Synthesis and hierarchical structures of amphiphilic polyphenylacetylenes carrying L-valine pendants

Article abstract of DOI:10.1021/ma0213091

In this work, we synthesized amino acid-containing poly(phenylacetylene)s and demonstrated the possibility of tuning their hierarchical structures by internal and external perturbations. A valine-acetylene adduct, 4-ethynylbenzoyl-L-valine methyl ester (5), was readily polymerized by [Rh(nbd)Cl]₂, and the resultant polyester (1) was selectively hydrolyzed by KOH to its polyacid congener, poly(4-ethynylbenzoyl-L-valine) (2). (Supra)molecular structures of the polymers were characterized by NMR, IR, UV, CD, and AFM techniques. The macromolecules took helical chain conformations in solutions. Their Cotton effects varied to different extents, when the polymer, solvent, and pH changed respectively from i to 2, from methanol to THF, and from neutral to basic. Upon evaporation of their methanol solutions, 1 and 2 self-assembled into micellar spheres and helical cables, respectively. Changing the solvent of i to THF changed its folding structure to helical cables, while increasing the pH of the methanol solution of 2 led to the formation of random threads. The denaturation from helical cables to random threads is probably caused by the cleavage of interstrand hydrogen bonds by base-mediated ionization of carboxyl groups of the valine pendants.

Full text of DOI:10.1021/ma0213091

Macromolecules 2003, 36, 77-85
77
Synthesis and Hierarchical Structures of Amphiphilic
Polyphenylacetylenes Carrying L-Valine Pendants
Bin g Sh i Li,†,‡ Kevin K. L. Ch eu k ,‡ Lia n sh en g Lin g,† J u n w u Ch en ,‡ Xu d on g Xia o,§
Ch u n li Ba i,*,† a n d Ben Zh on g Ta n g*,‡
Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100080, China; and Departments of Chemistry and Physics, Institute of Nano Science
and Technology, and Open Laboratory of Chirotechnology of the Institute of Molecular
Technology for Drug Discovery and Synthesis,| Hong Kong University of Science & Technology,
Clear Water Bay, Kowloon, Hong Kong, China
Received August 12, 2002; Revised Manuscript Received November 6, 2002
ABSTRACT: In this work, we synthesized amino acid-containing poly(phenylacetylene)s and demonstrated
the possibility of tuning their hierarchical structures by internal and external perturbations. A valine-
acetylene adduct, 4-ethynylbenzoyl-L-valine methyl ester (5), was readily polymerized by [Rh(nbd)Cl]2,
and the resultant “polyester” (1) was selectively hydrolyzed by KOH to its “polyacid” congener, poly(4-
ethynylbenzoyl-L-valine) (2). (Supra)molecular structures of the polymers were characterized by NMR,
IR, UV, CD, and AFM techniques. The macromolecules took helical chain conformations in solutions.
Their Cotton effects varied to different extents, when the polymer, solvent, and pH changed respectively
from 1 to 2, from methanol to THF, and from neutral to basic. Upon evaporation of their methanol
solutions, 1 and 2 self-assembled into micellar spheres and helical cables, respectively. Changing the
solvent of 1 to THF changed its folding structure to helical cables, while increasing the pH of the methanol
solution of 2 led to the formation of random threads. The denaturation from helical cables to random
threads is probably caused by the cleavage of interstrand hydrogen bonds by base-mediated ionization of
carboxyl groups of the valine pendants.
In tr od u ction
A on pH: its rate constant kcat peaks in a neutral
medium (pH ∼7) but sharply drops at lower (acidic) or
higher (basic) pH.8
Hierarchical organization of biopolymers forms a
structural basis of life in the natural world.1,2 While the
primary structures of the biopolymers are constructed
via covalent linkage of naturally occurring building
blocks such as amino acids, their higher-order (second-
ary, tertiary, and quaternary) structures are assembled
through intra- and interchain associations cooperatively
aided by noncovalent forces such as hydrogen bonding,
hydrophobic stacking, solvation effect, and electrostatic
interaction.3 The molecular information such as amino
acid sequence and chain chirality and amphiphilicity
encoded in the primary structures of the biomacromol-
ecules plays a primary role in determining their native
folding structures; for example, L-glutamic acid seg-
ments in proteins often give an R-helix structure, while
L-isoleucine segments most frequently induce â-sheet
formation.4,5 The folding structures can, however, be
varied or denatured by the changes in the environmen-
tal surroundings of the biopolymers, due to the nonco-
valent nature of the supramolecular assembling.3,6 Loss
of body fluid (an important biological medium or “sol-
vent”), for example, can transmute organizational struc-
tures of proteins by dehydration or deprivation. Varia-
tion in pH often causes changes in the active-site
structures of enzymes, which in turn alters their bio-
catalytic activities.7 This is best manifested by the bell-
shaped dependence of the enzymatic activity of RNase
Amino acids are the constitutional components of pro-
teins, which are everywhere in living systems: hair,
skin, muscle, and connective tissue are proteins; almost
all enzymes are proteins; ....5 Incorporation of the nat-
urally occurring building blocks into manmade polymers
is of interest because such a meld may create new
nonbiological macromolecules with biomimetic struc-
tures and properties.9,10 For example, molecular hybrid-
ization of hydrophilic amino acids with hydrophobic
synthetic polymers will give birth to amphiphilic off-
spring;11 the chirality of the amino acid moieties may
induce the macromolecular chains to rotate in a screw
sense,12-15 and self-assembling of the helical amphi-
pathic polymers may generate proteomimetic organi-
zational structures.16,17 The amino acid moieties may
be arranged, through judicious molecular design, in such
a way that they wrap a conjugated polymer chain as
pendant groups; in other words, a conductive molecular
wire may be sheathed in a cytophilic shroud. Such bio-
compatible nanowires may find innovative applications
in medical diagnosis as biosensors, in drug delivery
devices as control elements, in tissue engineering as
architectural scaffolds, and more exotically, in cytotech
and nanorobotics as artificial nerves, retinas, muscles,
etc.18
Intrigued by the captivating prospects, in this study,
we chose poly(phenylacetylene) (PPA), the best-known
photoconductive polyacetylene,19,20 as a model polymer,
and tried to attach L-valine, one of the 20 amino acids
commonly found in proteins, to the PPA chains as
pendants. By fusing the valine and acetylene units
together at the monomer stage, we succeeded in syn-
thesizing PPA derivatives with every one of their repeat
units being precisely appended with one amino acid
* Corresponding authors. E-mails: tangbenz@ust.hk (B.Z.T.);
clbai@infoc3.icas.ac.cn (C.B.).
† Center for Molecular Sciences, Chinese Academy of Sciences.
‡ Department of Chemistry, Hong Kong University of Science
& Technology.
§ Department of Physics, Hong Kong University of Science &
Technology.
| An Area of Excellence (AoE) Scheme administrated by the
University Grants Committee of Hong Kong.
10.1021/ma0213091 CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/12/2002

78 Li et al.
Macromolecules, Vol. 36, No. 1, 2003
Sch em e 1. Syn th esis of Va lin e-Con ta in in g P olyp h en yla cetylen es: P oly(4-eth yn ylben zoyl-L-va lin e m eth yl ester )
(1) a n d P oly(4-eth yn ylben zoyl-L-va lin e) (2)
solutions were calculated on the basis of the repeat units of
the polymers.
moiety (Scheme 1). The pendant chirality induced the
PPA backbone to helically spiral and the chain am-
phiphilicity enabled the polymer strands to self-as-
semble. Similar to proteins, the synthetic hybrids
changed their hierarchical structures in response to the
changes in their molecular structures and environmen-
tal surroundings. Changing the end-capped valine ester
in 1 to “free” valine acid in 2 enhanced the chain
hydrophility; this internal perturbation (structural
change), as well as external stimuli (solvent and pH
variations), affected the folding structures of the bio-
mimetic polymers to various extents.
The AFM samples were prepared by placing tiny amounts
(∼3-5 µL) of dilute polymer solutions (∼1-5 µg/mL) on the
new surfaces of freshly cleaved mica under ambient conditions.
The morphological structures formed by the polymers upon
natural evaporation of the solvents were imaged on a Nano
IIIa atomic force microscope (Digital Instruments) operating
in a tapping mode using hard silicon tips with a spring
constant of ∼40 N/m.
Mon om er Syn th esis. The valine-containing acetylene
monomer 5 was prepared by amidation of 4-ethynylbenzoyl
chloride (3) with L-valine methyl ester hydrochloride (4; cf.,
Scheme 1). Into a 100 mL round-bottom flask were added 1.02
g (6.1 mmol) of 4, 3 mL of pyridine, and 10 mL of DCM under
nitrogen. The contents were mixed by stirring and cooled with
an ice bath. A solution of 3 (1.00 g, 6.1 mmol) in 10 mL of
DCM was then slowly injected into the flask. The reaction
mixture was gradually warmed to room temperature and
stirred overnight. The mixture was then diluted with 100 mL
of DCM, and the resultant solution was washed twice with
dilute HCl solution and once with water. The organic layer
was dried over 5 g of magnesium sulfate. After filtration of
the solid and removal of the solvent, the crude product was
purified on a silica gel column using a mixture of CHCl3/
acetone (15:1 by volume) as eluent. Evaporation of the solvents
gave 1.03 g of product 5 as white solid (yield: 65.1%). IR (KBr),
ν (cm-1): 2105 (m, CtC), 1738 (s, CdO), 1652 (s, CdO). 1H
NMR (300 MHz, CDCl3), δ (TMS, ppm): 7.7 (m, 2H, aromatic
protons o to CdO), 7.4 (m, 2H, aromatic protons m to CdO),
6.8 (d, 1H, NH), 4.7 (m, 1H, NHCH), 3.7 (s, 3H, CO2CH3), 3.2
(s, 1H, tCH), 2.2 (m, 1H, CH), 0.9 [m, 6H, (CH3)2]. 13C NMR
(75 MHz, CDCl3), δ (TMS, ppm): 172.6 (CO2), 166.4 (CONH),
134.1 (aromatic carbon attached to CdO), 132.3 (aromatic
carbons m to CdO), 127.0 (aromatic carbons o to CdO), 125.6
(aromatic carbon p to CdO), 82.7 (PhCt), 79.6 (HCt), 57.5
Exp er im en ta l Section
Ma ter ia ls. Dichloromethane (DCM) was purchased from
Lab-Scan and was distilled over calcium hydride. Pyridine and
triethylamine (both from RdH) were dried and distilled over
KOH. THF (Aldrich) was dried over 4 Å molecular sieves and
distilled from sodium benzophenone ketyl immediately before
21
use. L-Valine methyl ester hydrochloride {4; [R]D +23.6° (c
2, methanol); Sigma} and phenylacetylene (PA; Aldrich) were
used as received without further purification. The rhodium
catalyst [Rh(nbd)Cl]2 (nbd ) 2,5-norbornadiene) was prepared
according to published procedures.21 4-Ethynylbenzoic acid was
prepared by palladium-catalyzed coupling of trimethylsilyl-
acetylene with methyl 4-bromobenzoate followed by based-
catalyzed desilylation, according to our previously published
synthetic procedures.22 The acid was converted to acid chloride
3, which was used in-situ for the preparation of the valine-PA
adduct or 4-ethynylbenzoyl-L-valine methyl ester 5 (see Mon o-
m er Syn th esis below).
In str u m en ta tion . The average molecular weights (Mw and
Mn) and polydispersity indexes (PDI; Mw/Mn) of the polymers
were estimated by gel permeation chromatography (GPC)
using a Waters Associates liquid chromatograph equipped with
a Water 510 HPLC pump, a column temperature controller, a
Waters 486 wavelength-tunable UV detector, and a Waters
410 differential refractometer. Styragel columns HT3, HT4,
and HT6 were used in the GPC system, which covers a
molecular mass range of 102-107 Da. Polymer solutions were
prepared in THF (∼2 mg/mL) and filtered with 0.45 µm PTFE
syringe-type filters before being injected into the GPC system.
THF was used as eluent at a flow rate of 1.0 mL/min. The
column temperature was maintained at 40 °C and the working
wavelength of the UV detector was set at 254 nm. Monodis-
perse polystyrene samples (Waters) were used as the calibra-
tion standards.
20
(NHCH), 52.3 (CO2CH3), 31.6 (CH), 19.0, 18.0 [(CH3)2]. [R]D
+54° (c 0.038, chloroform).
P olym er iza tion . The monomer was polymerized with [Rh-
(nbd)Cl]2 in Et3N/THF. Into a 20 mL Schlenk tube were added
0.2 mmol of 5 and 1 mL of THF. The catalyst solution was
prepared in another tube by dissolving 0.01 mmol of [Rh(nbd)-
Cl]2 in 1 mL of THF with a catalytic amount of Et3N, which
was transferred to the monomer solution using a hypodermic
syringe. The reaction mixture was stirred at room temperature
for 24 h. The mixture was then diluted with 2 mL of THF,
and the dilute solution was added dropwise to an acetone/ether
mixture (150 mL) under stirring. The precipitate was collected
by filtration and dried under vacuum at room temperature to
a constant weight. The polymeric product, i.e., poly(4-ethy-
nylbenzoyl-L-valine methyl ester) (1), was isolated as yellowish
fibrous solid in a high yield (93.2%). Mw: 371 000. Mw/Mn: 7.93
(GPC, polystyrene calibration). IR (KBr), ν (cm-1): 3032 (w,
dCH), 1740 (s, CdO), 1646 (s, CdO). 1H NMR (300 MHz,
acetone-d6), δ (TMS, ppm): 7.7 (NH), 7.6 (aromatic protons o
to CdO), 6.8 (aromatic protons m to CdO), 6.0 (cis olefin
The FT-IR spectra were recorded on a Perkin-Elmer 16 PC
FT-IR spectrometer. The 1H and 13C NMR spectra were
measured on a Bruker ARX 300 NMR spectrometer in chlo-
roform-d, acetone-d6, methanol-d4, and/or DMSO-d6. The deu-
terated solvents or tetramethylsilane (TMS) were used as the
internal references for the NMR analyses. Circular dichroism
(CD) measurements were performed on a J asco J -720 spec-
tropolarimeter in 1 mm quartz cuvettes using a step resolution
of 0.2 nm, a scan speed of 50 nm/min, a sensitivity of 0.1°,
and a response time of 0.5 s. Each spectrum was the average
of 5-10 scans. The molar concentrations of the polymer
proton), 4.5 (NHCH), 3.6 (CO2CH3), 2.2 (CH), 1.0 [(CH3)2]. 13
C
NMR (75 MHz, acetone-d6), δ (TMS, ppm): 173.1 (CO2), 167.7
(CONH), 146.0 (dC-), 139.7 (aromatic carbon p to CdO), 134.4
(aromatic carbon attached to CdO), 128.4 (H-Cd, aromatic

Macromolecules, Vol. 36, No. 1, 2003
Valine-Containing Poly(phenylacetylene)s 79
carbons o and m to CdO), 59.3, (NHCH), 52.2 (CO2CH3), 31.5
(CH), 19.6, 19.3 [(CH3)2]. UV (THF, 2.20 × 10-4 mol/L), λmax
(nm)/ꢀmax (mol-1 L cm-1): 272/6.67 × 103, 395/2.32 × 103. [R]D
-244.4° (c 0.126, chloroform).
:
20
Hyd r olysis. The valine methyl ester of 1 was partially or
completely hydrolyzed under controlled reaction conditions. A
typical experimental procedure for fully hydrolyzing the meth-
yl ester to the corresponding free acid is given below. To a 50
mL round-bottom flask were added 200 mg (0.77 mmol) of 1
and a methanolic solution of KOH (2 g KOH in 20 mL of
methanol). After 2 h of stirring at room temperature, the
mixture was poured into a dilute aqueous HCl solution. The
precipitate was collected by filtration and dried under vacuum
for at least 2 days to a constant weight. The “polyacid” or poly-
(4-ethynylbenzoyl-L-valine) (2) was isolated as yellowish
solid in 95.3% yield. Mw: 408 000. Mw/Mn: 8.47 (GPC, poly-
styrene calibration). IR (KBr), ν (cm-1): 3032 (w, dCH), 1732
(s, CdO), 1645 (s, CdO). 1H NMR (300 MHz, DMSO-d6), δ
(TMS, ppm): 12.6 (CO2H), 8.1 (NH), 7.6 (aromatic protons o
to CdO), 6.7 (aromatic protons m to CdO), 5.9 (cis olefin
proton), 4.3 (NHCH), 2.2 (CH), 0.9 [(CH3)2]. 13C NMR (75 MHz,
CD3OD), δ (TMS, ppm): 175.1 (CO2), 169.5 (CONH), 146.7
(dC-), 140.2 (aromatic carbon p to CdO), 134.0 (aromatic
carbon attached to CdO), 128.7 (H-Cd, aromatic carbons o
and m to CdO), 59.9 (NHCH), 31.6 (CH), 19.8, 19.4 [(CH3)2].
F igu r e 1. 1H NMR spectra of (A) an acetone-d6 solution of
poly(4-ethynylbenzoyl-L-valine methyl ester) (1) and (B) meth-
anol-d4 and (C) DMSO-d6 solutions of poly(4-ethynylbenzoyl-
L-valine) (2). The peaks of the residual nondeuterated solvents
and the water dissolved in the deuterated solvents are marked
with / symbols.
UV (THF, 1.39 × 10-4 mol/L), λmax (nm)/ꢀmax (mol-1 L cm-1):
20
272/8.04 × 103, 395/2.70 × 103. [R]D
: -423.3° (c 0.030,
chloroform).
panels B and C of Figure 1. The sharp peak associated
with the resonance of the methyl ester protons in 1 in
Figure 1A at δ 3.6 (peak g) completely disappeared in
the spectra of 2. As its replacement, a broad peak
related to the absorption of the newly formed carboxy
proton in 2 appeared at δ 12.6 (peak h). The resonance
signal of the amide proton (peak c) is still clearly seen,
although it has undergone a slight downfield shift due
to the change in its chemical environments caused by
the hydrolysis reaction. Comparison of the integrated
peak areas confirmed the intactness of the amide proton
or the selectivity of the hydrolysis reaction.
Ch a in Helicity. After confirming the molecular
structures of the polymers, we investigated their chain
conformations by chiroptical spectrometry. While mono-
mer 5 showed weak CD signals in the deep UV region
(<280 nm), its polymer (1) exhibited strong Cotton
effects in the long wavelength region, where its back-
bone absorbed (Figure 2).22,29 The CD and UV-vis
spectral data thus clearly confirm that the polyacetylene
backbone of 1 takes a helical conformation. Similarly,
polymer 2 was also CD-active in the low-energy region.
Its UV-vis absorption was, however, stronger than 1
over the whole spectral region. This hyperchromism is
probably associated with the difference in the chain
hydrophilicity caused by the difference in the pendant
polarity. In the polar solvent of methanol, the chains of
“polyacid” 2, which are more polar, may be more
extended, while those of the less polar “polyester” 1 may
be more compactly coiled, although they both take
helical conformations. The more extended chains of 2
allow its chiral pendants to be better exposed and enable
its polyene backbone to be better conjugated, which in
turn enhances the absorptions of its pendants in the UV
region and its backbone in the visible.
Resu lts a n d Discu ssion
P olym er Syn th esis. Two approaches have been
reported for attaching amino acid pendants to a poly-
acetylene backbone: (1) by macromolecular complex-
ation of amino acids with a preformed PPA acid23 and
(2) by covalent bonding of amino acid and acetylene
moieties at the monomer stage.24 The former approach
enjoys simplicity in polymer preparation, whereas the
latter route offers homogeneity in molecular structure.
Macromolecular complexation is an addition process,
which does not always proceed to completion (with every
repeat unit being complexed) due to the involved steric
effect.25 Polymerization of an amino acid-PA adduct,
however, affords inherent guarantee that every one of
the monomer repeat unit of the resultant polymer
carries one amino acid pendant. We thus in this work
took the latter approach and synthesized a valine-PA
monomer by an amidation reaction.
Using our previously developed synthetic method,22
we prepared a PPA derivative, 4-ethynylbenzoyl chlo-
ride (3). Simple coupling of 3 with a commercially
available L-valine salt, 4, gave a valine-PA adduct, 5
(cf., Scheme 1). The ester-capped valine-PA monomer
was readily converted by an acetylene polymerization
catalyst [Rh(nbd)Cl]226-28 to its polymer, 1, in excellent
yield (93%). The polymer exhibited well-defined NMR
spectra in polar solvents, an example of which is given
in Figure 1A. The resonance peaks well correspond to
the expected molecular structure of polymer 1, there
being no any unexpected or unidentifiable signals
originating from any side products or impurities. Using
a base-catalyzed hydrolysis reaction, we converted
“polyester” 1 to “polyacid” 2. Through optimization of
reaction conditions, we succeeded in selectively cleaving
the methyl ester groups of 1 while keeping its amide
bond unharmed. By changing the reaction conditions
(shortening reaction time, lowering reaction tempera-
ture, reducing KOH amount, etc.), we can now control
the ester hydrolysis to a desirable extent.
The chain helicity is obviously induced by the pendant
chirality.12-14,30 The asymmetric force field generated
by the chiral pendants may drive the polymer chains
to spiral in a screw sense, giving helical segments with
different pitches, lengths, and shapes, depending on the
backbone conformation31 (Chart 1A). Hydrogen bonding
between the segments of the same and/or different
Two examples of the 1H NMR spectra of a fully
hydrolyzed product, i.e., “polyacid” 2, are shown in

80 Li et al.
Macromolecules, Vol. 36, No. 1, 2003
by the polar solvent molecules to a somewhat better
extent, thus causing a slight increase in its UV-vis
absorptivity.
Su p r a m olecu la r Assem blin g. In the structural
hierarchy of biomacromolecules, chain helicity belongs
to the secondary structure, whose change often brings
about variations in their higher-order structures.1,4-6
We were able to tune the helical structures of our valine-
containing PPAs by altering their chain hydrophilicity
and environmental conditions, and we further investi-
gated how these internal and external perturbations
would affect their assembling behaviors and whether
the manipulation could be extended to, or amplified at,
the supramolecular level.
When a tiny drop of a dilute methanol solution of
“polyester” 1 was placed on newly cleaved mica, pearl-
shaped morphological structures were formed upon
natural evaporation of the solvent (Figure 4).35 Under
similar conditions, “polyacid” 2 assembled into helical
ropes, and a partially hydrolyzed polymer containing
both the ester and acid repeat units (∼1:1 in molar ratio)
gave an intermediate structure with spherical beads
strung up by a filamentary string. In methanol, the
coiled chains of “polyester” 1 may pack together to
minimize the exposure of its hydrophobic backbones to
the polar solvent, forming micellelike structures with
their outer shells decorated by the hydrophilic amide
and ester functional groups. During the solvent evapo-
ration, the micelles may grow in size and stick together
via intershell hydrogen bonding between the amide and
ester groups to give the clustered pearls. On the other
hand, the strong solvation power of the methanol
solvent toward “polyacid” 236 may force the helical
chains to take an extended conformation. When metha-
nol evaporates, the individual helical chains may ag-
gregate in a side-by-side fashion via interchain hydrogen
bonding to form spirally twisting fibrils, similar to the
assembling process followed by fibrous proteins such as
keratins and collagen in the formation of twisted cables.5
The partially hydrolyzed polymer consists of both
“polyester” and “polyacid” segments, and it is thus not
surprising that the morphological structure formed by
the polymer contains both assembling features of 1 and
2, because the association of the coiled “polyester”
segments would give the spherical knobs and spiraling
of the extended “polyacid” segments would yield the
helical string.
F igu r e 2. CD and UV-vis spectra of poly(4-ethynylbenzoyl-
L-valine methyl ester) (1) and (B) poly(4-ethynylbenzoyl-L-
valine) (2). The CD spectrum of ethynylbenzoyl-L-valine methyl
ester monomer (5) is shown for comparison. Solvent: metha-
nol. Concentration (mM): ∼1.5 (CD); ∼0.1-0.2 (UV). Tem-
perature: ∼23 °C (room temperature).
chains may compensate the entropic cost paid for the
formation of the regular helical structures, examples of
such hydrogen bonds being given in panels B and C of
Chart 1. The cisoid and transoid backbone conforma-
tions can be inter-changed by liable single-bond rotation
(atropisomerism),12h,32 and the entropic balance can be
disturbed and shifted to another equilibrium state by
breaking the noncovalent hydrogen bonds between the
chain segments.3-5,33 The susceptibility of these pro-
cesses suggests the possibility of manipulating the chain
helicity of the polyacetylene segments by environmental
changes or external perturbations.
This proved to be the case. When the solvent of the
solution of polymer 1 was changed from methanol to
THF, its Cotton effects were intensified and its molar
absorptivity was also enhanced (Figure 3A). Methanol
is a good solvent for the amino acid pendants but a
bad solvent for the PPA backbone. THF is, however, a
good solvent for both the pendants and the back-
bone. The polymer chains thus may take a coiled con-
formation in methanol but an extended one in THF. The
coiled chains may experience complex steric effects and
bury some chain segments inside the cores during the
folding process induced by the solvophobic effect.34
The extended chains, on the other hand, may allow
the formation of more regular helical structures and
better exposure of the chain segments to the photoex-
citation, thus leading to the observed higher [θ] and ꢀ
values.
After checking the effect of the structural change
(chain hydrophilicity), we examined the effects of the
environmental variations (solvent and pH). While the
methanol solution of polymer 1 gave pearl-shaped
structures upon natural evaporation, its THF solution
gave helical cables with a clear left-handed twist under
similar assembling conditions (Figure 5). As discussed
above (cf., Figure 3A), the polymer chains of 1 may take
an extended conformation in THF because THF is a
good solvent for both the backbone and the pendants.
During the aggregation process accompanying the THF
evaporation, the extended helical chains may twine
around each other via interchain hydrogen bonding to
give twisted strands, further association of which in
different multiplicities (doublet, triplet, etc.) will give
thicker fibrils of different diameterssthis assembling
mechanism is clearly suggested by the image shown in
Figure 5B. It is envisioned that evaporation of a
methanol/THF solution of 1 may generate transit mor-
phologies containing both micellar and fibrillar struc-
When 1 molar equiv of KOH was added into the
methanol solution of polymer 2, its CD signals were
dramatically weakened, while its UV-vis absorptions
were slightly enhanced (Figure 3B). When the base is
admixed with the “polyacid”, its carboxylic protons will
be replaced by the potassium ions. The ionization of the
carboxyl groups will break the hydrogen bonds (cf.,
Chart 2 and related discussion). Entropy-driven ran-
domization will destroy the regular helical structures,
resulting in the large decrease in the CD activity of the
polymer. The ionized polymer chains might be solvated

Macromolecules, Vol. 36, No. 1, 2003
Valine-Containing Poly(phenylacetylene)s 81
Ch a r t 1. Dia gr a m m a tic Illu str a tion s of (A) Helica l P olya cetylen e Ch a in s In d u ced by Ch ir a l P en d a n ts a n d (B, C)
Hyd r ogen Bon d s betw een Am in o Acid Moieties in P oly(4-eth yn ylben zoyl-L-va lin e m eth yl ester ) (1)
The pH effect on the assembling behaviors of poly-
mer 2 is shown in Figure 6. Changing the medium pH
from “neutral” to “basic” changed the organizational
morphology of the polymer from continuous helical
cables to discrete random threads. The helical cables
were several tens of nanometers in diameter and up to
several hundreds of micrometers in length. The ran-
dom threads were, however, much thinner and shorter.
Their “true” diameters, after subtracting the in-
volved “tip broadening effect” in the AFM measure-
ments,37,38 were close to the sizes of one or two single
polymer chains, but their lengths were, in many cases,
still much longer than a single chain. The random
threads showed almost no macroscopic screw sense. The
AFM images here appeared to be visual representa-
tions of the CD spectral data given in Figure 3B. The
strong Cotton effects of the methanol solution of 2
were magnified to fibrillary superhelicity via the evapo-
F igu r e 3. Effects of (A) solvent and (B) pH on the molar
ration-induced self-assembling process, while the ran-
ellipticity ([θ]) and molar absorptivity (ꢀ) of (A) poly(4-
dom coils with little CD activities in the basic solution
ethynylbenzoyl-L-valine methyl ester) (1) and (B) poly(4-
hardly organized into any regular morphological struc-
tures.
ethynylbenzoyl-L-valine) (2). Polymer concentration (mM):
∼1.5 (CD), ∼0.1-0.2 (UV). Temperature: ∼23 °C (room
temperature). In panel B, the methanol solutions of 2 without
and with KOH (1 molar equiv of 2) are defined as “neutral”
and “basic”, respectively.
The self-weaving of the helical chains of polymer 2
into the thick, long fibrillar cables is believed to be aided
by interchain hydrogen bonding, an example of which
is given in Chart 2A, which sketchily illustrates how
helical strands are associated through side-by-side and
head-to-tail interstrand hydrogen bonding. Ionization
of the carboxyl groups by the potassium ions breaks the
hydrogen bonds, and entropic chaos randomize the
tures. This was indeed the case: as shown in Figure
5C, the organizational morphologies obtained from the
mixture solvent system showed combined features of the
assembling structures obtained from the individual
solvent systems.

82 Li et al.
Macromolecules, Vol. 36, No. 1, 2003
Ch a r t 2. Sch em a tic Rep r esen ta tion s of (A) Su p r a m olecu la r Associa tion s of th e Helica l Ch a in s of
P oly(4-eth yn ylben zoyl-L-va lin e) (2) via La ter a l a n d Ter m in a l In ter str a n d Hyd r ogen Bon d in g a n d (B) Clea va ge of
th e Hyd r ogen Bon d s by Ba se-Med ia ted Ion iza tion of th e Ca r boxylic Acid P en d a n ts a n d Den a tu r a tion of th e
Helica l Str a n d s to Ra n d om Coils by En tr op y-Dr iven Ra n d om iza tion of th e P olyelectr olyte Ch a in s.
F igu r e 4. AFM images of (A) clustered pearls, (B) helical cables, and (C) string beads formed upon natural evaporation of methanol
solutions of (A) poly(4-ethynylbenzoyl-L-valine methyl ester) (1), (B) poly(4-ethynylbenzoyl-L-valine) (2), and (C) a partially
hydrolyzed 1 {or poly[(4-ethynylbenzoyl-L-valine methyl ester)-co-(4-ethynylbenzoyl-L-valine)]} on the surfaces of newly cleaved
mica. Scale bars (nm): (A) 250; (B) 500; (C) 100. Concentration: ∼1-5 µg/mL. Temperature: ∼23 °C (room temperature).
macromolecular chains. The polyelectrolyte chains car-
rying the same negative charges repulse each other and
can hardly aggregate into multistranded cables, which
may account for the observed single-chain dimension-
alities of the random threads. Some small fractions of
the carboxylic groups may survive the ionization due
to the steric hindrance and electrical repulsion involved
in the polymer reaction, and the chain segments with
the un-ionized carboxylic residues may still be able to
fold and assemble. When the segmental residues are
associated in a head-to-tail fashion, filamentary threads
with lengths longer than that of a single polymer chain
will be formed (Chart 2B). When the segments are
associated in a side-by-side fashion, twisting filaments
will be resulted. The knots of the looped threads marked
by the arrows in Figure 6B might be the consequence
of such segmental braiding.
Con clu d in g Rem a r k s
In this study, we hybridized naturally occurring
building blocks with synthetic conjugated polymer
chains. We generated the molecular blends by attaching
the hydrophilic amino acid pendants to the hydrophobic
conjugated PPA backbone. The pendant chirality, chain
amphiphilicity, and hydrogen-bonding capability en-
coded in the primary structure conferred a rich struc-
tural hierarchy on the hybrid polymers, enabling the
macromolecular chains to take helical conformations
and to self-assemble into biomimetic architectural mor-
phologies including micellar spheres and helical cables.

Macromolecules, Vol. 36, No. 1, 2003
Valine-Containing Poly(phenylacetylene)s 83
F igu r e 5. AFM images of (A) clustered pearls, (B) helical cables, and (C) clustered pearls plus helical cables formed upon natural
evaporation of (A) methanol, (B) THF, and (C) methanol/THF (1:7 by volume) solutions of poly(4-ethynylbenzoyl-L-valine methyl
ester) (1) on the surfaces of newly cleaved mica. Scale bars (nm): (A) 250; (B) 125; (C) 500. Concentration of polymer solutions:
∼1-5 µg/mL. Temperature: ∼23 °C (room temperature).
acetylenes containing naturally occurring species in-
cluding not only different amino acids but also various
saccharides and nucleosides.41,42
The novel structural feature of our hybrid polymers
is that a conjugated polyacetylene backbone is wrapped
up with a coat of naturally occurring pendants. Cyto-
toxicity assays reveal that all the polymers are cyto-
compatible.41 Some monosaccharide-containing poly-
acetylenes can even stimulate the growth of living HeLa
cells when the polymer solutions are added into the
culture media or when the polymer films are precoated
on the microtiter plates.41,43 We are currently exploring
the exciting possibility of utilizing our biomimetic
polymers as biocompatible nanowires for cytotech,
especially bioelectronics, applications.
Ack n ow led gm en t. This work was partly supported
by the Research Grants Council of Hong Kong (Project
Nos. HKUST 6187/99P, 6121/01P, and 6085/02P) and
the National Science Foundation of China (Project No.
F igu r e 6. AFM images of (A) helical cables and (B-D)
G2000077501). The financial support from the Univer-
random threads formed on natural evaporation of (A) “neutral”
sity Grants Committee (Hong Kong) through an Area
and (B-D) “basic” methanol solutions of poly(4-ethynylbenzoyl-
of Excellence Scheme (Project No. AoE/P-10/01-1-A) is
L-valine) (2) on the surfaces of newly cleaved mica. The “basic”
also acknowledged.
solution was prepared by adding 1 equiv of KOH (relative to
the molar amount of 2) to the “neutral” solution of the polymer
in methanol. Scale bars (nm): (A) 980; (B) 170; (C, D) 200.
Polymer concentration: ∼1-5 µg/mL. Temperature: ∼23 °C
(room temperature).
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