; amide NH).
Conclusion
The optically active poly(phenylacetylene)s bearing a series
of polypeptide pendants form a predominantly one-handed
helical structure in which the helical polypeptide pendants
are arranged in a helical array with a predominant screw-
sense along the helical poly(phenylacetylene) backbone. A
unique conformational change such as a helix helix transi-
tion of the polymer backbone takes place by response to the
conformational change of the pendant polypeptides, which
can be controlled by external stimuli such as the change in
the solvent or pH, although the relationships between the
Cotton effect patterns at the polymer backbone regions and
the conformations of the polymer remain unknown at pres-
ent. We expect this methodology to be useful in the con-
struction of novel switchable chiral materials such as chiral
sensors and chiral selectors.[1]
4-[l-Alanylamino]iodobenzene: 4-{N-[tert-Butoxycarbonyl]-l-alanylami-
no}iodobenzene (19.4 g, 49.6 mmol) was dissolved in formic acid
(350 mL) and 0.1n HCl (1.2 mL) and the solution was stirred at room
temperature for 5 h. The solution was concentrated to ca. 100 mL, the
aqueous layer neutralised with saturated aqueous NaHCO3, extracted
with chloroform, and the chloroform layer was washed with water and
dried over Na2SO4. After filtration, the solvent was removed by evapora-
tion. The residue was purified by chromatography on silica gel with
hexane/ethyl acetate (1:1 v/v), followed by hexane/ethyl acetate/methanol
(2:2:1 v/v/v) as the eluents to give 4-[l-alanylamino]iodobenzene (9.92 g,
34.2 mmol, 69.0%).[33] M.p. 113.2 114.08C; [a]23655 =+2.68 (c=0.5 in
chloroform); IR (KBr): n˜ =3293 (amide NH), 1670 (amide I), 1521 cmÀ1
(amide II); 1H NMR (300 MHz, CDCl3): d=1.43 (d, 3H; CH3), 1.57 (s,
2H; NH2), 3.60 (q, 1H; CH), 7.40 (d, 2H; aromatic), 7.61 (d, 2H; aro-
matic), 9.53 (s, 1H; amide NH).
Experimental Section
4-[l-Alanylamino]phenylacetylene (1): Bis(triphenylphosphine)palladium
dichloride (718 mg, 1.02 mmol), triphenylphosphine (266 mg, 1.02 mmol),
and copper(i) iodide (195 mg, 1.02 mmol) were added to a solution of 4-
[l-alanylamino]iodobenzene (9.90 g, 34.1 mmol) in triethylamine
(300 mL) and methanol (30 mL) and the reaction mixture was stirred
under nitrogen at room temperature for 1 h. (Trimethylsilyl)acetylene
(15 mL, 0.10 mmol) was then added to the reaction mixture, which was
stirred under nitrogen at room temperature for 3 h. The solvent was re-
moved under reduced pressure, and the residue was dissolved in ethyl
acetate; the insoluble ethyl acetate part was removed by filtration. The
filtrate was washed with saturated NaHCO3 aqueous solution and water,
and dried over Na2SO4. After the solvent was evaporated, the crude resi-
due was purified by column chromatography on silica gel with hexane/
ethyl acetate (1:4 v/v) as the eluent to give yellowish white crystals (1-[4-
[l-alanylamino]phenyl]-2-[trimethylsilyl]acetylene) (7.48 g, 84.0%). M.p.
138.3 139.38C; [a]23655 =À7.08 (c=0.5 in chloroform); IR (KBr): n˜ =3275
Instrumentation: NMR spectra were measured on a Varian VXR-500
1
1
(500 MHz for H) or Varian Mercury 300 (300 MHz for H) spectrometer
in CDCl3, D2O, [D6]DMSO, or [D7]DMF using tetramethylsilane (for
CDCl3), dioxane (for D2O), or a solvent residual peak (for [D6]DMSO
and [D7]DMF) as the internal standard. Melting points were measured
on a B¸chi melting point apparatus and are uncorrected. Optical rotation
was measured in a 5 cm quartz cell on a Jasco P-1030 polarimeter. IR
spectra were recorded using a Jasco Fourier Transform IR-620 spectro-
photometer. Absorption spectra were measured with a Jasco V-570 spec-
trophotometer in a 0.1 cm quartz cell. CD spectra were measured on a
Jasco J-725 spectropolarimeter with a liquid nitrogen-controlled quartz
cell (0.5 cm) in a cryostat. The temperature was controlled with a Jasco
PTC-348WI apparatus. SEC was performed with a Jasco PU-980 liquid
chromatograph equipped with a UV/Vis (254 nm, Jasco UV-970) detector
using Tosoh TSK-GEL a-3000 and a-5000 columns connected in series
(eluent: DMF containing LiCl (10 mm), standards: poly(ethylene oxide)s
and poly(ethylene glycol)s. DLS measurements were performed on a
DLS-7070YN (Otsuka Electronics Co. Ltd., Japan) equipped with a
10 mW He/Ne Laser (632.8 nm) at a fixed scattering angle of 908 at
258C.
(amide NH), 3189 ( NH2), 2154 (CꢀC), 1664 (amide I), 1533 cmÀ1
À
(amide II); 1H NMR (300 MHz, CDCl3): d=0.23 (s, 9H; Si(CH3)3), 1.40
(d, 3H; CH3), 2.03 (s, 2H; NH2), 3.61 (q, 1H; CH), 7.40 (d, 2H; aromat-
ic), 7.54 (d, 2H; aromatic), 9.60 (s, 1H; NH).
Chem. Eur. J. 2004, 10, 4000 4010
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¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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E. Yashima et al.
FULL PAPER
NaOH (1n, 20 mL) was added to a solution of 1-[4-[l-alanylamino]phen-
yl]-2-[trimethylsilyl]acetylene (6.94 g, 23.9 mmol) in methanol (200 mL)
and the solution was stirred at room temperature for 5 h. After the sol-
vent was removed under reduced pressure, the residue was dissolved in
chloroform, washed with water, and dried over Na2SO4. The solvent was
evaporated, and the resulting crude product was purified by recrystallisa-
tion from hexane/ethyl acetate (1:1 v/v) to give 1 as a white solid (3.48 g,
72.1%). M.p. 86.1 86.48C; [a]52757 =+2.98 (c=5 in chloroform); IR (KBr):
10 mm LiCl, and methanol was added to the solution as the precipitant.
The resulting yellow precipitate was collected by centrifugation, washed
with acetone, and dried in vacuo at 508C for 2 h to give poly(PBGA20)
showing a unimodal SEC trace (0.18 g, 20% yield). Poly(PBGA20) was
soluble in DMSO and DMF containing LiCl, but insoluble in other usual
organic solvents such as toluene, THF, chloroform, acetonitrile, and ace-
tone. The molecular weight (Mn) and the distribution (Mw/Mn) of the poly-
(PBGA20) were estimated to be 2.32î105 and 1.97, respectively, as deter-
mined by SEC (poly(ethylene oxide) standards using DMF containing
10 mm LiCl as the eluent).
À
n˜ =3382 (amide NH), 3315 ( NH2), 3147 (ꢀCH), 1655 (amide I),
1524 cmÀ1 (amide II); 1H NMR (300 MHz, CDCl3): d=1.37 (d, 3H;
CH3), 1.80 (s, 2H; NH2), 3.08 (s, 1H; ꢀCH), 3.56 (q, 1H; CH), 7.43 (d,
2H; aromatic), 7.53 (d, 2H; aromatic), 9.61 (s, 1H; NH); 13C NMR
(75 MHz, CDCl3): d=21.5 (CH3), 51.1 (CH), 76.8, 83.5 (HCꢀC), 117.1,
118.9, 132.7, 138.2 (aromatic), 173.8 (C=O); elemental analysis calcd (%)
for C11H12N2O (202.25): C 70.19, H 6.43, N 14.88; found: C 70.19, H 6.58,
N 14.64.
Spectroscopic data of poly(PBGA20): IR (KBr): n˜ =3295 (amide NH),
1736 (C=O of ester), 1655 (amide I), 1543 cmÀ1 (amide II); elemental
analysis calcd (%) for [(C12H13NO3)20¥C11H12N2On¥4H2O]n: C 64.90, H
6.08, N 6.63; found: C 64.75, H 5.95, N 6.83. For 1H NMR, see Figure S1
(Supporting Information).
Hydrolysis of PBGA: Poly(PBGA20) (28 mg) was dissolved in DMF
(10 mL) containing 10 mm LiCl and 1n NaOH aq.(10 mL) was added at
08C. After 10 min, 50 mL of water was added at 08C and the reaction
mixture was stirred at 08C for 1 h. The solution was neutralised with 1n
HCl aq. and freeze-dried. The residue was suspended in 0.3n NaOH
(70 mL) and stirred at room temperature until the suspended compound
was completely dissolved (2 h). The solution was then acidified with 1n
HCl aq. and the precipitated poly(PGA20) was collected by centrifuga-
tion, washed with water, and dried in vacuo at room temperature over
night. Yield 50.6% (8.6 mg).
g-Benzyl l-glutamate N-carboxyanhydride (BLG-NCA): This was pre-
pared by the reaction of g-benzyl l-glutamate with triphosgene and puri-
fied by repeated recrystallisation from a mixture of hexane and THF ac-
cording to the literature method (75.1% yield).[34] IR (KBr): n˜ =3300
À1
À
( NH), 1884, 1786 (C=O of acid anhydride), 1721 cm (C=O of ester);
1H NMR (300 MHz, CDCl3): d=2.13, 2.31 (m, 2H; g-CH2), 2.61 (m, 2H;
À
À
b-CH2), 4.37 (m, 1H; CHNH ), 5.15 (s, 2H; CH2Ph), 6.24 (s, 1H; NH),
7.36 (m, 5H; aromatic).
Synthesis of macromonomer PBGAm: PBGAm was synthesised by the
polymerisation of the BLG-NCA with
1 as initiator in chloroform
Spectroscopic data of poly(PGA20): IR (KBr): n˜ =1719 (C=O of carboxyl-
ic acid), 1655 (amide I), 1544 cmÀ1 (amide II); elemental analysis calcd
(%) for [(C5H7NO3)19¥C12H13NO3¥C11H12N2O¥2H2O]n: C 48.93, H 5.64, N
(Scheme 2). A typical polymerisation procedure (run 3 in Table 1) is de-
scribed below. Polymerisation was carried out in a dry glass ampoule
under a dry nitrogen atmosphere. BLG-NCA (3.99 g, 15.1 mmol) was
placed in a dry ampoule, which was then evacuated on a vacuum line
flushed with dry nitrogen. After this evacuation-flush procedure was re-
peated three times, a three-way stopcock was attached to the ampoule,
and chloroform was added with a syringe. To this was added a solution of
1 in chloroform at 308C. The concentrations of the monomer and the ini-
tiator were 0.25 and 0.017m, respectively. The polymerisation proceeded
homogeneously and the reaction mixture gelated within a few hours.
After 40 h, the resulting polymer was dissolved in DMF and precipitated
into a large amount of methanol, collected by centrifugation, and dried
in vacuo at room temperature over night (2.63 g, 74.5%). The methanol-
soluble fraction was concentrated under reduced pressure and reprecipi-
tated into a large amount of ether. The ether-insoluble part was collected
by centrifugation, and dried in vacuo at room temperature over night
(0.59 g, 16.8%).
1
10.64; found: C 48.99, H 5.72, N 10.21. For H NMR, see Figure S2 (Sup-
porting Information).
CD measurements–effect of solvent on ICD of poly(PBGAm): A stock
solution of poly(PBGA16) in DMF (4.7 mg/3 mL) containing 10 mm LiCl
was prepared in a 3 mL flask equipped with a stopcock and the initial
CD and absorption spectra were recorded. 900, 800, 700, and 600 mL ali-
quots of the stock solution were transferred to four flasks equipped with
a stopcock using a Hamilton microsyringe, and DMSO was added up to
mark. Absorption and CD spectra were taken for each flask with the use
of a 0.2 or 0.5 cm quartz cell. In a similar manner, the effect of chloro-
form on ICD of poly(PBGA16) was investigated.
Concentration effect of poly(PBGA) on ICD: A stock solution of poly-
(PBGA16) (2 mgmLÀ1) in DMSO was prepared in a 2 mL flask equipped
with a stopcock and the initial CD spectrum was recorded with a 0.4 cm
quartz cell. A 600 mL aliquot of the stock solution was transferred to a
vessel with a screw-cap using a Hamilton microsyringe and the solution
Spectroscopic data of PBGA20: IR (KBr): n˜ =3295 (amide NH), 1736 (C=
O of ester), 1655 (amide I), 1543 cmÀ1 (amide II); 1H NMR (500 MHz,
[D7]DMF): d=1.4 1.6 (m, 3H; CH3 of 1), 2.0 3.0 (br, 4Hî20; CH2CH2 î
20), 4.13 (brs, 1Hî20; a-CH (a-helix)î20), 4.42 (brs, 1Hî20; a-CH
(random coil)î20), 5.0 5.3 (m, 2Hî20; CH2Phî20), 7.36 (brs, 5H; aro-
matic), 7.45 (d, 2H; aromatic of 1), 7.96 (d, 2H; aromatic of 1), 8.53 (brs,
1Hî20; NHî20); elemental analysis calcd (%) for (C12H13NO3)20¥C11H12-
N2O¥2H2O (4608.99): C 65.41, H 6.04, N 6.69; found: C 65.45, H 6.00, N
6.89.
was diluted with DMSO, giving
a
0.08 mgmLÀ1 solution of poly-
(PBGA16). The CD spectrum of this solution was recorded in a 5 cm
quartz cell. Similarly, the concentration effect of poly(PBGA16) on ICD
in DMF containing LiCl (10 mm) was also investigated.
Effect of pH on ICD of poly(PGA20): Deionised, distilled water was used
after degassing with nitrogen. A stock solution of poly(PGA20) in alkaline
water (pH 10.7) (1.5 mg/3 mL) was prepared in a 3 mL flask equipped
with a stopcock and the initial CD and absorption spectra were recorded
with 0.05 (200 330 nm) and 1.0 cm (330 650 nm) quartz cells. The pH of
the solution was measured with a B-211 pH meter (Horiba). The pH of
the solution was adjusted with 1.0 or 0.5n HCl aq. The absorption and
CD spectra were then taken at pH 6.8, 6.1, 4.8, and 3.6 with 0.05 (200
330 nm) and 1.0 cm (330 650 nm) quartz cells.
Polymerisation of PBGAm: Polymerisation of the macromonomer
PBGAm was conducted according to Scheme 3. A typical polymerisation
procedure (run 6 in Table 2) is described below. Polymerisation was car-
ried out in a similar way for the polymerisation of BLG-NCA in a dry
glass ampoule under a dry nitrogen atmosphere with [Rh(nbd)Cl]2 as the
catalyst. PBGA20 (0.91 g, 0.19 mmol) was placed in a dry ampoule and
DMF and triethylamine were added with
a syringe. A solution of
Dynamic light scattering measurements: In the DLS measurements, we
used poly(PBGA16) (run 4 in Table 1). Solutions of poly(PBGA16)
(2 mgmLÀ1) in DMF containing 10 mm LiCl and in DMSO were sepa-
rately prepared in 2 mL flasks equipped with a stopcock and the solu-
tions were filtered using a 0.2 mm syringe filter (Toyo Roshi Co. Ltd.,
Japan). The DLS measurements of the samples were performed at a
fixed scattering angle of 908. The obtained autocorrelation functions
were analysed by the method of cumulants to give the translational diffu-
sion coefficients (D). The corresponding hydrodynamic radius (Rh) was
[Rh(nbd)Cl]2 in DMF was added at 308C. The concentrations of the
monomer and the rhodium catalyst were 0.15 and 0.006m, respectively.
The polymerisation proceeded homogeneously, and the solution became
viscous with time. After 17 h, the resulting polymer was precipitated into
a large amount of methanol, collected by centrifugation, and dried in
vacuo at 508C for 2 h (0.18 g, 20%). The SEC measurement of the ob-
tained methanol-insoluble fraction showed a trimodal SEC trace, suggest-
ing that the methanol-insoluble part consisted of three fractions with a
different molecular weight: high and low molecular weight polymeric
fractions, and the unreacted macromonomer PBGA20. In order to
remove the low molecular weight fraction and the unreacted macromo-
nomer, the methanol-insoluble part was dissolved in DMF containing
calculated using the Stokes Einstein equation: Rh
= kB T/(6phD),
where kB, h, and T are the Boltzmann constant, the solvent viscosity, and
the absolute temperature, respectively.
4008
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 4000 4010

Poly(phenylacetylene)s Bearing a Peptide Pendant
4000 4010
[6] a) E. Yashima, T. Nimura, T. Matsushima, Y. Okamoto, J. Am.
Chem. Soc. 1996, 118, 9800 9801; b) H. Kawamura, K. Maeda, Y.
Okamoto, E. Yashima, Chem. Lett. 2001, 58 59.
[7] H. Onouchi, K. Maeda, E. Yashima, J. Am. Chem. Soc. 2001, 123,
7441 7442.
Acknowledgement
This work was partially supported by Grant-in-Aid for Scientific Re-
search from the Japan Society for the Promotion of Science and the Min-
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Received: March 30, 2004
Published online: June 30, 2004
4010
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 4000 4010