19 research outputs found
Tuning Nanostructure Dimensions with Supramolecular Twisting
Peptide amphiphiles are molecules containing a peptide
segment
covalently bonded to a hydrophobic tail and are known to self-assemble
in water into supramolecular nanostructures with shape diversity ranging
from spheres to cylinders, twisted ribbons, belts, and tubes. Understanding
the self-assembly mechanisms to control dimensions and shapes of the
nanostructures remains a grand challenge. We report here on a systematic
study of peptide amphiphiles containing valine–glutamic acid
dimeric repeats known to promote self-assembly into belt-like flat
assemblies. We find that the lateral growth of the assemblies can
be controlled in the range of 100 nm down to 10 nm as the number of
dimeric repeats is increased from two to six. Using circular dichroism,
the degree of β-sheet twisting within the supramolecular assemblies
was found to be directly proportional to the number of dimeric repeats
in the PA molecule. Interestingly, as twisting increased, a threshold
is reached where cylinders rather than flat assemblies become the
dominant morphology. We also show that in the belt regime, the width
of the nanostructures can be decreased by raising the pH to increase
charge density and therefore electrostatic repulsion among glutamic
acid residues. The control of size and shape of these nanostructures
should affect their functions in biological signaling and drug delivery
Discovery of Y‑Shaped Supramolecular Polymers in a Self-Assembling Peptide Amphiphile System
Supramolecular polymers (SPs) formed by self-assembly
of peptide-based
molecular units assume a variety of interesting one-dimensional (1D)
morphologies. While the morphological complexity and phase behavior
of self-assembling peptide conjugates bear some resemblance to those
of low-molecular-weight and macromolecular surfactants, Y-junctions,
or three-way connected constructs, a topological defect observed in
traditional surfactants has not been identified, likely due to the
intolerance of defective packing by the strong, associative interactions
afforded by the peptide segments. Here we report our discovery of
branched SPs with Y-junctions and occasionally enlarged spherical
end-caps formed by micellization of a ferrocene-based peptide amphiphile
in water. Our results suggest that the incorporation of two ferrocenes
into the amphiphile design is key to ensure the formation of branched
SPs. We hypothesize that the complex interplay of internal interactions
limits the effective propagation of hydrogen bonding within the assemblies
and, consequently, creates fragmented β-sheets that are more
tolerant for supramolecular branching. Given the redox sensitivity
of the ferrocene units, sequential addition of reductants and oxidants
to the solution led the assemblies to reversibly transform between
branched SPs and spherical aggregates
π–π Stacking Mediated Chirality in Functional Supramolecular Filaments
While a great diversity of peptide-based
supramolecular filaments
have been reported, the impact of an auxiliary segment on the chiral
assembly of peptides remains poorly understood. Herein we report on
the formation of chiral filaments by the self-assembly of a peptide-drug
conjugate containing an aromatic drug camptothecin (CPT) in a computational
study. We find that the chirality of the filament is mediated by the
π–π stacking between CPTs, not only by the well-expected
intermolecular hydrogen bonding between peptide segments. Our simulations
show that π–π stacking of CPTs governs the early
stages of the self-assembly process, while a hydrogen bonding network
starts at a relatively later stage to contribute to the eventual morphology
of the filament. Our results also show the possible presence of water
within the core of the CPT filament. These results provide very useful
guiding principles for the rational design of supramolecular assemblies
of peptide conjugates with aromatic segments
Design and Construction of Supramolecular Nanobeacons for Enzyme Detection
Molecular beacons are typically water-soluble molecules that can convert specific chemical reactions or binding events into measurable optical signals, providing a noninvasive means to help understand cellular and subcellular activities at the molecular level. However, the soluble form of the current molecular beacon design often leads to their poor stability and facile degradation by nonspecific enzymes, and as a result, this undesired activation could give rise to false signals and thus poses a limitation for accurate detection of enzymatic activities. Here we report a proof-of-concept design and synthesis of a new type of supramolecular nanobeacon that is resistant to nonspecific enzymatic degradation in the self-assembled state but can be effectively cleaved by the target enzyme in the monomeric form. Our results show that the nanobeacon with a GFLG peptide linker could serve as an indicator for the presence of a lysosomal enzyme, cathepsin B
Enhanced Cellular Entry and Efficacy of Tat Conjugates by Rational Design of the Auxiliary Segment
Conjugation with a cell penetrating
peptide such as Tat presents
an effective approach to improve the intracellular accumulation of
molecules with low membrane permeability. This strategy, however,
leads to a reduced cellular entry of molecules that can cross cell
membrane effectively. We report here that covalent linkage of an additional
hydrophobic unit that mimics a hydrophobic domain near the Tat sequence
can further improve the cellular uptake of the parental conjugate
into cancer cells regardless of the membrane permeability of the unconjugated
molecule. Both fluorescent imaging and flow cytometry measurements
confirmed the effect of palmitoylation on the increased internalization
of the Tat conjugates with either 5-carboxyfluorescein (5-FAM), a
nonmembrane penetrating dye, or doxorubicin, an anticancer cancer
drug that can readily diffuse across cell membranes. In the case of
the Tat–doxorubicin conjugate, palmitoylation improves the
conjugate’s anticancer activity in both drug sensitive and
resistant cervical cancer cell lines. We further demonstrate that
modification of a Tat–5-FAM conjugate with a hydrophobic quencher
could not only efficiently quench the fluorescence outside of cancer
cell but also facilitate its entry into MCF-7 breast cancer cells.
These results highlight the importance of rational molecular design
of using peptide conjugation chemistry in cancer therapeutics and
diagnostics
Cellular Uptake and Cytotoxicity of Drug–Peptide Conjugates Regulated by Conjugation Site
Conjugation of anticancer drugs to
hydrophilic peptides such as
Tat is a widely adopted strategy to improve the drug’s solubility,
cellular uptake, and potency against cancerous cells. Here we report
that attachment of an anticancer drug doxorubicin to the <i>N</i>- or <i>C</i>-terminal of the Tat peptide can have a significant
impact on their cellular uptake and cytotoxicity against both drug-sensitive
and drug-resistant cancer cells. We observed higher cellular uptake
by both cell lines for <i>C</i>-terminal conjugate relative
to the <i>N</i>-terminal analogue. Our results reveal that
the <i>C</i>-terminal conjugate partially overcame the multidrug
resistance of cervical cancer cells, while the <i>N</i>-terminal
conjugate showed no significant improvement in cytotoxicity when compared
with free doxorubicin. We also found that both <i>N</i>-
and <i>C</i>-conjugates offer a mechanism to circumvent
drug efflux associated with multidrug resistance
Supramolecular Polymers Formed by ABC Miktoarm Star Peptides
We
report here the design and synthesis of an ABC miktoarm star peptide
connecting through a lysine junction a short peptide sequence and
two hydrophobic but immiscible blocks (a hydrocarbon and a fluorocarbon).
The designed molecule can self-assemble into one-dimensional nanostructures
with a great diversity of kinetically evolving morphologies in aqueous
solution, while molecules that contain only one of the two hydrophobic
blocks form structurally similar filaments. We believe the rich assembly
behavior and morphological evolution are a direct reflection of the
molecular frustration present within the filament core as a result
of the incompatibility of the fluorocarbon and hydrocarbon segments.
Our finding opens new opportunities for creating complex supramolecular
polymers through the architecture design of small molecular building
units
Amino Acid Sequence in Constitutionally Isomeric Tetrapeptide Amphiphiles Dictates Architecture of One-Dimensional Nanostructures
The switching of
two adjacent amino acids can lead to differences
in how proteins fold thus affecting their function. This effect has
not been extensively explored in synthetic peptides in the context
of supramolecular self-assembly. Toward this end, we report here the
use of isomeric peptide amphiphiles as molecular building blocks to
create one-dimensional (1D) nanostructures. We show that four peptide
amphiphile isomers, with identical composition but a different sequence
of their four amino acids, can form drastically different types of
1D nanostructures under the same conditions. We found that molecules
with a peptide sequence of alternating hydrophobic and hydrophilic
amino acids such as VEVE and EVEV self-assemble into flat nanostructures
that can be either helical or twisted. On the other hand, nonalternating
isomers such as VVEE and EEVV result in the formation of cylindrical
nanofibers. Furthermore, we also found that when the glutamic acid
is adjacent to the alkyl tail the supramolecular assemblies appear
to be internally flexible compared to those with valine as the first
amino acid. These results clearly demonstrate the significance of
peptide side chain interactions in determining the architectures of
supramolecular assemblies
Rational Design of MMP Degradable Peptide-Based Supramolecular Filaments
One-dimensional nanostructures formed
by self-assembly of small
molecule peptides have been extensively explored for use as biomaterials
in various biomedical contexts. However, unlike individual peptides
that can be designed to be specifically degradable by enzymes/proteases
of interest, their self-assembled nanostructures, particularly those
rich in β-sheets, are generally resistant to enzymatic degradation
because the specific cleavage sites are often embedded inside the
nanostructures. We report here on the rational design of β-sheet
rich supramolecular filaments that can specifically dissociate into
less stable micellar assemblies and monomers upon treatment with matrix
metalloproteases-2 (MMP-2). Through linkage of an oligoproline segment
to an amyloid-derived peptide sequence, we first synthesized an amphiphilic
peptide that can undergo a rapid morphological transition in response
to pH variations. We then used MMP-2 specific peptide substrates as
multivalent cross-linkers to covalently fix the amyloid-like filaments
in the self-assembled state at pH 4.5. Our results show that the cross-linked
filaments are stable at pH 7.5 but gradually break down into much
shorter filaments upon cleavage of the peptidic cross-linkers by MMP-2.
We believe that the reported work presents a new design platform for
the creation of amyloid-like supramolecular filaments responsive to
enzymatic degradation
Enzyme-Specific Doxorubicin Drug Beacon as Drug-Resistant Theranostic Molecular Probes
We
report here on the use of anticancer drug doxorubicin (Dox)
to construct a Förster resonance energy transfer (FRET)-based
theranostic molecular probe by covalently linking together through
a lysine junction a fluorescent drug, a black hole quencher, and a
cell-penetrating peptide. We show that upon cleavage by the target
lysosomal protease cathepsin B (CatB) the designed drug beacon could
release the fluorescent drug serving as an indicator for CatB. Our
cell studies suggest that the drug-beacon design can help to circumvent
the Dox drug resistance in NCI/ADR-Res ovarian cancer cells, showing
significant improvement in cell cytotoxicity compared to the free
drug. We believe our design opens up new opportunities to exploit
the new functional and structural features of anticancer drugs in
addition to their characteristic cytotoxicity