Sunlight mediated disruption of peptide-based soft structures decorated with gold nanoparticles

Article abstract of DOI:10.1039/c0cc02604b

This communication reports morphological studies of a novel C ₃-symmetric thiolated, tren-based ditryptophan conjugate. The exposed thiol groups on the soft structures interacted with gold nanoparticles and mediated the release of encapsulated fluorescent dye, when exposed to natural sunlight.

Full text of DOI:10.1039/c0cc02604b

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COMMUNICATION
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Sunlight mediated disruption of peptide-based soft structures decorated
with gold nanoparticlesw
Apurba Kr. Barman and Sandeep Verma*
Received 16th July 2010, Accepted 6th August 2010
DOI: 10.1039/c0cc02604b
This communication reports morphological studies of a novel
C3-symmetric thiolated, tren-based ditryptophan conjugate. The
exposed thiol groups on the soft structures interacted with
gold nanoparticles and mediated the release of encapsulated
fluorescent dye, when exposed to natural sunlight.
structures. A C3-symmetric ditryptophan-tren derivative was
conjugated to 3-mercaptopropionic acid and the product was
characterized by spectroscopic methods (Scheme 1).
A freshly prepared solution of 1 (1 mM; 9 : 1 methanol :
water) revealed a spherical morphology in SEM micrographs
(Fig. 1a), which was confirmed by AFM experiments on a
mica surface (Fig. 1b). The emergence of self-assembled
structures could be attributed to favorable interdigitation of
Trp indole rings supported by p–p stacking.4g Interestingly,
the 10-day incubation of this solution leads to the coalescence
of the soft structures (Fig. 1c). It is possible that exposed thiol
groups on surface of soft structures form disulfide cross-links
upon slow aerial oxidation due to prolonged incubation,
thus bringing the spherical structures into close proximity
followed by deformation. The latter process was reversed by
dithiothreitol treatment giving back the original spherical
morphology perhaps due to reduction of the disulfide cross-
links (Fig. 1d).
The diversity of peptide-based design of soft supramolecular
ensembles emanates from the properties engendered in the
side-chains of constituent amino acids that range from simple
alkyl groups to a host of acidic, basic, hydrophilic, aromatic
and heterocyclic groups.1 These functionalities confer self-
assembling and physicochemical properties through a variety
of noncovalent interactions such as hydrogen bonds, hydro-
phobic interactions, salt bridges and a number of other specific
interactions such as p–p stacking, cation–p and charge–dipole
interactions.2 Peptide-based soft structures respond to environ-
mental stimuli and offer exciting opportunities in material
synthesis, drug delivery, tissue engineering and sensing
applications.3
We next tried to exploit thiol–gold nanoparticle interaction
to decorate the surface of peptide soft structures. Gold nano-
particles (AuNP) of B15 nm in size were added to a solution
of 1 and UV-visible spectra were recorded. A slight red shift
and decrease in the peak height was observed for the plasmon
absorbance of AuNPs, but more importantly, there was no
colour change, and precipitation of AuNPs was not observed
in the presence of the peptide conjugate.8
One of the key interests in these soft structures concerns
the possibility of encapsulation, covalent attachment, or
chemiadsorption of bioactive agents for intracellular transport,
where the size scale may offer certain advantages during
administration via intravenous and subcutaneous routes and
circumvent issues related to solubility and dispersability. The
delivery of biologics could eventually be achieved by applying
suitable external stimuli, such as alteration of pH change,
exposure to reducing agents, or heat.4
Microscopy analysis of the pre-formed 1–AuNP interaction
was conducted to investigate the morphological details of the
hybrid structures. It was observed that a 1 h or longer
co-incubation resulted in the attachment of AuNP on to the
surface of spherical soft structures (Fig. 2a,b). The AuNP
decorated structures were able to retain their integrity for as
Recent excitement in metal nanoparticle research deals
with the fundamental concepts and possible applications of
plasmonic photothermal heating of diseased tissues by using
visible radiation or laser excitation at resonant frequencies of
plasmonic metal nanoparticles.5 It was also shown that the size
of the nanoparticles plays a significant role in determining the
thermal transduction efficiencies.6 Thus, it could be envisaged
that thermal transduction via plasmonic heating could serve as
an external stimulus for soft biological structures.
We have described the design and self-assembly of peptide-
based soft structures to study prion octarepeats, as delivery
vehicles and in ion-beam milling experiments.3d,7 In one of the
studies, a synthetic triskelion self-assembled to form spherical
objects which responded to pH stimuli.4g We decided to
explore the possibility of using a redox environment as a
stimulus to exert pressure on the morphology of peptide soft
Department of Chemistry, Indian Institute of Technology Kanpur,
Kanpur-208016 (UP), India. E-mail: sverma@iitk.ac.in
w Electronic supplementary information (ESI) available: Experimental
details, UV-VIS spectra, additional microscopic images. See DOI:
10.1039/c0cc02604b
Scheme 1 Structure of the conjugate (Mpa-Trp-Trp)3 tren (1).
Mpa = 3-mercaptopropionic acid and Trp = tryptophan
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We decided to investigate if plasmonic heating will have an
effect on the morphology of peptide-based soft self-assembled
structures, as we have an interest in discovering newer stimuli-
responsive structures for containment, release and delivery.
AuNP immobilized spherical structures from Fig. 2a were
directly exposed to sunlight for a period of 6 h in eppendorf
tubes. Subsequently, the sample was analyzed by scanning
electron microscopy which revealed some deformation in the
overall morphology and in a few instances, the spherical
objects coalesced resulting in irregular shapes (Fig. 2c).
Interestingly, AuNPs were detached and release in the bulk
solution, as a result of exposure to sunlight (Fig. 2d). In a
control experiment, the size and overall shape of spherical
objects from 1 remained unperturbed when exposed to
sunlight in the absence of gold nanoparticles under similar
conditions.8
Based on literature precedent, it could be proposed that a
direct exposure to sunlight results in the plasmonic heating of
AuNP-decorated peptide soft structures. This thermal energy
gets most likely transferred to the self-assembled peptide
scaffold thus modifying integrity of their structure. Although,
laser beams or near-IR (NIR) sources have the advantage of
tunable power for plasmonic heating experiments, a 2 h
exposure to sunlight was also sufficient to achieve photothermal
heating in the present example.
Fig. 1 (a) and (b) SEM and AFM micrographs of 1, respectively;
SEM micrographs of (c) 10 days aged sample, and (d) 10 days old
sample treated with a reducing agent, DTT.
Morphological changes of the nanospheres as a result of
SPR heating encouraged us to investigate whether they can
possibly be used for delivery related applications. For latter
studies, fluorescent dyes were used for encapsulation and
release, with the aid of an external stimulus.13 We performed
a simple experiment to follow the interaction of a fluorescent
dye with self-assembled structures of 1, followed by the fate of
dye labeled, AuNP conjugated soft structures upon plasmonic
heating.
1 was dissolved in a rhodamine solution (1 mM) and the
resulting solution was incubated for 2 h. As the dye interacted
with the spherical structures, punctated bright red coloured
objects were visualized in the fluorescence microscope
(Fig. 3a). These rhodamine encapsulated spheres were then
co-incubated with AuNP for 1 h as mentioned earlier.
Fluorescence microscopy of rhodamine encapsulated AuNP
immobilised spheres, after exposing to sunlight for 6 h, revealed
that the intensity of the dye decreased and it was confined to the
periphery of the spherical objects (Fig. 3c). Similar observations
have also been reported for laser and NIR-mediated plasmonic
heating of metal nanoparticle modified polyelectrolyte polymer
capsules and synthetic liposomes.13
Fig. 2 SEM micrographs: (a) and (b) pre-formed spheres of 1
co-incubated with AuNPs (B15 nm) for 1 h and 3 days, respectively;
(c) and (d) 1 co-incubated with AuNPs for 1 h and then exposed to
sunlight for 2 h and 6 h, respectively (arrow shows swelling of vesicles
and release of AuNPs due to SPR heating).
long as 16 days of incubation. EDAX analysis of these spheres
further confirmed the presence of gold on the surface.8
Gold nanoparticles and nanorods are known to intensely
absorb visible light due to the surface plasmon resonance
(SPR) effect. The exposure of metal nanoparticles to electro-
magnetic radiation facilitates plasmonic heat generation, with
a distinct correlation to the shape, number and organisation of
the metal nanoparticles.6 Besides the biological therapeutic
uses suggested earlier,5 plasmonic heating of gold nano-
particles has been used to control the size, shape, and phases
of the nanoparticles,9 for optothermal imaging,10 and catalysis.11
Such studies are mostly conducted with the help of lasers
corresponding to the SPR maxima, but the use of sunlight and
simulated sunlight for plasmonic heating of gold nanoparticles
is also reported.11a,12
For control studies, dye encapsulated spheres in the absence
of AuNP were exposed to sun light8 and dye encapsulated
AuNP immobilised spheres were kept in dark, for 6 h. In both
cases dyes were uniformly distributed in the spheres (Fig. 3b),
confirming that plasmonic heating could be used as a stimulus
for releasing encapsulated material from these hybrid spherical
self-assembled structures.
In summary, a new triskelion peptide conjugate was
synthesised and its self-assembly process was studied in detail
with various microscopy techniques. Time-dependent studies
of this conjugate revealed agglomeration of these self-assembled
spheres due to thiol oxidation, which was reversed when
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Fig. 3 Fluorescence microscopy images: (a) 1 co-incubated with
rhodamine B for 2 h (scale bar: 3 mm); (b) dye encapsulated spheres
co-incubated with AuNPs; (c) dye encapsulated AuNP immobilized
spheres exposed to sunlight for 6 h; (d) zoomed-in image of structures
with rhodamine fluorescence at the periphery.
treated with DTT. Further, we showed that thiol-containing
spheres serve as a scaffold for AuNP interaction. Interestingly,
AuNP decorated soft structures responded to sunlight-mediated
plasmonic heating and as an application, remote release of
encapsulated rhodamine B from the hybrid spherical objects
was demonstrated.
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We realize that considering the smaller size of the AuNPs
used in this study, less thermal effect is expected. As a follow-up,
we would like to investigate if low thermal effects are sufficient
in inducing morphological changes of soft structures. Finally,
it is surmised that the tuning of such hybrid structures may
open possibilities for discovering biocompatible drug delivery
compartments for targeted release of drugs on exposure to
sunlight as a release mechanism.
AKB thanks CSIR, India, for a Junior Research Fellow-
ship. This work is supported by DST, India (SV).
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c
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This journal is The Royal Society of Chemistry 2010