12 research outputs found
One-Pot Peptide LigationāOxidative Cyclization Protocol for the Preparation of Short-/Medium-Size Disulfide Cyclopeptides
Native chemical ligation (NCL) employing
the <i>N</i>-methylbenzimidazolinone (MeNbz) linker readily
provided the linear
precursor of a 16-mer peptide that is difficult to obtain by stepwise
solid-phase peptide synthesis. NCL and the workup conditions were
improved toward a protocol that allows for quantitative removal of
the 4-hydroxymercaptophenol additive and subsequent formation of the
disulfide bridge in the NCL cocktail by oxidation in air, tolerated
by the presence of trisĀ(hydroxypropyl)Āphosphine
Imidazole-1-sulfonyl Azide-Based Diazo-Transfer Reaction for the Preparation of Azido Solid Supports for Solid-Phase Synthesis
An efficient, standard, mild, and
copper-free imidazole-1-sulfonyl
azide hydrochloride-based diazo-transfer method was implemented in
a set of four resins that cover a broad range of hydrophobicity. The
imidazole-1-sulfonyl azide hydrochloride is easily prepared/commercially
available, stable upon storage at 4 Ā°C, and proved to be a suitable
alternative to triflyl azide for diazo-transfer reactions in amine
functionalized resins. We have successfully applied the azido resins
for the conjugation of a TFA-labile Wang-type linker using Click Chemistry
Chemical Protein Synthesis Using a Second-Generation <i>N</i>āAcylurea Linker for the Preparation of Peptide-Thioester Precursors
The
broad utility of native chemical ligation (NCL) in protein
synthesis has fostered a search for methods that enable the efficient
synthesis of C<i>-</i>terminal peptide-thioesters, key intermediates
in NCL. We have developed an <i>N-</i>acylurea (Nbz) approach
for the synthesis of thioester peptide precursors that efficiently
undergo thiol exchange yielding thioester peptides and subsequently
NCL reaction. However, the synthesis of some glycine-rich sequences
revealed limitations, such as diacylated products that can not be
converted into <i>N</i>-acylurea peptides. Here, we introduce
a new <i>N-</i>acylurea linker bearing an <i>o-</i>aminoĀ(methyl)Āaniline (MeDbz) moiety that enables in a more robust
peptide chain assembly. The generality of the approach is illustrated
by the synthesis of a pentaglycine sequence under different coupling
conditions including microwave heating at coupling temperatures up
to 90 C, affording the unique and desired <i>N-</i>acyl-<i>N</i>ā²-methylacylurea (MeNbz) product. Further extension
of the method allowed the synthesis of all 20 natural amino acids
and their NCL reactions. The kinetic analysis of the ligations using
model peptides shows the MeNbz peptide rapidly converts to arylthioesters
that are efficient at NCL. Finally, we show that the new MeDbz linker
can be applied to the synthesis of cysteine-rich proteins such the
cyclotides Kalata B1 and MCoTI-II through a one cyclization/folding
step in the ligation/folding buffer
Chemical Protein Synthesis Using a Second-Generation <i>N</i>āAcylurea Linker for the Preparation of Peptide-Thioester Precursors
The
broad utility of native chemical ligation (NCL) in protein
synthesis has fostered a search for methods that enable the efficient
synthesis of C<i>-</i>terminal peptide-thioesters, key intermediates
in NCL. We have developed an <i>N-</i>acylurea (Nbz) approach
for the synthesis of thioester peptide precursors that efficiently
undergo thiol exchange yielding thioester peptides and subsequently
NCL reaction. However, the synthesis of some glycine-rich sequences
revealed limitations, such as diacylated products that can not be
converted into <i>N</i>-acylurea peptides. Here, we introduce
a new <i>N-</i>acylurea linker bearing an <i>o-</i>aminoĀ(methyl)Āaniline (MeDbz) moiety that enables in a more robust
peptide chain assembly. The generality of the approach is illustrated
by the synthesis of a pentaglycine sequence under different coupling
conditions including microwave heating at coupling temperatures up
to 90 C, affording the unique and desired <i>N-</i>acyl-<i>N</i>ā²-methylacylurea (MeNbz) product. Further extension
of the method allowed the synthesis of all 20 natural amino acids
and their NCL reactions. The kinetic analysis of the ligations using
model peptides shows the MeNbz peptide rapidly converts to arylthioesters
that are efficient at NCL. Finally, we show that the new MeDbz linker
can be applied to the synthesis of cysteine-rich proteins such the
cyclotides Kalata B1 and MCoTI-II through a one cyclization/folding
step in the ligation/folding buffer
Multimodal Characterization of a Linear DNA-Based Nanostructure
Designer DNA structures have garnered much interest as a way of assembling novel nanoscale architectures with exquisite control over the positioning of discrete molecules or nanoparticles. Exploiting this potential for a variety of applications such as light-harvesting, molecular electronics, or biosensing is contingent on the degree to which various nanoarchitectures with desired molecular functionalizations can be realized, and this depends critically on characterization. Many techniques exist for analyzing DNA-organized nanostructures; however, these are almost never used in concert because of overlapping concerns about their differing character, measurement environments, and the disparity in DNA modification chemistries and probe structure or size. To assess these concerns and to see what might be gleaned from a multimodal characterization, we intensively study a single DNA nanostructure using a multiplicity of methods. Our test bed is a linear 100 base-pair double-stranded DNA that has been modified by a variety of chemical handles, dyes, semiconductor quantum dots, gold nanoparticles, and electroactive labels. To this we apply a combination of physical/optical characterization methods including electrophoresis, atomic force microscopy, transmission electron microscopy, dynamic light scattering, FoĢrster resonance energy transfer, voltammetry, and structural modeling. In general, the results indicate that the differences among the techniques are not so large as to prevent their effective use in combination, that the data tend to be corroborative, and that differences observed among them can actually be quite informative
Cytotoxicity of Quantum Dots Used for <i>In Vitro</i> Cellular Labeling: Role of QD Surface Ligand, Delivery Modality, Cell Type, and Direct Comparison to Organic Fluorophores
Interest
in taking advantage of the unique spectral properties
of semiconductor quantum dots
(QDs) has driven their widespread use in biological applications such
as <i>in vitro</i> cellular labeling/imaging and sensing.
Despite their demonstrated utility, concerns over the potential toxic
effects of QD core materials on cellular proliferation and homeostasis
have persisted, leaving in question the suitability of QDs as alternatives
for more traditional fluorescent materials (e.g., organic dyes, fluorescent
proteins) for <i>in vitro</i> cellular applications. Surprisingly,
direct comparative studies examining the cytotoxic potential of QDs
versus these more traditional cellular labeling fluorophores remain
limited. Here, using CdSe/ZnS (core/shell) QDs as a prototypical assay
material, we present a comprehensive study in which we characterize
the influence of QD dose (concentration and incubation time), QD surface
capping ligand, and delivery modality (peptide or cationic amphiphile
transfection reagent) on cellular viability in three human cell lines
representing various morphological lineages (epithelial, endothelial,
monocytic). We further compare the effects of QD cellular labeling
on cellular proliferation relative to those associated with a panel
of traditionally employed organic cell labeling fluorophores that
span a broad spectral range. Our results demonstrate the important
role played by QD dose, capping ligand structure, and delivery agent
in modulating cellular toxicity. Further, the results show that at
the concentrations and time regimes required for robust QD-based cellular
labeling, the impact of our in-house synthesized QD materials on cellular
proliferation is comparable to that of six commercial cell labeling
fluorophores. Cumulatively, our results demonstrate that the proper
tuning of QD dose, surface ligand, and delivery modality can provide
robust <i>in vitro</i> cell labeling reagents that exhibit
minimal impact on cellular viability
Quantum Dots as Simultaneous Acceptors and Donors in Time-Gated FoĢrster Resonance Energy Transfer Relays: Characterization and Biosensing
The unique photophysical properties of semiconductor
quantum dot
(QD) bioconjugates offer many advantages for active sensing, imaging,
and optical diagnostics. In particular, QDs have been widely adopted
as either donors or acceptors in FoĢrster resonance energy transfer
(FRET)-based assays and biosensors. Here, we expand their utility
by demonstrating that QDs can function in a simultaneous role as acceptors
and donors within time-gated FRET relays. To achieve this configuration,
the QD was used as a central nanoplatform and coassembled with peptides
or oligonucleotides that were labeled with either a long lifetime
luminescent terbiumĀ(III) complex (Tb) or a fluorescent dye, Alexa
Fluor 647 (A647). Within the FRET relay, the QD served as a critical
intermediary where (1) an excited-state Tb donor transferred energy
to the ground-state QD following a suitable microsecond delay and
(2) the QD subsequently transferred that energy to an A647 acceptor.
A detailed photophysical analysis was undertaken for each step of
the FRET relay. The assembly of increasing ratios of Tb/QD was found
to linearly increase the magnitude of the FRET-sensitized time-gated
QD photoluminescence intensity. Importantly, the Tb was found to sensitize
the subsequent QDāA647 donorāacceptor FRET pair without
significantly affecting the intrinsic energy transfer efficiency within
the second step in the relay. The utility of incorporating QDs into
this type of time-gated energy transfer configuration was demonstrated
in prototypical bioassays for monitoring protease activity and nucleic
acid hybridization; the latter included a dual target format where
each orthogonal FRET step transduced a separate binding event. Potential
benefits of this time-gated FRET approach include: eliminating background
fluorescence, accessing two approximately independent FRET mechanisms
in a single QD-bioconjugate, and multiplexed biosensing based on spectrotemporal
resolution of QD-FRET without requiring multiple colors of QD
Proteolytic Activity at Quantum Dot-Conjugates: Kinetic Analysis Reveals Enhanced Enzyme Activity and Localized Interfacial āHoppingā
Recent studies show that polyvalent, ligand-modified
nanoparticles
provide significantly enhanced binding characteristics compared to
isolated ligands. Here, we assess the ability of substrate-modified
nanoparticles to provide enhanced enzymatic activity. Energy transfer
assays allowed quantitative, real-time measurement of proteolytic
digestion at polyvalent quantum dot-peptide conjugates. Enzymatic
progress curves were analyzed using an integrated MichaelisāMenten
(MM) formalism, revealing mechanistic details, including deviations
from classic MM-behavior. A āhoppingā mode of proteolysis
at the nanoparticle was identified, confirming enhanced activity
Complex FoĢrster Energy Transfer Interactions between Semiconductor Quantum Dots and a Redox-Active Osmium Assembly
The ability of luminescent semiconductor quantum dots (QDs) to engage in diverse energy transfer processes with organic dyes, light-harvesting proteins, metal complexes, and redox-active labels continues to stimulate interest in developing them for biosensing and light-harvesting applications. Within biosensing configurations, changes in the rate of energy transfer between the QD and the proximal donor, or acceptor, based upon some external (biological) event form the principle basis for signal transduction. However, designing QD sensors to function optimally is predicated on a full understanding of all relevant energy transfer mechanisms. In this report, we examine energy transfer between a range of CdSeāZnS coreāshell QDs and a redox-active osmium(II) polypyridyl complex. To facilitate this, the Os complex was synthesized as a reactive isothiocyanate and used to label a hexahistidine-terminated peptide. The Os-labeled peptide was ratiometrically self-assembled to the QDs <i>via</i> metal affinity coordination, bringing the Os complex into close proximity of the nanocrystal surface. QDs displaying different emission maxima were assembled with increasing ratios of Osāpeptide complex and subjected to detailed steady-state, ultrafast transient absorption, and luminescence lifetime decay analyses. Although the possibility exists for charge transfer quenching interactions, we find that the QD donors engage in relatively efficient FoĢrster resonance energy transfer with the Os complex acceptor despite relatively low overall spectral overlap. These results are in contrast to other similar QD donorāredox-active acceptor systems with similar separation distances, but displaying far higher spectral overlap, where charge transfer processes were reported to be the dominant QD quenching mechanism
Probing the Quenching of Quantum Dot Photoluminescence by Peptide-Labeled Ruthenium(II) Complexes
Charge transfer processes with semiconductor
quantum dots (QDs)
have generated much interest for potential utility in energy conversion.
Such configurations are generally nonbiological; however, recent studies
have shown that a redox-active rutheniumĀ(II)āphenanthroline
complex (Ru<sup>2+</sup>-phen) is particularly efficient at quenching
the photoluminescence (PL) of QDs, and this mechanism demonstrates
good potential for application as a generalized biosensing detection
modality since it is aqueous compatible. Multiple possibilities for
charge transfer and/or energy transfer mechanisms exist within this
type of assembly, and there is currently a limited understanding of
the underlying photophysical processes in such biocomposite systems
where nanomaterials are directly interfaced with biomolecules such
as proteins. Here, we utilize
redox reactions, steady-state absorption, PL spectroscopy, time-resolved
PL spectroscopy, and femtosecond transient absorption spectroscopy
(FSTA) to investigate PL quenching in biological assemblies of CdSe/ZnS
QDs formed with peptide-linked Ru<sup>2+</sup>-phen. The results reveal
that QD quenching requires the Ru<sup>2+</sup> oxidation state and
is not consistent with FoĢrster resonance energy transfer, strongly
supporting a charge transfer mechanism. Further, two colors of CdSe/ZnS
core/shell QDs with similar macroscopic optical properties were found
to have very different rates of charge transfer quenching, by Ru<sup>2+</sup>-phen with the key difference between them appearing to be
the thickness of their ZnS outer shell. The effect of shell thickness
was found to be larger than the effect of increasing distance between
the QD and Ru<sup>2+</sup>-phen when using peptides of increasing
persistence length. FSTA and time-resolved upconversion PL results
further show that exciton quenching is a rather slow process consistent
with other QD conjugate materials that undergo hole transfer. An improved
understanding of the QDāRu<sup>2+</sup>-phen system can allow
for the design of more sophisticated charge-transfer-based biosensors
using QD platforms