26 research outputs found
Counting Single Redox Turnovers: Fluorogenic Antioxidant Conversion and Mass Transport Visualization via Single Molecule Spectroelectrochemistry
We
report herein the use of single molecule spectroelectrochemistry
(SMS-EC) to optically detect electrochemically activated individual
molecules of the redox-active, tocopherol-like fluorogenic probe H<sub>2</sub>B-PMHC. Single molecule detection combined with a fluorescence
burst frequency analysis is shown to provide a most sensitive method
to monitor subnanomolar concentrations of the electrochemically activated
probe. By measuring changes in activated probe concentration over
time and distance from a planar electrode, mass transport characteristics
of a prototype infinite planar electrode system are retrieved as a
showcase study. We show that fluorescence activation may be achieved
under both oxidative (direct oxidation of H<sub>2</sub>B-PMHC) and
reductive (via O<sub>2</sub><sup>•‑</sup> production)
conditions. Our results conducted in organic solvents broaden the
scope of electrochemical reactions that can be studied by SMS-EC.
The methodology we describe may potentially aid the study of mass
transport in systems with complex electrode geometries or hampered
diffusion
Redox-Based Photostabilizing Agents in Fluorescence Imaging: The Hidden Role of Intersystem Crossing in Geminate Radical Ion Pairs
Here
we report transient absorption studies on the ground-state
recovery dynamics of the single-molecule fluorophore Cy3B in the presence
of four different photostabilizing agents, namely β-mercaptoethanol
(β-ME), Trolox (TX), <i>n</i>-propyl gallate (<i>n</i>-PG), and ascorbic acid (AA). These are triplet-state quenchers
that operate via photoinduced electron transfer (PeT). While quantitative
geminate recombination was recorded following PeT for β-ME (∼100%),
for Trolox, <i>n</i>-propyl gallate, and ascorbic acid the
extent of geminate recombination was >48%, >27%, and >13%,
respectively.
The results are rationalized in terms of the rates of intersystem
crossing (ISC) in the newly formed geminate radical ion pairs (GRIPs).
Rapid spin relaxation in the radicals formed accounts for quantitative
geminate recombination with β-ME and efficient geminate recombination
with TX. Our results illustrate how the interplay of PeT quenching
efficiency and geminate recombination dynamics may lead to improved
photostabilization strategies, critical for single-molecule fluorescence
and super-resolution imaging
Cy3 Photoprotection Mediated by Ni<sup>2+</sup> for Extended Single-Molecule Imaging: Old Tricks for New Techniques
The photostability of reporter fluorophores
in single-molecule
fluorescence imaging is of paramount importance, as it dictates the
amount of relevant information that may be acquired before photobleaching
occurs. Quenchers of triplet excited states are thus required to minimize
blinking and sensitization of singlet oxygen. Through a combination
of single-molecule studies and ensemble mechanistic studies including
laser flash photolysis and time-resolved fluorescence, we demonstrate
herein that Ni<sup>2+</sup> provides a much desired physical route
(chemically inert) to quench the triplet excited state of Cy3, the
most ubiquitous green emissive dye utilized in single-molecule studies
Improving the Photostability of Red- and Green-Emissive Single-Molecule Fluorophores via Ni<sup>2+</sup> Mediated Excited Triplet-State Quenching
Methods
to improve the photostability/photon output of fluorophores
without compromising their signal stability are of paramount importance
in single-molecule fluorescence (SMF) imaging applications. We show
herein that Ni<sup>2+</sup> provides a suitable photostabilizing agent
for three green-emissive (Cy3, ATTO532, Alexa532) and three red-emissive
(Cy5, Alexa647, ATTO647N) fluorophores, four of which are regularly
utilized in SMF studies. Ni<sup>2+</sup> works via photophysical quenching
of the triplet excited state eliminating the potential for reactive
intermediates being formed. Measurements of survival time, average
intensity, and mean number of photons collected for the six fluorophores
show that Ni<sup>2+</sup> increased their photostability 10- to 45-fold,
comparable to photochemically based systems, without compromising
the signal intensity or stability. Comparative studies with existing
photostabilizing strategies enabled us to score different photochemical
and photophysical stabilizing systems, based on their intended application.
The realization that Ni<sup>2+</sup> allowed achieving a significant
increase in photon output both for green- and red-emissive fluorophores
positions Ni<sup>2+</sup> as a widely applicable tool to mitigate
photobleaching, most suitable for multicolor single-molecule fluorescence
studies
How Lipid Unsaturation, Peroxyl Radical Partitioning, and Chromanol Lipophilic Tail Affect the Antioxidant Activity of α-Tocopherol: Direct Visualization via High-Throughput Fluorescence Studies Conducted with Fluorogenic α-Tocopherol Analogues
The preparation of two highly sensitive fluorogenic α-tocopherol
(TOH) analogues which undergo >30-fold fluorescence intensity enhancement
upon reaction with peroxyl radicals is reported. The probes consist
of a chromanol moiety coupled to the <i>meso</i> position
of a BODIPY fluorophore, where the use of a methylene linker (BODIPY-2,2,5,7,8-pentamethyl-6-hydroxy-chroman
adduct, H<sub>2</sub>B-PMHC) vs an ester linker (<i>meso</i>-methanoyl BODIPY-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic
acid, H<sub>2</sub>B-TOH) enables tuning their reactivity toward H-atom
abstraction by peroxyl radicals. The development of a high-throughput
fluorescence assay for monitoring kinetics of peroxyl radical reactions
in liposomes is subsequently described where the evolution of the
fluorescence intensity over time provides a rapid, facile method to
conduct competitive kinetic studies in the presence of TOH and its
analogues. A quantitative treatment is formulated for the temporal
evolution of the intensity in terms of relative rate constants of
H-atom abstraction (<i>k</i><sub>inh</sub>) from the various
tocopherol analogues. Combined, the new probes, the fluorescence assay,
and the data analysis provide a new method to obtain, in a rapid,
parallel format, relative antioxidant activities in phospholipid membranes.
The method is exemplified with four chromanol-based antioxidant compounds
differing in their aliphatic tails (TOH, PMHC, H<sub>2</sub>B-PMHC,
and H<sub>2</sub>B-TOH). Studies were conducted in six different liposome
solutions prepared from poly- and mono-unsaturated and saturated (fluid
vs gel phase) lipids in the presence of either hydrophilic or lipophilic
peroxyl radicals. A number of key insights into the chemistry of the
TOH antioxidants in lipid membranes are provided: (1) The relative
antioxidant activities of chromanols in homogeneous solution, arising
from their inherent chemical reactivity, readily translate to the
microheterogeneous environment at the water/lipid interface; thus
similar values for <i>k</i><sub>inh</sub><sup>H<sub>2</sub>B‑PMHC</sup>/<i>k</i><sub>inh</sub><sup>H<sub>2</sub>B‑TOH</sup> in the range of 2–3 are recorded both in homogeneous solution
and in liposome suspensions with hydrophilic or lipophilic peroxyl
radicals. (2) The relative antioxidant activity between tocopherol
analogues with the same inherent chemical reactivity but bearing short
(PMHC) or long (TOH) aliphatic tails, <i>k</i><sub>inh</sub><sup>PMHC</sup>/<i>k</i><sub>inh</sub><sup>TOH</sup>, is
∼8 in the presence of hydrophilic peroxyl radicals, regardless
of the nature of the lipid membrane into which they are embedded.
(3) Antioxidants embedded in saturated lipids do not efficiently scavenge
hydrophilic peroxyl radicals; under these conditions wastage reactions
among peroxyl radicals become important, and this translates into
larger times for antioxidant consumption. (4) Lipophilic peroxyl radicals
show reduced discrimination between antioxidants bearing long and
short aliphatic tails, with <i>k</i><sub>inh</sub><sup>PMHC</sup>/<i>k</i><sub>inh</sub><sup>TOH</sup> in the range
of 3–4 for most lipid membranes. (5) Lipophilic peroxyl radicals
are scavenged with the same efficiency by all four antioxidants studied,
regardless of the nature of their aliphatic tail or the lipid membrane
into which they are embedded. These data underpin the key role the
lipid environment plays in modulating the rate of reaction of antioxidants
characterized by similar inherent chemical reactivity (arising from
a conserved chromanol moiety) but differing in their membrane mobility
(structural differences in the lipophilic tail). Altogether, a novel,
facile method of study, new insights, and a quantitative understanding
on the critical role of lipid diversity in modulating antioxidant
activity in the lipid milieu are reported
Conformational Changes Spanning Angstroms to Nanometers via a Combined Protein-Induced Fluorescence Enhancement–Förster Resonance Energy Transfer Method
Förster
resonance energy transfer (FRET)-based single-molecule
techniques have revolutionized our understanding of conformational
dynamics in biomolecular systems. Recently, a new single-molecule
technique based on protein-induced fluorescence enhancement (PIFE)
has aided studies in which minimal (<3 nm) displacements occur.
Concerns have been raised regarding whether donor fluorophore intensity
(and correspondingly fluorescence quantum yield Φ<sub>f</sub>) fluctuations, intrinsic to PIFE methods, may adversely affect FRET
studies when retrieving the donor–acceptor dye distance. Here,
we initially show through revisions of Förster’s original
equation that distances may be calculated in FRET experiments regardless
of protein-induced intensity (and Φ<sub>f</sub>) fluctuations
occurring in the donor fluorophore. We additionally demonstrate by
an analysis of the recorded emission intensity and competing decay
pathways that PIFE and FRET methods may be conveniently combined,
providing parallel complementary information in a single experiment.
Single-molecule studies conducted with Cy3- and ATTO647N-labeled RNA
structures and the HCV-NS5B polymerase protein undergoing binding
dynamics along the RNA backbone provide a case study to validate the
results. The analysis behind the proposed method enables for PIFE
and FRET changes to be disentangled when both FRET and PIFE fluctuate
over time following protein arrival and, for example, sliding. A new
method, intensity-FRET, is thus proposed to monitor conformational
changes spanning from angstroms to nanometers
Rate of Lipid Peroxyl Radical Production during Cellular Homeostasis Unraveled via Fluorescence Imaging
Reactive oxygen species (ROS) and
their associated byproducts have
been traditionally associated with a range of pathologies. It is now
believed, however, that at basal levels these molecules also have
a beneficial cellular function in the form of cell signaling and redox
regulation. Critical to elucidating their physiological role is the
opportunity to visualize and quantify the production of ROS with spatiotemporal
accuracy. Armed with a newly developed, extremely sensitive fluorogenic
α-tocopherol analogue (H<sub>4</sub>BPMHC), we report herein
the observation of steady concentrations of lipid peroxyl radicals
produced in live cell imaging conditions. Imaging studies with H<sub>4</sub>BPMHC indicate that the rate of production of lipid peroxyl
radicals in HeLa cells under basal conditions is 33 nM/h within the
cell. Our work further provides indisputable evidence on the antioxidant
role of Vitamin E, as lipid peroxidation was suppressed in HeLa cells
both under basal conditions and in the presence of Haber–Weiss
chemistry, generated by the presence of cumyl hydroperoxide and Cu<sup>2+</sup> in solution, when supplemented with the α-tocopherol
surrogate, PMHC (2,2,5,7,8-pentamethyl-6-hydroxy-chromanol, an α-tocopherol
analogue lacking the phytyl tail). H<sub>4</sub>BPMHC has the sensitivity
needed to detect trace changes in oxidative status within the lipid
membrane, underscoring the opportunity to illuminate the physiological
relevance of lipid peroxyl radical production during cell homeostasis
and disease
Fluorogenic α‑Tocopherol Analogue for Monitoring the Antioxidant Status within the Inner Mitochondrial Membrane of Live Cells
We
report here the preparation of a lipophilic fluorogenic antioxidant
(Mito-Bodipy-TOH) that targets the inner mitochondrial lipid membrane
(IMM) and is sensitive to the presence of lipid peroxyl radicals,
effective chain carriers in the lipid chain autoxidation. Mito-Bodipy-TOH
enables monitoring of the antioxidant status, i.e., the antioxidant
load and ability to prevent lipid chain autoxidation, within the inner
mitochondrial membrane of live cells. The new probe consists of 3
segments: a receptor, a reporter, and a mitochondria-targeting element,
constructed, respectively, from an α-tocopherol-like chromanol
moiety, a BODIPY fluorophore, and a triphenylphosphonium cation (TPP).
The chromanol moiety ensures reactivity akin to that of α-tocopherol,
the most potent naturally occurring lipid soluble antioxidant, while
the BODIPY fluorophore and TPP ensure partitioning within the inner
mitochondrial membrane. Mechanistic studies conducted either in homogeneous
solution or in liposomes and in the presence of free radical initiators
show that the antioxidant activity of Mito-Bodipy-TOH is on par with
that of α-tocopherol. Studies conducted on live fibroblast cells
further show the antioxidant depletion in the presence of methyl viologen
(paraquat), a known agent of oxidative stress and source of superoxide
radical anion (and indirectly, a causative of lipid peroxidation)
within the mitochondria matrix. We recorded a ca. 8-fold emission
enhancement with Mito-Bodipy-TOH in cells stressed with methyl viologen,
whereas no enhancement was observed in control studies with untreated
cells. Our findings underscore the potential of the new fluorogenic
antioxidant Mito-Bodipy-TOH to study the chemical link between antioxidant
load, lipid peroxidation and mitochondrial physiology
Exploiting Conjugated Polyelectrolyte Photophysics toward Monitoring Real-Time Lipid Membrane-Surface Interaction Dynamics at the Single-Particle Level
Herein we report
the real-time observation of the interaction dynamics
between cationic liposomes flowing in solution and a surface-immobilized
charged scaffolding formed by the deposition of conjugated polyanion
polyÂ[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylenevinylene (MPS-PPV)
onto 100-nm-diameter SiO<sub>2</sub> nanoparticles (NPs). Contact
of the freely floating liposomes with the polymer-coated surfaces
led to the formation of supported lipid bilayers (SLBs). The interaction
of the incoming liposomes with MPS-PPV adsorbed on individual SiO<sub>2</sub> nanoparticles promoted the deaggregation of the polymer conformation
and led to large emission intensity enhancements. Single-particle
total internal reflection fluorescence microscopy studies exploited
this phenomenon as a way to monitor the deformation dynamics of liposomes
on surface-immobilized NPs. The MPS-PPV emission enhancement (up to
25-fold) reflected on the extent of membrane contact with the surface
of the NP and was correlated with the size of the incoming liposome.
The time required for the MPS-PPV emission to reach a maximum (ranging
from 400 to 1000 ms) revealed the dynamics of membrane deformation
and was also correlated with the liposome size. Cryo-TEM experiments
complemented these results by yielding a structural view of the process.
Immediately following the mixing of liposomes and NPs the majority
of NPs had one or more adsorbed liposomes, yet the presence of a fully
formed SLB was rare. Prolonged incubation of liposomes and NPs showed
completely formed SLBs on all of the NPs, confirming that the liposomes
eventually ruptured to form SLBs. We foresee that the single-particle
studies we report herein may be readily extended to study membrane
dynamics of other lipids including cellular membranes in live cell
studies and to monitor the formation of polymer-cushioned SLBs
Enhancing the Emissive Properties of Poly(<i>p</i>-phenylenevinylene)-Conjugated Polyelectrolyte-Coated SiO<sub>2</sub> Nanoparticles
Here we describe single-particle imaging studies conducted
on the
conjugated polyelecrolyte polyÂ[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylenevinylene]
(MPS-PPV) supported on SiO<sub>2</sub> nanoparticles. The particles
are subjected to a time-programmed sequence involving addition and
removal of different additives, including excited-triplet-state quenchers
and scavengers of singlet oxygen as well as ground-state oxygen. Our
studies show that these additives enhance the emission intensity and
photostability of the nanoparticles and may further repair photodamaged
conjugated polymer. The ability to monitor the emission from individual
particles along multiple cycles under a range of conditions provides
a mechanistic insight into the action of these additives