Photoresponsive supramolecular architectures based on polypeptide hybrids

Article abstract of DOI:10.1021/ma501601r

Self-aggregation has recently emerged as an efficient tool for the production of well-ordered supramolecular structures at the nanometric scale. In this framework, peptides offer important advantages as building blocks because of their biocompatibility and 3D-structural/functional diversities. The chemical diversity of peptides may be further expanded by use of noncoded amino acids. In the present work, we focused our attention on two known photoswitchable azobenzene-containing α-amino acids and used them as initiators for the reversible modulation of the cis/trans conformational states of two poly(γ-benzyl-l-glutamate)-based hybrid molecules with either C₂ or C₃ symmetry. The microscopic photoresponsive self-assembly of these compounds was examined in detail. Moreover, these hybrids were exploited in the construction of macroscopic supramolecular architectures via the electrospinning technique. Finally, after appropriate thiol functionalization, we fabricated and characterized dimeric and trimeric gold nanoparticle/polypeptide hybrid systems.

Full text of DOI:10.1021/ma501601r

Article
pubs.acs.org/Macromolecules
Photoresponsive Supramolecular Architectures Based on
Polypeptide Hybrids
†
†
†
,‡
†
Daniela Mazzier, Marco Maran, Omar Polo Perucchin, Marco Crisma,* Mirco Zerbetto,
†
†,‡
,†,‡
Valerio Causin, Claudio Toniolo, and Alessandro Moretto*
†
Department of Chemical Sciences, University of Padova, 35131 Padova, Italy
‡
Institute of Biomolecular Chemistry, Padova Unit, CNR, 35131 Padova, Italy
*
S Supporting Information
ABSTRACT: Self-aggregation has recently emerged as an
efficient tool for the production of well-ordered supra-
molecular structures at the nanometric scale. In this frame-
work, peptides offer important advantages as building blocks
because of their biocompatibility and 3D-structural/functional
diversities. The chemical diversity of peptides may be further
expanded by use of noncoded amino acids. In the present
work, we focused our attention on two known photo-
switchable azobenzene-containing α-amino acids and used
them as initiators for the reversible modulation of the cis/trans
conformational states of two poly(γ-benzyl-L-glutamate)-based hybrid molecules with either C or C symmetry. The microscopic
2
3
photoresponsive self-assembly of these compounds was examined in detail. Moreover, these hybrids were exploited in the
construction of macroscopic supramolecular architectures via the electrospinning technique. Finally, after appropriate thiol
functionalization, we fabricated and characterized dimeric and trimeric gold nanoparticle/polypeptide hybrid systems.
INTRODUCTION
ring-opening polymerization (ROP) of NCAs, and new
16
■
applications have been discovered and explored.
The construction of “smart” supramolecular architectures with
stimulus-responsive behavior is gaining interest due to their
1
,2
Poly(γ-benzyl-L-glutamate) (PBLG) is a polypeptide that
3,4
adopts a highly stable α-helical secondary structure in common
potential exploitation in areas such as drug delivery, con-
17,18
organic solvents and even in the solid state.
The uni-
5
6
trolled release, and nanoreactor applications. Among the
various external stimuli, e.g. pH, solvent, or temperature changes,
light is particularly attractive as it can be controlled/utilized
directional alignment of the intramolecular hydrogen bonds
that stabilize the α-helix, parallel to its main axis, promotes the
19
formation of a macroscopic dipole in this rodlike polymer.
7
cleanly, easily, and rapidly. A common method used for
It is worth recalling that PBLG and related α-helical poly
obtaining photocontrol on the 3D structures of molecules is the
incorporation of photoresponsive moieties. In this connection,
many molecules have been designed with different locations of
the responsive groups. Azobenzene is one of the most widely
used photoswitch chromophores, as it can isomerize rapidly,
reversibly, and with high quantum yield upon irradiation be-
tween the cis and trans forms (Scheme 1, lower part), originating
(
α-amino acids) are considered to have the highest single-
20,21
molecule electric dipole among all organic compounds.
appropriate conditions PBLG displays liquid-crystal behavior.
Apart from the ability to assemble in a variety of supra-
molecular structures
unique characteristic of rod−polypeptide segments such as
PBLG is the possibility to endow various functionalities with
photophysical and electrochemical properties to the supra-
Under
22,23
2
4,25
at nanoscale dimensions, another
8−10
geometrical and conformational changes in the molecule.
26−28
In recent years, the syntheses of block copolymers with peptide
domains and polypeptides functionalized with photoresponsive
molecular materials.
The present work focuses on the study of the effect of
geometry (cis/trans isomerization) on the response behavior of
azobenzene-functionalized PBLG microstructures. To this aim,
we exploited either bifunctional (Scheme 1, upper part) or
trifunctional (Figure 1, left part) photochromic amine initiators
for the preparation of PBLG-based hybrids with C or C type
symmetry, respectively. These compounds were studied in
11−13
moieties have been extensively explored.
Polypeptides are
attractive not only for their ability to self-assemble into ordered
structures but also for the considerable chemical diversity they
can offer. Beyond the 20 canonical amino acids, a variety of
nonprotein or synthetic amino acids are readily accessible from
commercial sources, and others can be designed and synthe-
sized to tailor specific properties. Since in the past decade
substantial progress has been made in the synthesis of poly-
2
3
Received: August 4, 2014
Revised: October 16, 2014
1
4,15
peptides via N-carboxyanhydrides (NCAs),
novel photo-
active peptide-based materials have been easily prepared by
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ma501601r | Macromolecules XXXX, XXX, XXX−XXX

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a
Scheme 1. Synthesis of H-bis-azo-L-Phe-OMe (5), C Symmetry
2
a
(
top) Reagents and conditions: (i) Boc O, TEA, 1:1 CH CN/H O, rt, 18 h; (ii) Pd/C, H , MeOH, rt, 40 min; (iii) Zn, NH Cl, 5:1 EtOH/H O, rt,
2 3 2 2 4 2
2
h; (iv) FeCl , 5:1 EtOH/H O, 0 °C, 30 min; (v) AcOH, rt, 24 h; (vi) 50% TFA in CH Cl , rt, 40 min. (bottom) Schematic representation of the
3
2
2
2
light-driven, reversible photoisomerization process occurring for compound 5.
terms of microscopic photoresponsive self-aggregation and as
precursors for the fabrication of macroscopic materials via the
electrospinning technique. The preparation and character-
ization of dimeric and trimeric gold nanoparticle/polypeptide
hybrids are also described.
High-Resolution Mass Spectroscopy. Mass spectra were obtained
by electrospray ionization (ESI) on a Perseptive Biosystem Mariner
ESI-ToF 5220 spectrometer. Data were collected in the positive mode.
High-Performance Liquid Chromatography. The HPLC measure-
ments were performed on an Agilent 1200 apparatus equipped with a
UV detector at 226 nm. Conditions: Phenomenex C18 (100 Å)
stationary phase), 5−80% B, 25 min, 1 mL/min (eluants: A = 9:1
(
EXPERIMENTAL SECTION
H
2
O/CH
Ultraviolet−Visible Absorption. UV−vis absorption spectra were
recorded using a Shimadzu model UV-2501 PC spectrophotometer. A
3
CN, 0.05% TFA; B = 1:9 H O/CH CN, 0.05% TFA).
2
3
■
4
1
Materials. γ-Benzyl-L-glutamate (BLG), p-nitrobenzylamine, and
-(dimethylamino)pyridine (DMAP) were obtained from Fluka.
-Ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride
1
cm path length quartz cell was used.
Electronic Circular Dicroism. ECD measurements were carried out
at room temperature using a Jasco J-715 spectropolarimeter. A fused
quartz cell of 1 mm path length (Hellma) was used.
Dynamic Light Scattering. DLS measurements were carried out at
(
EDC·HCl) was purchased from GL Biochem (Shanghai). Triphos-
gene, di-tert-butyl dicarbonate (Boc O), triethylamine (TEA),
2
p-aminobenzoic acid, 2,2′-dimethoxy-2-phenylacetophenone (DMPA),
β-mercaptoethanol, p-nitro-L-phenylalanine methyl ester hydrochlor-
2
5 °C on a Malvern Zetasizer Nano-S instrument using a He−Ne laser
ide [H-L-Phe(pNO )-OMe·HCl], 2,4,6-triallyloxy-1,3,5-triazine, acetic
2
(633 nm) and a scattering angle of 176°.
acid (AcOH), trifluoroacetic acid (TFA), and α-pinene were obtained
Transmission Electron Microscopy. Samples were analyzed on a
Jeol 300 PX TEM instrument. A glow discharged carbon coated grid
was floated on a small drop of the nanosphere suspension and excess
was removed by #50 hardened Whatman filterpaper.
Scanning Electron Microscopy. A Carl Zeiss Merlin field emission
SEM operating at 5 kV accelerating voltage was used. A small drop of
the nanosphere suspension was placed on a microscope glass coverslip
and allowed to dry overnight.
from Sigma-Aldrich. The deuterated solvents CDCl3 and DMSO
(
1
dimethyl-d6 sulfoxide) were purchased from Euriso-Top. Catalyst
0% Pd/C was obtained from Acros-Janssen. All other chemicals and
solvents are Sigma-Aldrich, Fluka, or Acros products and were used as
provided (without further purification).
General Methods. Nuclear Magnetic Resonance. H and 13C
1
NMR spectra were recorded at room temperature on a Bruker AC-200
(
200 MHz) instrument using TMS (tetramethylsilane) as the internal
Size Exclusion Chromatography. SEC analyses were performed on
an Agilent 1260 Infinity system equipped with 1260 isopump, 1260
TCC, 1260 VWD VL, 1260 RID, Phenogel 5 μm linear/mixed guard
column (30 × 4.6 mm), followed by Phenomenex Phenogel 5 μm
reference. The multiplicity of a signal is indicated as s, singlet; d,
doublet; t, triplet; m, multiplet; and br, broad. Chemical shifts (δ) are
expressed in ppm.
Fourier Transform Infrared Absorption. FT-IR absorption spectra
in KBr disk were recorded with a PerkinElmer 1720X spectropho-
tometer. The v maxima values for the main absorption bands are given.
4
10 Å (300 × 4.6 mm) column working at 60 °C. DMF was used as
eluant at a flow rate of 1 mL/min. Before SEC analysis, the samples
were filtered through a 0.2 μm PTFE filter (15 mm, Phenomenex).
B
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Figure 1. Synthesis of initiator (11), C symmetry. (left) Reagents and conditions: (i) Boc O, TEA, 1:1 CH CN/H O, rt, 18 h; (ii) Zn, NH Cl, 5:1
3
2
3
2
4
EtOH/H O, rt, 2 h; (iii) FeCl , 5:1 EtOH/H O, 0 °C, 40 min; (iv) AcOH, p-aminobenzoic acid, rt, 48 h; (v) 10% mol DMPA, 2-mercaptoethanol,
2
3
2
hv = 365 nm, 40 min; (vi) EDC, DMAP, DMF, rt, 18 h; (vii) 50% TFA in CH Cl , rt, 2 h. (right) (A) Schematic representation of the light-driven,
2
2
reversible photoisomerization process occurring for 11. (B) HPLC profile of a few-second-irradiated (at 365 nm) solution of 11, which revealed
formation of all (four) of its possible isomers.
The MWs were calculated according to a calibration using polystyrene
standards.
Microstructure Preparation. Typically, 20 mg of polymer was
dissolved in 10 mL of a DMF/THF 3:7 (v/v) solvent mixture. This
solution was put into a cellulose dialysis tubing with a MW cutoff of
the solvent was removed under reduced pressure, and the compound was
obtained as a solid (5.9 g, 97%). FT-IR absorption: υ 3358, 2985, 1732,
̅
−1
1689, 1524, 1346 cm . HRMS (ESI+): m/z calcd for C H N O
1
5
20
2
6
+
1
324.1321; found 325.3298 [M + H] . H NMR (200 MHz, CDCl ):
3
δ (ppm) 1.40 (s, 9H), 3.11 (dd, J = 13.7, 6.2 Hz, 1H), 3.27 (dd, J = 13.7,
6.2 Hz 1H), 3.73 (s, 3H), 4.58−4.68 (m, 1H), 5.06 (d, J = 7.7 Hz, 1H),
1
2 kDa (flat width 35 mm, Sigma). The dialysis process was carried
out against deionized water for 48 h.
7
.31 (d, J = 8.7 Hz, 2H), 8.6 (d, J = 8.7 Hz, 2H).
29,30
Electrospinning of PBLG Fibers. Three solutions at different
concentrations (4, 8, and 12 wt %) were prepared by dissolving PBLG
in dicloroacetic acid at 50 °C. A 2 mL SGE syringe with stainless steel
needle was used as an electrode. Syringes with 8 mL of each
concentration were loaded in a syringe pump (Genei Plus Syringe
pump, Kent Scientific) to control the flow rate of the polypeptide
solutions. Electrospinning was performed at ambient conditions with a
constant applied voltage of +8 kV at needle and −5 kV at collector
with the flow rate set at 0.01 mL/min to maintain a constant size of
droplet at the tip of the syringe needle. A circular aluminum plate was
used as a collector. The tip-to-collector distance was kept at 15 cm.
Electrospinning was performed for about 2 h to obtain a nonwoven,
continuous fiber mat. All fibers were dried prior to use for the
photostimuli experiments.
Boc-L-Phe(pNH )-OMe (2).
Boc-L-Phe(pNO )-OMe (2.40 g,
2
2
7
.4 mmol) was dissolved in MeOH (50 mL), and N was bubbled in
2
the solution for 10 min. Then, Pd/C (300 mg) was added, and the
mixture was stirred at rt under an atmospheric pressure of H gas for
2
1
h. The catalyst was removed by filtration through Celite. The solvent
was evaporated, and the residue was purified via flash chromatography
(
(
1
eluant: 1:1 EtOAc/hexane). The product was recovered as a solid
2.1 g, 96%). FT-IR absorption: υ 3397, 3374, 3232, 1741, 1690,
̅
−1
515 cm . HRMS (ESI+): m/z calcd. for C H N O 294.1579;
15
22
2
4
+
1
found 295.1732 [M + H] . H NMR (200 MHz, CDCl ): δ (ppm)
3
1
4
6
.40 (s, 9H), 2.96 (d, J = 5.7 Hz, 2H), 3.44 (s br, 2H), 3.70 (s, 3H),
.45−4.55 (m, 1H), 4.95 (d, J = 7.9 Hz, 1H); 6.61 (d, J = 8.4 Hz, 2H),
.89 (d, J = 8.4 Hz, 2H).
29,30
Boc-L-Phe(pNO)-OMe (3).
Boc-L-Phe(pNO )-OMe (1.07 g,
2
Synthesis of Initiators. a. H-bis-azo-L-Phe-OMe (5), C
2
29,30
3.3 mmol) and NH Cl (790 mg, 14.8 mmol) were dissolved in a 5:1
Symmetry Initiator (Scheme 1, Upper Part).
Boc-L-Phe(pNO )-
4
2
EtOH/H O (42 mL) solvent mixture. Then, Zn dust (600 mg,
OMe (1). H-L-Phe(pNO )-OMe·HCl (4.9 g, 18.8 mmol) was dissolved
2
2
in an 1:1 CH CN/H O solvent mixture (20 mL), and TEA was added
9.2 mmol) was added. The resulting suspension was stirred at rt for
2 h, then filtered through Celite, and washed with a cold mixture of
3
2
to pH = 9. Then, Boc O (5.50 g, 25.2 mmol), dissolved in CH CN
2
3
(
10 mL), was added. The mixture was stirred at rt for 18 h. The
5:1 EtOH/H
was cooled to 0 °C, and a cold solution of FeCl
23.4 mmol) in 5:1 EtOH/H O (35 mL) was added. The mixture was
2
O (40 mL) to remove the excess of Zn. The filtrate
·6H O (6.33 g,
mixture was concentrated under reduced pressure, and the residue was
3
2
diluted with EtOAc (ethyl acetate). The organic layer was washed with
2
5
% KHSO4(aq), 5% NaHCO3(aq), and brine. After drying over Na SO ,
stirred at 0 °C for 30 min, diluted with brine, and extracted with EtOAc.
2
4
C
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Scheme 2. Synthesis of PBLG Hybrid with C Symmetry (12)
2
The combined organic phases were washed with brine, dried over
dissolved in 5:1 EtOH/H O (25 mL). Then, Zn dust (940 mg,
2
anhydrous Na SO , and filtered. The solvent was removed under
14.4 mmol) was added, and the obtained suspension was stirred at rt for
2 h. The suspension was filtered through Celite, the filtrate was cooled
to 0 °C, and a cold solution of FeCl ·6H O (6.33 g, 23.4 mmol) in 5:1
2
4
reduced pressure, and the residue was purified rapidly via flash
chromatography (eluant: 30:1 CH Cl /EtOH). Compound 3 was
2
2
3
2
obtained as a green oil (910 mg, 89%), which was immediately used
EtOH/H O (35 mL) was added. The mixture was stirred at 0 °C for
2
for the next synthetic step.
Boc-Bis-Azo-L-Phe-OMe (4).
30 min, diluted with brine, and extracted with EtOAc. The combined
organic phases were washed with brine, dried over anhydrous Na SO ,
29,30
A solution of freshly prepared
2
4
compound 2 (1.0 g, 3.4 mmol) in glacial AcOH (15 mL) was added
to a solution of compound 3 (1.15 g, 3.7 mmol) in glacial AcOH
and filtered. The solvent was evaporated, and the crude was rapidly
purified via flash chromatography (eluant: 100:1 CH Cl /MeOH),
2
2
(
15 mL). The mixture was stirred at rt for 24 h. The solvent was
affording compound 7 as a green solid (880 mg), which was immedi-
ately used for the next synthetic step. This compound was dissolved
in glacial AcOH (10 mL), and a solution of p-aminobenzoic acid
(0.746 g, 5.4 mmol) in glacial AcOH (5 mL) was added. After 48 h
under stirring at rt, the mixture was concentrated and diluted with
EtOAc. The organic solution was washed with 5% NaHCO3(aq), 5%
evaporated, and the crude product was purified via flash chromatog-
raphy (eluant: 50:1 CH Cl /EtOH). After precipitation from EtOAc/
hexane, compound 4 was recovered as an orange solid (830 mg, 43%).
FT-IR absorption: υ 3356, 1752, 1735, 1688, 1520 cm . HRMS
2
2
−
1
̅
+
1
(
(
(
ESI+): m/z calcd for 584.2846; found 585.2832 [M + H] . H NMR
200 MHz, CDCl ): δ (ppm) 1.42 (s, 18H), 3.14−3.20 (m, 4H), 3.73
s, 6H), 4.58−4.68 (m, 2H), 5.03 (d, J = 7.27 Hz, 2H), 7.27 (d, J =
KHSO4(aq), and brine. After drying over MgSO
concentrated and the crude purified via flash chromatography (eluant:
CH Cl /EtOAc, increasing the solvent mixture polarity from 9:1 to
, the filtrate was
3
4
8
.3 Hz, 4H), 7.84 (d, J = 8.3 Hz, 4H).
2
2
H-Bis-Azo-L-Phe-OMe (5). The removal of the Boc protecting group
was obtained by treating Boc-bis-azo-L-Phe-OMe (175 mg, 0.29 mmol)
1:1). The compound was obtained as an orange solid after
precipitation from EtOAc/petroleum ether (370 mg, 28%). FT-IR
−1
with an 1:1 TFA/CH Cl2 mixture (10 mL) at rt under stirring
absorption: υ 3343, 2980, 1682, 1507, 1426, 1168 cm . HRMS
2
̅
for 40 min. Then, the solvent was evaporated, and the residue
suspended in water and lyophilized. Product 5 was obtained as an
(ESI+): m/z calcd for C19
H
N
21
3
O
4
355.1532; found 356.1691 [M +
6
+
1
H] . H NMR (200 MHz, DMSO, d ): δ (ppm) 1.41 (s, 9H), 4.24
orange solid (170 mg, 98%). FT-IR absorption: υ 3425, 3007, 1748,
(d, J = 6 Hz, 2H), 7.47 (d, J = 8.4 Hz, 2H), 7.88−7.97 (m, 4H), 8.14
(d, J = 8.4 Hz, 2H), 13.18 (s, 1H).
Compound 9. 2,4,6-Triallyloxy-1,3,5-triazine (5.03 g, 20.2 mmol),
2-mercaptoethanol (8.66 g, 110.8 mmol), and DMPA (2,2-dimethoxy-
2-phenylacetophenone) (376 mg, 1.5 mmol) were mixed in a beaker
and stirred under irradiation at 365 nm with a UV lamp for 40 min.
̅
−1
1
674 cm . HRMS (ESI+): m/z calcd for 384.1797; found 385.2035
+
1
[
M + H] . H NMR (200 MHz, DMSO, d ): δ (ppm) 3.21 (d, J =
6
5
(
.3 Hz, 4H), 3.70 (s, 6H), 4.42 (t, 2H), 7.46 (d, J = 8.4 Hz, 4H), 7.86
d, J = 8.4 Hz, 4H), 8.59 (s br, 4H). 13C NMR (50 MHz, DMSO-d6)
δ: 169.41, 151.22, 138.51, 130.66, 122.84, 53.05, 52.81, 35.82.
b. Compound (11), C Symmetry Initiator (Figure 1, Left Part).
The mixture was diluted with MeOH (10 mL), and Et O was added
2
3
Boc-p-Nitrobenzylamine (6). Solid Boc O (4.5 g, 20.8 mmol) was
until the precipitation of an oily compound took place. The
supernatant was removed and the oily compound was washed twice
2
added to a solution of p-nitrobenzylamine (2 g, 10.6 mmol) and TEA
(
2 mL, 14.4 mmol) in CH Cl (45 mL) cooled to 0 °C. The mixture
with Et O. Compound 9 was recovered as a colorless oil (8.2 g, 84%).
FT-IR absorption: 3389, 2920, 1567, 1417, 1332, 1140 cm . HRMS
2
2
2
−1
was stirred at rt for 18 h. Then, the mixture was diluted with CH Cl2
2
and washed with 5% KHSO4(aq) and brine. The organic phase was
(ESI+): m/z calcd for C H N O S 483.1531; found 484.1626 [M +
18
33
3
6 3
+
1
dried over MgSO , filtered, and concentrated under reduced pressure.
H] . H NMR (200 MHz, CDCl ): δ (ppm) 2.00−2.13 (m, 6H), 2.30
4
3
After precipitation from CH Cl /petroleum ether, compound 6 was
(s br, 3H), 2.65−2.75 (m, 12H), 3.72 (t, J = 6.2 Hz, 6H), 4.49 (t, J =
2
2
recovered as a solid (1.8 g, 67%). FT-IR absorption: υ 3325, 2984,
6.2 Hz, 6H).
̅
−1
2
935, 2914, 1688, 1520, 1165 cm . HRMS (ESI+): m/z calcd
Compound 10. To a solution of compound 8 (320 mg, 0.91 mmol),
compound 9 (120 mg, 0.25 mmol), and DMAP (36 mg, 0.29 mmol) in
10 mL of anhydrous DMF cooled to 0 °C, EDC·HCl (190 mg, 0.97
mmol) was added, and the reaction mixture was stirred at rt for 48 h.
The solvent was removed under reduced pressure, the residue
+
1
for C H N O 252.1110; found 197.0652 [M-tBu + H] . H NMR
12
16
2
4
(
4
200 MHz, CDCl ): δ (ppm) 1.46 (s, 9H), 4.40 (d, J = 6.2 Hz, 2H),
3
.98 (s br, 1H), 7.44 (d, J = 8.7 Hz, 2H), 8.19 (d, J = 8.7 Hz, 2H).
31
Boc-4-(aminomethyl)phenylazobenzoic Acid (8). Boc-p-nitro-
benzylamine (1 g, 4 mmol) and NH Cl (690 mg, 13 mmol) were
dissolved in CH Cl and washed with 5% KHSO
and brine. After
4
2
2
4(aq)
D
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Scheme 3. Synthesis of PBLG Hybrid with C Symmetry (13)
3
drying over MgSO , the filtrate was concentrated and the crude was
solvent mixture was added, and the mixture was stirred at 25 °C for
48 h. The mixture was diluted with 1,4-dioxane, and the polypeptide
hybrid was precipitated by adding MeOH. The obtained solid was
4
purified via flash chromatography (eluant: CH Cl /MeOH, increasing
2
2
the solvent mixture polarity from 60:1 to 40:1). Compound 10 was
recovered as an orange solid (270 mg, 70%). FT-IR absorption:
filtered, washed with MeOH and Et O, and dried in vacuo. The
polypeptide hybrid was recovered as a light yellow solid (680 mg,
2
−1
υ 3335, 2975, 1716, 1684, 1513, 1167 cm . HRMS (ESI+): m/z calcd
̅
+
1
for C H N O S 1494.5810; found 1495.6822 [M + H] . H NMR
62%). SEC MW: 63 500 (PDI 1.16). FT-IR absorption: υ 3295, 3003,
75
90 12 15
3
̅
−
1
(
(
(
200 MHz, CDCl ): δ (ppm) 1.48 (s, 27H), 2.01−2.20 (m, 6H), 2.78
2951, 1734, 1653, 1546, 1163 cm .
3
t, J = 7.1 Hz, 6H), 2.90 (t, J = 7.1 Hz, 6H), 4.39 (s, 6H), 4.46−4.53
m, 12H), 4.95 (s br, 3H), 7.43 (d, J = 8.4 Hz, 6H), 7.89−7.85 (m,
C3 Symmetry PBLG Hybrid (13) (Scheme 3). BLG-NCA (1.00 g,
3.81 mmol) was dissolved in dry 1,4-dioxane (2.3 mL) under an N
2
1
2H), 8.17 (d, J = 8.6 Hz, 6H).
atmosphere, and the solution was cooled to 0 °C. Then, a solution of
compound 11 (12 mg, 0.010 mmol) in DMF (1 mL) was added, and
the mixture was stirred at 25 °C for 72 h. The mixture was diluted with
1,4-dioxane, and the polypeptide hybrid was precipitated by adding
MeOH. The obtained solid was filtered, washed with MeOH and
Compound 11. The removal of the Boc protecting group was
obtained by treating compound 10 (61 mg, 0.041 mmol) with an 1:1
TFA/CH Cl mixture (4 mL) at rt under stirring for 2 h. The solvent
2
2
mixture was evaporated; the residue was suspended in water and
lyophilized. Product 11 was obtained as an orange solid (47 mg, 96%).
Et
2
O, and dried in vacuo. The polypeptide hybrid was recovered as a
−1
FT-IR absorption: υ 3423, 2955, 1715, 1561, 1271 cm . HRMS (ESI
light yellow solid (750 mg, 76%). SEC MW: 46 800 (PDI 1.56). FT-IR
absorption: υ 3331, 3064, 2940, 1734, 1653, 1545, 1161 cm . This
synthesis was repeated under similar conditions by using a higher
monomer/initiator ratio (500:1) and a prolonged reaction time
(5 days). The polypeptide hybrid was recovered as a light yellow solid
(550 mg, 76%). SEC MW: 154 000 (PDI 1.66). FT-IR absorption: υ
̅
−1
+
): m/z calcd for C H N O S 1194.4237; found 1195.5364 [M +
60
66 12
9
3
̅
+
1
H] . H NMR (200 MHz, DMSO, d ): δ (ppm) 1.95−2.01 (m, 6H),
6
2
1
(
.67−2.74 (m, 6H), 2.89−2.85 (m, 6H), 4.17 (s, 6H), 4.34−4.48 (m,
2H), 7.70 (m, 6H), 7.98 (m, 12H), 8.13−8.17 (m, 6H), 8.41
s, 12H). 13C NMR (50 MHz, DMSO-d ): δ (ppm) 172.52, 164.92,
6
̅
−1
1
6
54.38, 151.69, 149.90, 138.19, 131.66, 130.58, 130.01, 122.98, 122.75,
6.34, 64.17, 59.30, 41.88, 32.56, 29.67, 28.17, 27.64.
3329, 3062, 2941, 1732, 1651, 1542, 1161 cm .
Gold Nanoparticles−PBLG Hybrids. Thiol Derivatives. Trt
(trityl)-β-mercaptopropionic acid (120 mg, 0.34 mmol) and HOAt
(7-aza-1-hydroxy-1,2,3-benzotriazole) (48 mg, 0.34 mmol) were
dissolved in 3 mL of anhydrous DMF and cooled to 0 °C. Then,
EDC·HCl (66 mg, 0.34 mmol) was added. The mixture was stirred for
PBLG Derivatives. γ-Benzyl-L-glutamate N-Carboxyanhydride
32
(
BLG-NCA). BLG (5.05 g, 21.3 mmol) and α-pinene (6.64 g, 48.7
mmol) were dissolved in EtOAc (70 mL) in a three-neck flask and
heated under reflux. Triphosgene (4.24 g, 14.2 mmol) was dissolved in
EtOAc (25 mL) and added slowly with a dropping funnel once the
reflux started. After 4 h of reaction, heating was interrupted and most
of the solvent was evaporated under reduced pressure. The compound
was precipitated by addition of petroleum ether, and the recovered
solid was recrystallized twice, subsequently filtered, and washed with
15 min at 0 °C. The C symmetry polypeptide hybrid (12) (150 mg,
2.36 μmol) or the C symmetry polypeptide hybrid (13) (110 mg,
2
3
2.36 μmol) was dissolved in 2 mL of DMF and added with TEA
(100 μL, 0.72 mmol) to the solution of the active ester. After stirring
the solution at rt for 48 h, the polypeptide hybrids were precipitated by
adding MeOH, then filtered, and washed several times with MeOH.
In both cases, the polypeptide hybrids were obtained in almost
quantitative yield after drying.
Gold Nanoparticles. Gold nanoparticles (GNP) (13 nm size) were
prepared using the standard reduction of tetrachloroauric(III) acid
(HAuCl ·3H O) with trisodium citrate bis-hydrate. Typically, 10 mL
petroleum ether. BLG-NCA was recovered as a solid (4.6 g, 84%).
1
H NMR (200 MHz, CDCl ): δ (ppm) 7.35 (s, 5H), 6.61 (s, 1H),
3
5
.14 (s, 2H), 4.37 (t, J = 6 Hz, 1H), 2.59 (t, J = 6.8 Hz, 2H), 2.36−
2.02 (m, 2H).
C2 Symmetry PBLG Hybrid (12) (Scheme 2). BLG-NCA (1.10 g,
.2 mmol) was dissolved in dry 1,4-dioxane (2 mL) under an N2
3
3
4
4 2
atmosphere, and the solution was cooled to 0 °C. Then, a solution of
compound 5 (5 mg, 0.013 mmol) in 2:1 1,4-dioxane/DMF (600 μL)
of 38.8 mM sodium citrate solution was rapidly added to a boiling
water solution of HAuCl ·3H O (1.0 mM, 100 mL) under vigorous
4
2
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Figure 2. (A) Schematic representation of the cis/trans photoisomerization for the C symmetry PBLG hybrid (12). (B) UV−vis absorption spectra
2
of the cis and trans isomers of the C symmetry PBLG hybrid in THF solution (two cycles of irradiation are shown). (C) SEC analysis which shows
2
the different elution profiles occurring for the cis and the trans forms of the C symmetry PBLG hybrid. (D) ECD spectra of the cis and trans isomers
2
of the C symmetry PBLG hybrid in 1,1,1,3,3,3-hexafluoroisopropanol solution (HFIP) (the cis red trace was slightly shifted up for clarity).
2
stirring, and the mixture was kept boiling for an additional 10 min.
Then, the mixture was allowed to cool to rt while stirring for an
additional 15 min. After their syntheses, red GNP colloids (1.25 ×
and β-mercaptoethanol. Condensation of compounds 8 and 9
using the EDC/DMAP C-activation, followed by Boc-
deprotection, gave the trifunctional amine initiator (11). In
Figure 1A (right), the four isomers expected from the photo-
isomerization of (11) are shown. Indeed, four species could be
detected by HPLC after 40 s of irradiation at 365 nm (Figure 1B,
right). A more prolonged irradiation (≥2 min) at 365 nm is
able to reduce the population of the all-trans form to less than
7%. Back-conversion to about 70% population of the all-trans
form (and 0% of the all-cis isomer; the remaining 30%
population being distributed between the trans−cis−cis and
−
8
10
mol/L, assuming a density for gold colloids identical to that of
gold in bulk) were kept in the dark at 5 °C.
Citrate−Thiol Exchange. After various attempts, the citrate−thiol
exchange experiments were performed as follows. To a 100 mL of
−8
1
.25 × 10 mol/L water solution of GNP, a DMF solution of the C2
symmetry polypeptide hybrid (12) (20 mL, 0.01 μmol) or C3
symmetry polypeptide hybrid (13) (20 mL, 1 nmol) was slowly
added using a syringe pump system, under vigorously stirring, for
2−6 h. TEM images were recorded after each preparation.
trans−trans−cis isomers) was achieved by 30 min irradiation at
1
RESULTS AND DISCUSSION
4
20 nm (Figures S4 and S5). Again, H NMR and UV−vis
■
absorption spectra after two cycles of irradiation at 365 and
20 nm (Figures S6 and S7) clearly support the reversibility of
The bis-aminoester (5) (initiator of the C symmetry) was
prepared through a modification of a reported procedure.
2
2
9,30
4
The synthesis involved the condensation of two L-Phe deriv-
atives, bearing an amino (2) and a nitroso (3) moiety, res-
pectively, as para-substituents (Scheme 1). The HPLC profiles,
monitoring the time-dependent photoinduced trans/cis isomer-
the process.
The PBLG hybrids with C
and C symmetries were obtained
3
2
via NCA-ROP using compounds 5 and 11, respectively, as the
initiators. BLG-NCA was synthesized via phosgenation of BLG
31
1
ization of 5 along with the H NMR and UV−vis absorption
following a reported protocol. The polymerizations using BLG-
NCA were performed in 1,4-dioxane or DMF. The solid PBLG
hybrids, obtained after precipitation with MeOH, were
characterized by SEC analysis. We obtained a MW distribution
of 63 500 Da, PDI (polydispersity index) 1.16 for the C2
symmetry PBLG hybrid. Two different polymerization con-
spectra after two cycles of irradiation at 365 and 420 nm, are
reported in the Supporting Information (Figures S1−S3). The
synthesis of the initiator with C symmetry (11) initially involved
3
the preparation of the azo compound 8 (Figure 1, left). The
C -platform (9) was easily obtained via thiol−ene “click” chemistry
3
starting from the tris-alkene core 2,4,6-triallyloxy-1,3,5-triazine
ditions used for the preparation of the C symmetry PBLG
3
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Figure 3. Upper part: schematic representation of the thiol-functionalized C symmetry PBLG hybrid and the corresponding dimeric GNP/PBLG
2
hybrid system. (A) TEM images of single and monodisperse (12 nm), citrate passivated GNP. (B) TEM images of multimers of the GNP/PBLG
hybrid system, obtained by using high concentration and fast addition experimental conditions. (C, D) TEM images of the dimeric GNP/PBLG
hybrid system, obtained by using more diluted and slow addition experimental conditions. (E, F) TEM images showing details of the dimeric GNP/
PBLG hybrid system, obtained by two different experiments.
hybrid afforded polypeptides characterized by a MW distribution
of 46 800 Da, PDI 1.56, and 154 000 Da, PDI 1.66, respectively.
characterized by a longer elution time, indicative of a more
compact 3D shape, with respect to the trans form. Conversely, as
expected, cis/trans isomerization does not affect the polypeptide
secondary structure of the PBLG hybrid, the ECD spectra of
which are almost identical for both isomers and indicative of an
overwhelmingly prevailing α-helical conformation (Figure 2D).
The reliability of the MW distributions of the C and the
2
lower MW C3 symmetry PBLG hybrids was checked by
cleaving each of them at the level of the azo group(s) through
34
reaction with sodium hydrosulfite. SEC analysis of the linear
fragments derived from the cleavage of the C symmetry PBLG
We decided to exploit the C symmetry of this PBLG hybrid
2
2
hybrid gave a MW of 31 500 Da (PDI: 1.21), nearly 1/2 the
MW of the original material. Similarly, the fragments obtained
for the construction of a more complex, dimeric GNP/
polypeptide hybrid systems. To this aim, both N-termini of the
PBLG hybrid were functionalized with β-mercaptopropionic acid
(Figure 3, upper part) following a protocol previously reported
from the 46 800 Da C symmetry PBLG hybrid gave a MW
3
distribution of 13 000 Da (PDI: 1.18), a value not far from 1/3
the MW of the original PBLG hybrid.
2
6
by some of us. Monodisperse, citrate-passivated GNP
33
C Symmetry PBLG Hybrid. The C symmetry PBLG
(diameter 13 nm, Figure 3A) were prepared and subsequently
subjected to a thiol/citrate exchange reaction with the bis-β-
mercaptopropionic acid functionalized PBLG hybrid. By running
this experiment under appropriate diluted conditions, we were
able to detect by TEM the formation of dimeric species.
Specifically, a fast addition of a DMF solution of the thiol-
functionalized PBLG hybrid to an aqueous GNP solution
promoted the formation of GNP/PBLG hybrid multimers
(Figure 3B), while a slow addition, combined with a high
dilution, allowed the onset of dimeric units (Figure 3C−F).
2
2
hybrid (12) was studied in terms of photoresponsive behavior
(
Figure 2A). After UV irradiation at λ = 365 nm of a PBLG
hybrid solution in THF, isomerization of the azobenzene
moiety from trans-to-cis was observed, as indicated by the
decrease of the π−π* transition band at 328 nm and the
concomitant increase of the n−π* transition band at 440 nm
(
Figure 2B). This process was found to be reversible for at least
four cycles of repeated irradiation at 365 and 420 nm. Two
cycles of irradiation are shown in Figure 2B.
Interestingly, the cis and trans isomers can be differentiated by
SEC analysis (Figure 2C). Specifically, the cis form is
The self-assembly properties of the C symmetry PBLG
2
16
hybrid were investigated in aqueous environment. Starting
G
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Figure 4. TEM (A) and SEM (B) images of the self-assembled structures obtained from the C symmetry PBLG hybrid in aqueous suspension. (C)
2
Tentative, vesicle-like model of the self-assembled structures. (D−G) TEM images of the microstructure transitions occurring after different
irradiation times under UV light at λ = 365 nm (20, 40, 60, and 80 s, respectively).
from a solution of this hybrid in an organic solvent mixture
20 mg in 10 mL of 3:7 DMF/THF v/v), a milk-like sus-
pension was obtained after dialysis against ultrapure water for
8 h (membrane cutoff, 12 kDa). A few drops of this sus-
We recorded the fluorescence spectra of a diluted water
suspension of the spherical aggregates prior and after exposure
to UV light at λ = 365 nm (Figure 5A). The UV-induced
collapse of the aggregates (demonstrated above) is accom-
panied by a dramatic enhancement in the CF fluorescence. This
result strongly supports the view that our spherical aggregates
(
4
pension examined by TEM (without uranyl-acetate treatment)
and SEM revealed formation of aggregates which display
polydisperse spherical morphologies with diameters of 100−
of the C symmetry PBLG hybrid are indeed vesicles, hosting in
2
5
00 nm (Figure 4A,B). The relative high and positive value of
their inner cavity CF molecules at relatively high concentration.
This phenomenon gives rise to a low fluorescence emission
owing to the self-quenching effect. Upon disruption of the
vesicles, diluted CF is released into the medium, fully restoring
its strong fluorescence. As shown in Figure 5B, the maximum
fluorescence intensity is reached after 6 min irradiation at
365 nm. This time is much longer than the 20−80 s required
for the collapse of the empty vesicles reported in Figure 4. Such
a discrepancy might be rationalized by observing that CF
absorbs part of the irradiating light. Disruption/destabilization
of the vesicles, leading to CF release within a few minutes, can
be also achieved by addition to the aqueous suspension of
either MeOH (which acts as a precipitant) or a diluted NaOH
solution which neutralizes the positive charges on the vesicle
surface that are instrumental in preventing the aggregation of
this colloidal form (Figure 5B).
C3 Symmetry PBLG Hybrid. The strategy described above
for the preparation of the dimeric GNP/PBLG hybrid was also
exploited for the synthesis of a trimeric system starting from the
C3 symmetry PBLG hybrid (Figure 6, left). The TEM images
reported in Figure 6, panels A−F, from samples obtained by six
independent preparations in which the thiol/citrate exchange
was performed at very high dilution, clearly show the formation
35
the Z-potential function (+30 mV) (related to the degree of
electrostatic repulsion between adjacent, similarly charged
particles) obtained from these experiments provided evidence
for the presence of electrostatic net charges, due to the N-
terminal, protonated amino groups, present at least in the outer
layer of these microstructures. Moreover, a high Z-potential
value is often associated with double-layer systems, such as
vesicles. Indeed, our rod-like PBLG hybrid, largely hydrophobic
but carrying protonated amino groups at both termini, might
give rise to a vesicle-like self-assembly, as shown in Figure 4C.
Considering that this hybrid has an average MW of 63 500,
which corresponds to about 280 BLG units, and that the
36
elevation per residue in the α-helix is 0.15 nm, the wall
thickness of the vesicles might be of the order of 40 nm.
Upon UV irradiation (λ = 365 nm) of the suspension, a rapid
and progressive collapse of these ordered structures was
observed (Figure 4D−G). In all probability, the change in
the 3D geometry which occurs as a consequence of the trans-to-
cis azobenzene isomerization, even if taking place for a fraction
of the polypeptide hybrid molecules, is able to destabilize the
self-assembled microstructures.
Evidence for the vesicular nature of the self-assembled
structures was obtained by the carboxyfluorescein (CF)-
of GNP/PBLG hybrid trimers with C symmetry. It has to be
3
37
entrapped technique. The self-assembly process described
above was repeated in the presence of CF in the starting
organic medium. After dialysis, the resulting suspension was
submitted to SEC, thus allowing removal of the nonentrapped
CF and isolation of a red-orange colored main fraction. This
fraction consisted of spherical self-assembled structures,
homogeneous in size (200 nm) according to TEM and DLS
analyses (Figure 5, upper part, and Supporting Information,
Figure S8, respectively).
noted that for both the dimeric and trimeric nanoparticle
hybrid systems, the distances between nanoparticles within the
assemblies appear from the TEM images (Figures 3 and 6) to
be much shorter than those expected on the basis of the
polypeptide lengths. This observation may suggest that GNPs
could be attached to the polypeptide side rather than to the
thiol-functionalized ends. However, in control experiments
carried out using the nonthiol functionalized C and C
2
3
symmetry PBLG hybrids, no formation of GNP dimers and
H
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Figure 5. Upper part: TEM image of purified (by gel-exclusion chromatography) spherical aggregates of the C symmetry PBLG hybrid formed in
2
the presence of CF. Lower part: (A) Comparison of the fluorescence emission spectra of the CF-encapsulated aggregates before (red line) and
after (black line) UV irradiation at 365 nm. (B) Time evolution of the release of CF from the spherical aggregates as a result of either UV irradiation
(
365 nm) or denaturation by addition of 10% MeOH (v/v) or 5% 0.1 M NaOH (v/v).
Figure 6. Left: schematic representation of the trimeric GNP/PBLG hybrid system based on the thiol-functionalized C symmetry PBLG
3
hybrid. (A−F) TEM images showing details of the trimeric GNP/PBLG hybrid system (from samples obtained by six different experiments at very
high dilution).
trimers, respectively, could be detected. Tentatively, the short
interparticle distances may be ascribed to the solid-state level at
which TEM analyses are carried out, i.e., in the absence of
solvation. Covalently linked GNPs may experience enhanced
cohesion forces as the result of the drying process, possibly
strong enough to overcome the stiffness of the polypeptide
chains connecting them.
We also investigated the self-assembly properties of the C3
symmetry PBLG hybrid (average MW 46 800) in aqueous
environment. The microstructures formed starting from an
I
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Figure 7. Upper part: TEM (center) and SEM (right) images of the self-assembled structures obtained from the C symmetry PBLG hybrid (left) in
3
aqueous suspension. Central part, left: TEM images of the microstructures after UV irradiation (λ = 365 nm) for different times (A, no irradiation; B,
5
min; C and D, 10 min; E, 20 min; F, 30 min). Central part, right: schematic representation of the light-induced microstructure reorganization.
Lower part: SEM images showing the microstructures before (left) and after (right) 30 min UV irradiation.
organic solution upon dialysis against water are characterized by
an oval shape and dimensions of 100−400 nm (Figure 7, top).
After UV irradiation for increasing times up to 30 min, we
observed the reorganization of the microstructures into smaller
aggregates as detected by TEM analyses (Figure 7A−F).
In particular, a partial disassembly of the oval aggregates was
detected after 5 min irradiation, whereas prolonged irradiation
variation of morphology could be detected even after prolonged
UV irradiation (Supporting Information, Figure S9). We
assume that in this case the noncovalent interactions among
the polypeptide chains would be strong enough to prevent a
conformational change to take place as a result of the photo-
switch process. Therefore, inspired by a recent publication on
38
electrospun α-helical PBLG fibers and considering the
(
up to 20−30 min) led to the formation of smaller
characteristics of our C symmetry PBLG hybrid (it has a
3
organizations of spherical nature (20−40 nm) as shown by
SEM experiments (Figure 7, lower part). Since each molecule
of the initial aggregates contains three azobenzene units in the
trans disposition, in all probability the disassembly is related to
a partial conversion to the cis form. The reorganization into
smaller microstructures that takes place upon prolonged
irradiation is quite surprising. A deeper understanding of the
entire process will require additional investigations, possibly
including a computational approach. In any case, the oval
aggregates are not of vesicular nature, since their formation in
the presence of CF did not lead to encapsulation of the dye.
Finally, we investigated the photoresponsive behavior of a C3
symmetry PBLG hybrid characterized by a much higher MW
MW comparable to that reported in ref 38), we decided to
process our high-MW C symmetry PBLG hybrid via the
3
electrospinning technique. The characterization of the resulting
material is illustrated in Figure 8. The densely packed fibrillar
network detected by SEM (Figure 8A) corresponds, to the
macroscopic level, to a solid layer (Figure 8B). We tested the
photoresponsive behavior of this solid layer under UV
irradiation. The fibers, originally remarkably homogeneous in
shape, tend to progressively “swell” into larger domains after
30−60 min of irradiation (Figure 8C,D). A total collapse of the
fiber morphology was achieved after 200 min UV irradiation
(Figure 8E).
Interestingly, application of the electrospinning technique to
(
154 000 Da). In this case, the self-assembly process obtained
the C symmetry PBLG hybrid also led to the formation of a
2
starting from an organic solution upon dialysis against water led
again to the formation of oval microstructures. At variance with
the lower MW polypeptide, however, for this architecture no
fibrillar network. In this case, however, the fiber morphology did
not change even after prolonged UV irradiation (up to 3 h). In
all probability, the packing of polypeptide chains within the fibers
J
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Figure 8. (A) SEM image of the fibers obtained from the high-MW C symmetry PBLG hybrid after processing by the electrospinning technique.
3
(
B) Image showing the macroscopic material collected by electrospinning. (C−E) SEM images of the microstructure transition occurring at different
irradiation time at λ = 365 nm (30, 60, and 200 min, respectively).
of this linearly organized material is tighter than in the case of the
C3 symmetry PBLG hybrid, the molecules of the latter being
characterized by three arms that can give rise to a 3D network.
ASSOCIATED CONTENT
Supporting Information
■
*
S
Figures S1−S3: compound 5 (photoswitch behavior); Figures
S4−S7: compound 11 (photoswitch behavior); Figure S8: DLS
of CF vesicles from C symmetry PBLG; Figures S9−S11: C
CONCLUSIONS
■
2
3
We took advantage from two known azobenzene-based amino
acids as polymerization initiators for the construction of “smart”
supramolecular systems endowed with photoswitchable behav-
symmetry PBLG characterizations; Figures S12−S14: FT-IR
1
absorption spectra of polypeptides; Figures S15−S26: H NMR
spectra of the compounds reported in this work. This material
is available free of charge via the Internet at http://pubs.acs.org.
α
ior. The two starting materials are an N -protected derivative of
the achiral Moroder’s (4-aminomethyl)phenylazobenzoic acid
3
1
(
8) and the Sewald/Cativiela’s azo-bridged di-α-amino-
AUTHOR INFORMATION
29,30
■
*
*
diester 4,4′-diazenediyldi-L-Phe (5).
In particular, the
Corresponding Authors
E-mail: marco.crisma@unipd.it (M.C.).
E-mail: alessandro.moretto.1@unipd.it (A.M.).
NCA polymerization methodology, using the BLG heterocyclic
derivative as substrate, was utilized to prepare two polypeptide
hybrid molecules characterized by either C or C symmetry. In
2
3
the first case, the bis-initiator was the di-α-amino-diester (5)
itself, while in the second case the tris-initiator was the three-
branched compound 11 with an 1,3,5-triazine central core.
Moreover, both polypeptide hybrids were doubly or triply
modified at their N-termini upon treatment with excess
β-mercaptopropionic acid. This appropriate functionalization
allowed us to covalently link them to gold nanoparticles.
These azobenzene-containing compounds and materials were
reversibly trans-to-cis photoisomerized, and the micro- and
macroscopic changes in their 3D structures, self-aggregation
properties, and overall characteristics were carefully analyzed by
chromatography and UV−vis absorption/CD spectroscopies as
well as by TEM/SEM nanomaterial techniques. In particular,
the results reported here clearly indicate that the morphology
of the self-aggregated systems can be finely tuned by a geo-
metric (trans vs cis) modification of one (or more) azobenzene
units of the building blocks. As the observed phenomena are
hierarchical processes, conformational variations of a single
azobenzene amino acid component propagate to the final 3D
structures and macroscopic properties. The novel polypeptide
hybrids presented in this paper are promising materials to
expand the repertoire of photocontrolled releasers of bio-
logically relevant molecules.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the University of Padova (grant PRAT
A. M. C91 J11003560001) and MIUR, Ministero dell’Istru-
̀
zione, Universita e Ricerca, (grant FIRB 2012) is gratefully
acknowledged. A.M. thanks Dr. F. Caicci for assistance with
TEM data recording.
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dx.doi.org/10.1021/ma501601r | Macromolecules XXXX, XXX, XXX−XXX