Anisotropy Decay Research Articles

Orientational distribution of DPH in lipid membranes: a comparison of molecular dynamics calculations and experimental time-resolved anisotropy experiments.

Characterization of the membrane phases is a crucial task in cell biology. Cells differ in composition of the lipids and consequently in adopted phases. The phases can be discriminated based upon lipid ordering and molecular diffusion and their identification could be used for characterization of cell membranes. Here we used molecular dynamics (MD) simulations to study the behavior of the fluorescent reporter molecule diphenylhexatriene (DPH) in different lipid phases - liquid disordered (Ld), liquid ordered (Lo), and solid ordered (So) composed of phosphatidylcholines (Ld and So) or a sphingomyelin/cholesterol (SM/Chol) mixture (Lo). To the best of our knowledge, this is the first simulation of DPH in Lo SM/Chol and So DPPC membranes. For the considered membrane compositions DPH is mostly oriented parallel to lipid tails. In the Lo phase we observed a significant fraction of DPH positioned in between membrane leaflets, which agrees with experimental findings, but which has not been observed in previous MD simulations of DPH in phosphatidylcholine membranes. Further, we calculated rotational autocorrelation functions (ROTACF) from our MD simulations in order to model the time-resolved fluorescence anisotropy decay. We observed that order parameters P2 and P4 are sufficient to fully describe the orientation distribution of DPH. We analyzed the ROTACFs by a so-called general model for the time-resolved fluorescence anisotropy [W. van der Meer et al., Biophys. J., 1984, 46, 515] and observed an overestimation of P4. We suggest a rescaling of the recovered P4 yielding an orientation distribution of DPH close to the one observed in our MD simulations.

Comment on ""Where does the fluorescing moiety reside in a carbon dot?"- Investigations based on fluorescence anisotropy decay and resonance energy transfer dynamics" by A. Das, D. Roy, C. K. De and P. K. Mandal, Phys. Chem. Chem. Phys., 2018, 20, 2251.

In a recent paper published in Physical Chemistry Chemical Physics (Phys. Chem. Chem. Phys., 2018, 20, 2251-2259), Förster resonance energy transfer (FRET) between carbon dots and rhodamine 123 has been reported. The FRET rates and hence donor acceptor distances estimated from acceptor rise time for various acceptor concentrations appear to be incorrect. In the present comment need for a correct analysis in the case of such a scenario is addressed.

Reply to the 'Comment on ""Where does the fluorescing moiety reside in a carbon dot?"- Investigations based on fluorescence anisotropy decay and resonance energy transfer dynamics"' by H. C. Joshi, Phys. Chem. Chem. Phys., 2019, 21, DOI: 10.1039/c9cp00136k.

The claim that the analysis regarding resonance energy transfer should have been made using different equations than those that we have used is negated based on the following points: (1) we are well aware of the equations the author has provided in his comment. The equation (eqn (3) mentioned below) that the author has written is undoubtedly too simple to describe the complex system delineated in our original paper. This particular equation is perhaps OK for simple dye (donor and acceptor) systems; however, such a simple equation is never enough for nanoparticle/quantum dot systems. (2) Another equation suggested by the author in his comment (eqn (2)) contains a parameter called donor concentration in excited state. We have categorically described in page 6-7 of our original paper why it is difficult to measure the donor concentration accurately even in the ground state. When the donor concentration can't be known accurately it can't be used in the suggested equation. (3) Donor-acceptor distance calculated by eqn (3)/Table 1 provided by the author deviates more than 100% from the distance that is physically feasible. Such kinds of problems are well documented in the literature. (4) One of the papers cited by the author in his comment and many other published papers clearly mention that in the case when all donor molecules/particles do not take part in the resonance energy transfer process or the stoichiometry of a donor-acceptor complex is not known or deviates strongly from 1 : 1, especially in quantum dots or any other nanomaterial system, it is not possible to extract accurate dynamical information related to RET from donor decay. Instead risetime of acceptor yields much more accurate information. Such situations do arise in our system as well.

Ion-Molecule Complex Dissociation and Formation Dynamics in LiCl Aqueous Solutions from 2D IR Spectroscopy.

Ion-molecule complex dynamics as well as water dynamics in concentrated lithium chloride (LiCl) solutions are examined using ultrafast two-dimensional infrared (2D IR) spectroscopy with the CN stretching mode of methyl thiocyanate (MeSCN) as the vibrational probe. In pure water, MeSCN has a narrow symmetric absorption line shape. 2D IR spectral diffusion measurements of the CN stretch give the identical time dependence of water dynamics, as previously observed using the OD stretch of HOD in H2O. In concentrated LiCl solutions, the IR absorption spectrum of MeSCN displays two distinct peaks, one corresponding to water H-bonded to the N lone pair of MeSCN (W) and the other corresponding to Li+ associated with the N (L). These two species are in equilibrium, and switching of the CN bonding partner from Li+ to H2O and vice versa was observed and explicated with 2D IR chemical exchange spectroscopy. The MeSCN·Li+ complex dissociation time constant, τLW, and the MeSCN·H2O dissociation time constant, τWL, were determined. The observed τLW chemical exchange dissociation time constant changes from 60 to 40 ps as the LiCl concentration decreases from ∼10.7 to ∼7.7 M, mainly due to the increase of the water concentration as the LiCl concentration is reduced. The observed time constants are independent of the model for the chemical reaction. With the assumption of a simple chemical equation, MeSCN·Li+ + H2O ⇄ MeSCN·H2O + Li+, the equilibrium equation rate constants were obtained from the observed chemical exchange time constants. It was determined that the equilibrium rate constants barely change even though the viscosity changes by a factor of 2 and the ionic strength changes by a factor of 1.4. Extrapolation to dilute LiCl solution estimates the τLW to be ∼30 ps. The orientational relaxation (anisotropy decay) of both the W and L complexes was measured using polarization selective 2D IR experiments. The lithium-bonded species undergoes orientational relaxation ∼3 times slower than the water-bonded species in each LiCl solution studied. The difference demonstrates the distinct interactions with the medium experienced by the neutral and charged species in the concentrated salt solutions.

Hadronic Paschen–Back effect

We find a novel phenomenon induced by the interplay between a strong magnetic field and finite orbital angular momenta in hadronic systems, which is analogous to the Paschen–Back effect observed in the field of atomic physics. This effect allows the wave functions to drastically deform. We discuss anisotropic decay from the deformation as a possibility to measure the strength of the magnetic field in high-energy heavy-ion collisions, which has not been measured experimentally. As an example we investigate charmonia with a finite orbital angular momentum in a strong magnetic field. We calculate the mass spectra and mixing ratios. To obtain anisotropic wave functions, we apply the cylindrical Gaussian expansion method. There we use different extension parameters for the parallel and transverse directions to the magnetic field.

Spontaneous Exciton Valley Coherence in Transition Metal Dichalcogenide Monolayers Interfaced with an Anisotropic Metasurface.

The control of the exciton intervalley coherence renders transition metal dichalcogenides monolayers promising candidates for quantum information science. So far, generating intervalley coherence has the need for an external coherent field. Here, we theoretically demonstrate spontaneous generation (i.e., without any external field) of exciton intervalley coherence. We achieve this by manipulating the vacuum field in the vicinity of the monolayer with a designed polarization-dependent metasurface, inducing an anisotropic decay rate for in-plane excitonic dipoles. Harnessing quantum coherence and interference effects in two-dimensional materials may provide the route for novel quantum valleytronic devices.

Mapping lattice-induced plasmon modes in metallic nanoantenna arrays using fluorescence decay of semiconductor quantum dot bioconjugates

We use the fluorescence decay of quantum dot bioconjugates and their size distribution as near field probes to investigate lattice-induced plasmonic resonances and their interaction with excitons. The quantum dots are biologically immobilized on gold nanoantenna arrays while their spontaneous emission decay rates are spectrally resolved. Our results show that the periodic arrangement of the nanoantennas can generate or optically activate certain plasmonic resonances, different from their intrinsic plasmon modes. This includes formation of a hybrid photonic-plasmonic dipole mode that can strongly couple with the quantum dots, making their decay rates anisotropic and wavelength dependent. The anisotropic decay is decided by the shapes and sizes of the nanoantennas and by the direction of the quantum dot dipoles. The results suggest lattice-induced plasmonic resonances can be used as engineered states to drive transfer of energy between quantum dots at certain wavelengths.

Viscoelastic and Dielectric Relaxation of Reptating Type-A Chains Affected by Reversible Head-to-Head Association and Dissociation

For entangled linear polymer having type A dipoles and undergoing head-to-head association and dissociation reaction, viscoelastic and dielectric behavior is theoretically analyzed on the basis of the reptation dynamics combined with the reaction kinetics. Specifically, for the dissociated unimer and associated dimer (indexed with j = 1 and 2, respectively), the normalized complex modulus gj*(ω) and the normalized complex dielectric permittivity ej*(ω) are analytically calculated via eigenfunction expansion of the orientational anisotropy and orientational memory defined in terms of the bond vectors u of entanglement segments. The reaction activates mutual conformational transfer between the unimer and dimer. Multiple coupling occurs for the anisotropy decay modes of the unimer and dimer due to this transfer, and the viscoelastic g1* and g2* of the unimer and dimer, respectively, exhibit considerably retarded and accelerated relaxation compared to the pure reptation case. In contrast, the memory decay mo...

(Invited) Ultrafast Generation and Characterization of Hot Carriers in Plasmonic Nanostructures for Solar Light Harvesting

Photoexcitation of plasmonic materials can create energetic electrons and holes that are of strong interest for solar energy conversion, photocatalysis, and light harvesting applications. Since intraband photoexcitation can create energetic (other terminology includes “hot” or “nonthermal”) carriers that are up to the photon excitation energy above the Fermi energy, their potential for increasing the efficiency of optical energy conversion is clear. However, in practice there are several challenges to realizing the potential of hot carriers. First, the efficiency of hot carrier generation is low in many plasmonic materials. Second, the energetic carriers generally thermalize very quickly, on the order of 10-100 femtoseconds, thereby making their extraction before energy loss difficult. Third, the energetic carriers tend to produce small changes in the permittivity of the metal that require advanced spectroscopic experimentation or devices in order to characterize them. In this talk I begin with describing the principles of hot carrier generation and the large advantages of using nanostructured metals to improve their generation efficiency. This is followed by our efforts to spectroscopically characterize hot electrons on an ultrafast timescale, as well as to design new nanostructures for creating hot electrons in greater numbers. Physical insights into how to extract the hot carriers before loss of energy to thermalization is given, and our efforts to understand the anisotropic decay of hot carriers is described. Recent efforts to directly extract the energetic distribution of hot carriers as a function of time are also discussed. I further describe recent experiments on specific photocatalytic processes and new biomimetic nanostructures for light harvesting. These biomimetic structures enable unique opportunities for tuning structure and energy transport properties. This work focuses on the use of chromophore-doped peptide amphiphiles to create varied and potentially self-healing structures for transporting energy. A closing description of the research opportunities and capabilities at the Center for Nanoscale Materials user facility is also given. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, and supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.

Resolving Mixtures in Solution by Single-Molecule Rotational Diffusivity.

Sensing the size of individual molecules in an ensemble has proven to be a powerful tool to investigate biomolecular interactions and association-dissociation processes. In biologically relevant solution environments, molecular size is often sensed by translational or rotational diffusivity. The rotational diffusivity is more sensitive to the size and conformation of the molecules as it is inversely proportional to the cube of the hydrodynamic radius, as opposed to the inverse linear dependence of the translational diffusion coefficient. Single-molecule rotational diffusivity has been measured with time-resolved fluorescence anisotropy decay, but the ability to sense different sizes has been restricted by the limited number of photons available or has required surface attachment to observe each molecule longer, and the attachment may be perturbative. To address these limitations, we show how to measure and monitor single-molecule rotational diffusivity by combining the solution-phase Anti-Brownian ELectrokinetic (ABEL) trap and maximum likelihood analysis of time-resolved fluorescence anisotropy based on the information inherent in each detected photon. We demonstrate this approach by resolving a mixture of single- and double-stranded fluorescently labeled DNA molecules at equilibrium, freely rotating in a native solution environment. The rotational diffusivity, fluorescence brightness and lifetime, and initial and steady-state anisotropy are simultaneously determined for each trapped single DNA molecule. The time resolution and precision of this method are analyzed using statistical signal analysis and simulations. We present key parameters that define the usefulness of a particular fluorescent label for extracting molecular size information from single-molecule rotational diffusivity measurements.

Fine Structure of Beta Decay Strength Function and Anisotropy of Isovector Nuclear Dencity Component Oscillations in Deformed Nuclei

The experimental measurement data on the fine structure of beta-decay strength function Sβ(E) in spherical, transitional, and deformed nuclei are analyzed. Modern high-resolution nuclear spectroscopy methods made it possible to identify the splitting of peaks in Sβ(E) for deformed nuclei. By analogy with splitting of the peak of E1 giant dipole resonance (GDR) in deformed nuclei, the peaks in Sβ(E) are split into two components from the axial nuclear deformation. In this report, the fine structure of Sβ(E) is discussed. Splitting of the peaks connected with the oscillations of neutrons against protons (E1GDR), of proton holes against neutrons (peaks in Sβ(E) of β+/EC-decay), and of protons against neutron holes (peaks in Sβ(E) of β–-decay) is discussed.

Lq‐estimates for the stationary Oseen equations on the exterior of a rotating obstacle

We study the Dirichlet problem for the stationary Oseen equations around a rotating body in an exterior domain. Our main results are the existence and uniqueness of weak and very weak solutions satisfying appropriate Lq‐estimates. The uniqueness of very weak solutions is shown by the method of cut‐off functions with an anisotropic decay. Then our existence result for very weak solutions is deduced by a duality argument from the existence and estimates of strong solutions. From this and interior regularity of very weak solutions, we finally establish the complete D1,r‐result for weak solutions of the Oseen equations around a rotating body in an exterior domain, where 4/3<r <4. Here, D1,r is the homogeneous Sobolev space.

Luminescent mesoporous silica nanoparticles for biomedical applications: Synthesis and characterization

We report the synthesis and detailed characterization of intrinsically/lable-free luminescent mesoporous silica nanoparticles (L-MSN), which may have useful biomedical applications. These particles were prepared by modified Stober's method followed by calcination. By optimizing the ratio of (3-aminopropyl triethoxysilane) APTS to tetraetthoxysaline (TEOS), particles with size ~ 43 nm and size dispersion ~ 12% were obtained. The luminescence of L-MSN is suggested to be due to the formation of carbonaceous compounds, giving particles yellowish brown color, during the process of calcination at optimum temperatures ~ 400 °C. Interestingly, our studies suggest that these carbonaceous impurities are carbon dots (C-Dots) and fluorescence anisotropy decay measurements indicate C-Dots as integral part of L-MSN. Luminescent properties of L-MSN were found to be stable as a function of pH as well as long time light exposure. Our results suggest that L-MSN may be explored for biomedical imaging as well as drug delivery applications.

Supramolecular host-guest interaction of antibiotic drug ciprofloxacin with cucurbit[7]uril macrocycle: Modulations in photophysical properties and enhanced photostability

Supramolecularly assisted modulations in the photophysical properties of the well known and practised antibiotic drug ciprofloxacin (Cpf) with versatile macrocyclic cavitand molecule cucurbit[7]uril (CB7) has been explored under different pH conditions. Both the cationic (Cpf-H+) and neutral (Cpf) forms of the drug exhibits strong binding interaction with CB7. Ground state absorption, steady-state fluorescence, time-resolved emission and fluorescence anisotropy decay measurements documented the host assisted intriguing changes in the photophysical properties of the drug, occurred through the successive formations of the 1:1 and 1:2 (drug to host) inclusion complexes, and thereby causing the intriguing small initial enhancement in fluorescence intensity at lower host concentrations and subsequently leading to a strong fluorescence quenching at higher CB7 concentration. Inferences drawn from the photophysical studies are nicely supported by the results obtained from the 1H NMR measurements. Investigations on the photodegradation studies involving the Cpf-H+ and Cpf forms of the drug, both in the absence and in the presence of CB7 host, revealed that there is a large enhancement in the photostability for both the prototropic forms of the drug, by about 3 times, on their inclusion complex formation with the CB7 host. Observed results from the supramolecular interactions of the ciprofloxacin drug with CB7 macrocycle are suggested to have implications in improving drug stability and in the possible deployments in drug formulations and drug delivery systems.

Pre-micellar interaction or direct monomer to micelle transition for zwitterionic sulfobetaine surfactant in water? A comparative fluorescence study with cationic surfactant

Interfacial properties of zwitterionic sulfobetaine surfactant (e.g. SB-12, SB-16) micelles are remarkably similar with those of cationic alkyl-ammonium surfactant (e.g. CTAB, DTAB) micelles and both interact strongly with an anionic fluorophore 8-hydroxypyrene-1,3,6-trisulphonate (HPTS). Does the similarity of their interaction extents into the pre-micellar region? At low concentrations (below the critical micelle concentration or cmc) of the cationic surfactants, fluorescence intensity of HPTS strongly quenches indicating HPTS-surfactant association. However, no net fluorescence quenching was observed for the sulfobetaine surfactants at any concentration. HPTS concentration was varied from 0.8 μM to 80 μM to assess any possibility of HPTS-induced pre-micellar association. The ratio of the protonated and deprotonated emission intensities varies quite differently with the concentrations of sulfobetaine surfactant, particularly, at the micellar transition region, for different initial concentrations of HPTS. This HPTS concentration-dependent behavior apparently indicates HPTS-surfactant association below cmc, however, the observed changes can be satisfactorily justified by micelle-water partitioning of HPTS without invoking pre-micellar interactions. Furthermore, fluorescence anisotropy measurements of MPTS (methyl derivative of HPTS) revealed no significant change in the fluorescence anisotropy decay below cmc, supporting the absence of pre-micellar interaction. This study along with our previous work may provide new insights on the general response of an anion towards zwitterionic surfactants at different concentration ranges.

Continuous Fluorescence Depletion Anisotropy Measurement of Protein Rotation.

Protein rotation in viscous environments can be measured by fluorescence depletion anisotropy (FDA) which combines long lifetimes of chromophore triplet states with the sensitivity of fluorescence excitation and detection. FDA achieves sensitivity well beyond that attainable by the more common technique of time-resolved phosphorescence anisotropy (TPA). We have now combined benefits of both time-domain and frequency-domain FDA into a single continuous technique (CFDA). Intensity and polarization of a single laser beam are modulated continuously according to a complex, repeating waveform. Fluorescence signals excited from triplet-forming fluorescent probes are digitized over recurring waveform periods by a high-speed signal averager. CFDA experiments typically involve substantial ground state depletion. Thus signals, unlike those of TPA, are not linear in the exciting light intensity and simple data analysis based on such linearity is not appropriate. An exact solution of the coupled diffusion and triplet production/decay equation describing CFDA within individual data points has been combined with simulated annealing optimization to extract triplet and anisotropy decay kinetics from experimental data. Related calculations compare possible excitation waveforms with respect to rotational information provided per fluorescence photon. We present CFDA results for the model system of eosin conjugates of carbonic anhydrase, BSA and immunoglobulin G in 90% glycerol at various temperatures and initial cellular results on eosin-IgE bound to 2H3 cell Type I Fcε receptors. We explore how CFDA reflects rotational parameters of heterogeneous systems and discuss challenges of extending this method to single cell microscopic measurements.

Mehanisms in Cancer Signaling: The Role of the Membrane in the Recruitment of the Oncogene KRas4b

Ras proteins are a key element of signal transduction where extracellular growth factors control nuclear transcription events involved in cell division, proliferation, and apoptosis. Ras activity is controlled, and signaling mediated by, critical protein-protein interactions. GTPase activating proteins (GAPs), such as p120, bind to activated Ras, dramatically increasing the rate of GTP hydrolysis thus returning the system to the inactive GDP bound state. Guanine exchange factors (GEFs), such as Son of Sevenless (SOS), bind and effect the exchange of GDP for GTP, thus turning “on” KRas4b. Most importantly, these multi-protein complexes all operate on a membrane surface, which is a critical partner in signaling. Despite this critical role of the membrane, there is incomplete knowledge as to the role of the bilayer composition in anchoring the protein to the membrane and the importance of specific lipid type in dictating the final orientation of KRas4b on the surface. Using a fully post-translationally modified KRAS4b provided by the Frederick National Laboratory, we investigated the role of specific lipids in the recruitment of KRas4b to a Nanodisc membrane surface of defined composition. Application of a single frequency fluorescence anisotropy decay experiment to this system revealed that KRas4b has a significant binding preference for Nanodisc bilayers containing PIP2. We conducted molecular dynamics simulations to look for an origin of this specificity. In the case of membranes containing PIP2 the protein formed long-lived salt bridges with PIP2 head groups but not the monovalent DMPS, explaining the experimentally observed lipid specificity. Additionally, we report that PIP2 forms key contacts with Helix-4 on the catalytic domain of KRas4b that orient the protein in a manner expected to facilitate association with upstream and downstream signaling partners.

Probing the Conformational Dynamics of the Disordered 4E-BP2 Protein in Different Phosphorylation States using Single-Molecule Fluorescence

Intrinsically disordered proteins (IDPs) are a class of proteins that lack a well-defined 3D structure while carrying out a diverse range of biological functions. They play a crucial role in regulatory pathways and in mediating interactions with multiple partners. Cap-dependent initiation of translation is regulated by the interaction of 120-residue eukaryotic initiation factor 4E (eIF4E) with disordered eIF4E binding proteins (4E-BPs) in a phosphorylation-dependent manner. Fluorescence correlation spectroscopy (FCS) and fluorescence anisotropy decay (FAD) were used to study the conformations, dynamics and binding of 4E-BP2. Anisotropy data informed the local chain flexibility at various points within the 4E-BP2. The segmental flexibility is hindered in the folded 18-62 region upon phosphorylation, whereas the rest of the chain becomes more flexible. The local segments become more flexible upon denaturation, which suggests that the native protein, whereas not having a stable 3D fold, contains significant secondary structure. Segmental rotational correlation times and wobbling cone angles extracted for different labelling sites along the chain provide a rigidity map of 4E-BP2 and are essential for elucidating the binding mode to eIF4E. Longer range intra-chain reorganization dynamics were assessed by FCS via photoblinking/quenching of the fluorophore by aromatic residues. Heterogeneous quenching kinetics were observed: multi-site phosphorylation of the protein slows the proximal chain motions and modulates the kinetics of the distal regions. Our results paint a complex behavior of 4E-BP2 upon phosphorylation and binding and suggest that electrostatics play a crucial role in modulating its dimensions and compactness and thus its activity.

Rotational Dynamics of Solutes with Multiple Single Bond Axes Studied by Infrared Pump-Probe Spectroscopy.

To investigate the relationship between the structural degrees of freedom around a vibrational probe and the rotational relaxation process of a solute in solution, we studied the anisotropy decays of three different N3-derivatized amino acids in primary alcohol solutions. By performing polarization-controlled IR pump-probe measurements, we reveal that the anisotropy decays of the vibrational probe molecules in 1-alcohol solutions possess two decay components, at subpicosecond and picosecond time scales. On the basis of results showing that the fast relaxation component is insensitive to the vibrational probe molecule, we suggest that the anisotropy decay of the N3 group on a subpicosecond time scale results from a local, small-amplitude fluctuation of the flexible vibrational probe, which does not depend on the details of its molecular structure. However, the slow relaxation component depends on the solute: with longer alkyl chains attached to the N3 group, the anisotropy decay of the slow component is faster. Consequently, we conclude that the slow relaxation component corresponds to the reorientational motion of the N3 group correlated with other intramolecular rotational motions (e.g., rotational motions of the neighboring alkyl chain). Our experimental results provide important insight into understanding the rotational dynamics of solutes with multiple single bond axes in solution.

Nanoantenna-Enhanced Radiative and Anisotropic Decay Rates in Monolayer-Quantum Dots

Nanoantenna-enhanced ultrafast emission from colloidal quantum dots as quantum emitters is required for fast quantum communications. On-chip integration of such devices requires a scalable and high-throughput technology. We report self-assembly lithography technique of preparing hybrid of gold nanorods antenna over a compact CdSe quantum dot monolayer. We demonstrate resonant and nonresonant gold nanorod antenna-enhanced radiative and anisotropic decay. Extensive simulations explain the mechanism of the decay rates and the role of antenna in both random and compact monolayers of quantum dots. The study could find applications in quantum dot display and quantum communication devices.