(397bd) Kinetically Controlled Self-Assembly of Redox-Active Ferrocene-Diphenylalanine: From Nanospheres to Nanofibers

Self-assembly is a common
process in biological and synthetic systems, which has been emerging as a
powerful, bottom-up approach in the construction of well-ordered
nanostructures. The inherent biocompatibility, ease of synthesis and facile
chemical modification of peptide molecules render them excellent building
blocks in the construction of well-defined supramolecular architectures. A
well-known example is that of the diphenylalanine
peptide (L-Phe-L-Phe, FF),  which is the core recognition motif of
Alzheimer's Ab polypeptide.¹ Through a combination of specific molecular interactions such
as van der vaals, hydrogen bonding, hydrophobic and ¦Ð-¦Ð stacking under different
conditions (e. g. solvent or surface), Phe-Phe peptide can self-assemble into
various nanostructures such as nanotubes,¹ nanowires,² microcrystals,³ and vertically aligned nanoforest.⁴ Due to the specific molecular interactions, Phe-Phe can be
employed as a recognition module to design novel molecular building blocks. It
is suggested that the modified motifs play a determinant role in the creation
of novel functional peptide nanomaterials.

Putting metals into
self-assembling peptides may provide us an opportunity to design molecular
building blocks with new desirable properties. Ferrocene (Fc),⁵ one of the most stable and useful organometallic compounds, has
been used as metal-containing component for the construction of various hybrid
molecules from small molecules to polymers with various applications in biosensor,
magnetic and stimuli-responsive materials, and asymmetric catalysis. Here, we
showed the kinetically controlled self-assembly of a novel bioorganometallic
molecule via the conjugation of ferrocene with diphenylalanine (ferrocene-diphenylalanine,
Fc-FF, Figure 1a), and directly observed a morphological transition from
metastable nanospheres to well-defined nanofibers by introducing an external
mechanical force (i.e. shaking), which finally led to the formation of a stable
self-supporting hydrogel (Figure 1b and c). Moreover, as the Fc moiety in this
hybrid molecule serves as a redox-active site, its self-assembly process can be
reversibly controlled by altering the redox state of Fc group (Figure 1d and e).
The main ideas and conclusions are summarized as follows:

(1) Strong hydrophobic
interaction among molecular units may lead to the formation of nanomaterials
that depend largely on the pathway or kinetic control. For Fc-FF assemblies,
after diluting the concentrated Fc-FF methanol solution in water, the solvent
environment around Fc-FF molecules changed intensively, giving rise to strong
hydrophobic interactions which were adequate for achieving efficient kinetic
trapping, thus leading to formation of nanospheres. After exerting slight
shaking, the metastable Fc-FF nanospheres can transform into thermodynamically
stable nanofibers with simultaneously molecular rearrangement from unordered
structure to ¦Á-helix. It is obvious that the self-assembly of Fc-FF is a
kinetically controlled process. High-uniform nanospheres were formed by the
fast self-assembly, whereas the nanofibers with helical superstructures were
obtained from the slow self-assembly. External mechanical force can be used to
overcome the energy barrier and thus to initiate the 'slow' self-assembly
process. The
strong hydrophobic interactions introduced by Fc group in water may play an
essential role in this transition process.

(2) For Fc-FF molecule, the Fc group in its reduced state is hydrophobic, while its oxidized
form (Fc⁺) is hydrophilic (Fig. 1d). This can give rise to a remarkable change in the hydrophobic-hydrophilic
balance of Fc-FF, thus altering its self-assembly behavior. To demonstrate this, 1 mg ml^-1 Fc-FF nanofibers was
dispersed in 10% CH₃OH/90% H₂O. After introducing an appropriate amount of CeSO₄ as
oxidant, the yellow turbid Fc-FF nanofibers suspension turned into a blue
transparent solution, and continuous addition of GSH to the solution
transformed it back to yield a yellow turbid suspension again (Figure 1e,
insets). SEM analysis revealed that the Fc-FF units could reversibly
self-assemble into nanofibers after the oxidation-reduction cycle (Figure 1e).
After the oxidation of Fc-FF into Fc⁺-FF,
apart from an amorphous film deposited on the surface of the microscope glass
coverslip, no characteristic morphology was observed. This indicated that the
Fc-FF nanofibers may disassemble or disrupt into monomers or much smaller
aggregates as the oxidation of Fc-FF into Fc⁺-FF. Moreover,
continuous reduction of the Fc⁺-FF molecule into Fc-FF recovered its
self-assembling ability, thus leading to the reassemble of well-defined Fc-FF
nanofibers.

(3) Due to the stimulus (i.e. mechanical force, redox) responsive
properties, the Fc-FF nanostructures may have great potential applications in
areas such as biosensor, tissue engineering, controlled drug release. Moreover,
considering the structural diversity of FF assemblies, the Fc-FF molecule may have the ability to form other novel
nanosturcutres by controlling the self-assembly conditions.

Figure 1. Self-assembly behavior of ferrocene-diphenylalanine. a) Molecular structure of
ferrocene-diphenylalanine (Fc-FF). b) Photograph of the mixture incubated
without any external disturbance. c) Photograph of the mixture after incubation
by introducing slight shaking. d) Redox-control of the ferrocenyl group. e) SEM
analysis showed the self-assembling behavior of Fc-FF molecules in their
reduced and oxidized state. The insets showed the photographic images of 1mg ml^-1
Fc-FF nanofibers dispersed in 10% CH₃OH/90% H₂O before
and after oxidation.

This work was supported by the NSF of China
(51173128, 31071509), the Ministry of Science and Technology of China (Nos.
2012YQ090194, 2013AA102204, 2012BAD29B05), the Program for New Century
Excellent Talents in Chinese University (NCET-08-0386), and Beiyang Young
Scholar Program (2012).

References

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