Extending single molecule fluorescence observation time by amplitude-modulated excitation

Spectral Characteristics of Water-Soluble Rhodamine Derivatives for Laser-Induced Fluorescence

We present a comprehensive fluorescence characterization of seven water-soluble rhodamine derivatives for applications in laser-induced fluorescence (LIF) techniques. Absorption and emission spectra for these dyes are presented over the visible spectrum of wavelengths (400 to 700 nm). Their fluorescence properties were also investigated as a function of temperature for LIF thermometry applications. Rhodamine 110 depicted the least fluorescence emission sensitivity to temperature at -0.11%/°C, while rhodamine B depicted the most with a -1.55%/°C. We found that the absorption spectra of these molecules are independent of temperature, supporting the notion that the temperature sensitivity of their emission only comes from changes in quantum yield with temperature. Conversely, these rhodamine fluorophores showed no change in emission intensities with pH variations and are, therefore, not suitable tracers for pH measurements. Similarly, fluorescent lifetime, which is also a property sensitive to local environmental changes in temperature, pH, and ion concentration, measurements were conducted for these fluorophores. It was found that rhodamine B and kiton red 620 have shorter fluorescence timescales compared to those of the other five rhodamine dyes, making them least suitable for applications where temporal changes in emission are monitored. Lastly, we conducted experiments to assess the physicochemical absorption characteristics of these dyes' molecules into polydimethylsiloxane (PDMS), the most common material for microfluidic devices. Rhodamine B showed the highest diffusion into PDMS substrates as compared to the other derivative dyes.

Native diffusion of fluorogenic turn-on dyes accurately report interfacial chemical reaction locations

Single-molecule fluorescence microscopy with "turn-on" dyes that change fluorescent state after a reaction report on the chemistry of interfaces relevant to analytical and bioanalytical chemistry. Paramount to accurately understanding the phenomena at the ultimate detection limit of a single molecule is ensuring fluorophore properties such as diffusion do not obscure the chemical reaction of interest. Here, we develop Monte Carlo simulations of a dye that undergoes reduction to turn-on at the cathode of a corroded iron surface taking into account the diffusion of the dye molecules in a total internal reflection fluorescence (TIRF) excitation volume, location of the cathode, and chemical reactions. We find, somewhat counterintuitively, that a fast diffusion coefficient of D = 10⁸ nm²/s, corresponding to the dye in aqueous solution, accurately reports the location of single reaction sites. The dyes turn on and are present for the acquisition of a single frame allowing for localization before diffusing out of the thin TIRF excitation volume axially. Previously turned-on (i.e., activated) dyes can also randomly hit the surface surrounding the reaction site leading to a uniform increase in the background. Using concentrations that lead to high turnover rates at the reaction site can achieve signal-to-background ratios of  ~100 in our simulation. Therefore, the interplay between diffusion, turn-on reaction rate, and concentration of the dye must be strategically considered to produce accurate images of reaction locations. This work demonstrates that modeling can assist in the design of single-molecule microscopy experiments to understand interfaces related to analytical chemistry such as electrode, nanoparticle, and sensor surfaces.

Competitive multicomponent anion exchange adsorption of proteins at the single molecule level

Super-resolution imaging of multicomponent, competitive adsorption demonstrates that competitors block certain ligands from the analyte without changing analyte adsorption kinetics.

S. Indrasekara AS, Chen S, Wang LY, Derry PJ, Liopo A, Zubarev ER, Landes CF, Link S. Adsorption and Unfolding of a Single Protein Triggers Nanoparticle Aggregation

The response of living systems to nanoparticles is thought to depend on the protein corona, which forms shortly after exposure to physiological fluids and which is linked to a wide array of pathophysiologies. A mechanistic understanding of the dynamic interaction between proteins and nanoparticles and thus the biological fate of nanoparticles and associated proteins is, however, often missing mainly due to the inadequacies in current ensemble experimental approaches. Through the application of a variety of single molecule and single particle spectroscopic techniques in combination with ensemble level characterization tools, we identified different interaction pathways between gold nanorods and bovine serum albumin depending on the protein concentration. Overall, we found that local changes in protein concentration influence everything from cancer cell uptake to nanoparticle stability and even protein secondary structure. We envision that our findings and methods will lead to strategies to control the associated pathophysiology of nanoparticle exposure in vivo.

Characterization of Porous Materials by Fluorescence Correlation Spectroscopy Super-resolution Optical Fluctuation Imaging

Porous materials such as cellular cytosol, hydrogels, and block copolymers have nanoscale features that determine macroscale properties. Characterizing the structure of nanopores is difficult with current techniques due to imaging, sample preparation, and computational challenges. We produce a super-resolution optical image that simultaneously characterizes the nanometer dimensions of and diffusion dynamics within porous structures by correlating stochastic fluctuations from diffusing fluorescent probes in the pores of the sample, dubbed here as "fluorescence correlation spectroscopy super-resolution optical fluctuation imaging" or "fcsSOFI". Simulations demonstrate that structural features and diffusion properties can be accurately obtained at sub-diffraction-limited resolution. We apply our technique to image agarose hydrogels and aqueous lyotropic liquid crystal gels. The heterogeneous pore resolution is improved by up to a factor of 2, and diffusion coefficients are accurately obtained through our method compared to diffraction-limited fluorescence imaging and single-particle tracking. Moreover, fcsSOFI allows for rapid and high-throughput characterization of porous materials. fcsSOFI could be applied to soft porous environments such hydrogels, polymers, and membranes in addition to hard materials such as zeolites and mesoporous silica.

Ensemble and single-molecule biophysical characterization of D17.4 DNA aptamer-IgE interactions

BACKGROUND

The IgE-binding DNA aptamer 17.4 is known to inhibit the interaction of IgE with the high-affinity IgE Fc receptor FcεRI. While this and other aptamers have been widely used and studied, there has been relatively little investigation of the kinetics and energetics of their interactions with their targets, by either single-molecule or ensemble methods.

METHODS

The dissociation kinetics of the D17.4/IgE complex and the effects of temperature and ionic strength were studied using fluorescence anisotropy and single-molecule spectroscopy, and activation parameters calculated.

RESULTS

The dissociation of D17.4/IgE complex showed a strong dependence on temperature and salt concentration. The koff of D17.4/IgE complex was calculated to be (2.92±0.18)×10(-3) s(-1) at 50 mM NaCl, and (1.44±0.02)×10(-2) s(-1) at 300 mM NaCl, both in 1 mM MgCl2 and 25°C. The dissociation activation energy for the D17.4/IgE complex, Ea, was 16.0±1.9 kcal mol(-1) at 50 mM NaCl and 1 mM MgCl2. Interestingly, we found that the C19A mutant of D17.4 with stabilized stem structure showed slower dissociation kinetics compared to D17.4. Single-molecule observations of surface-immobilized D17.4/IgE showed much faster dissociation kinetics, and heterogeneity not observable by ensemble techniques.

CONCLUSIONS

The increasing koff value with increasing salt concentration is attributed to the electrostatic interactions between D17.4/IgE. We found that both the changes in activation enthalpy and activation entropy are insignificant with increasing NaCl concentration. The slower dissociation of the mutant C19A/IgE complex is likely due to the enhanced stability of the aptamer.

GENERAL SIGNIFICANCE

The activation parameters obtained by applying transition state analysis to kinetic data can provide details on mechanisms of molecular recognition and have applications in drug design. Single-molecule dissociation kinetics showed greater kinetic complexity than was observed in the ensemble in-solution systems, potentially reflecting conformational heterogeneity of the aptamer. This article is part of a Special Issue entitled: Physiological Enzymology and Protein Functions.

Charge-dependent transport switching of single molecular ions in a weak polyelectrolyte multilayer

The tunable nature of weak polyelectrolyte multilayers makes them ideal candidates for drug loading and delivery, water filtration, and separations, yet the lateral transport of charged molecules in these systems remains largely unexplored at the single molecule level. We report the direct measurement of the charge-dependent, pH-tunable, multimodal interaction of single charged molecules with a weak polyelectrolyte multilayer thin film, a 10 bilayer film of poly(acrylic acid) and poly(allylamine hydrochloride) PAA/PAH. Using fluorescence microscopy and single-molecule tracking, two modes of interaction were detected: (1) adsorption, characterized by the molecule remaining immobilized in a subresolution region and (2) diffusion trajectories characteristic of hopping (D ∼ 10(-9) cm(2)/s). Radius of gyration evolution analysis and comparison with simulated trajectories confirmed the coexistence of the two transport modes in the same single molecule trajectories. A mechanistic explanation for the probe and condition mediated dynamics is proposed based on a combination of electrostatics and a reversible, pH-induced alteration of the nanoscopic structure of the film. Our results are in good agreement with ensemble studies conducted on similar films, confirm a previously-unobserved hopping mechanism for charged molecules in polyelectrolyte multilayers, and demonstrate that single molecule spectroscopy can offer mechanistic insight into the role of electrostatics and nanoscale tunability of transport in weak polyelectrolyte multilayers.

High ionic strength narrows the population of sites participating in protein ion-exchange adsorption: a single-molecule study

The retention and elution of proteins in ion-exchange chromatography is routinely controlled by adjusting the mobile phase salt concentration. It has repeatedly been observed, as judged from adsorption isotherms, that the apparent heterogeneity of adsorption is lower at more-eluting, higher ionic strength. Here, we present an investigation into the mechanism of this phenomenon using a single-molecule, super-resolution imaging technique called motion-blur Points Accumulation for Imaging in Nanoscale Topography (mbPAINT). We observed that the number of functional adsorption sites was smaller at high ionic strength and that these sites had reduced desorption kinetic heterogeneity, and thus narrower predicted elution profiles, for the anion-exchange adsorption of α-lactalbumin on an agarose-supported, clustered-charge ligand stationary phase. Explanations for the narrowing of the functional population such as inter-protein interactions and protein or support structural changes were investigated through kinetic analysis, circular dichroism spectroscopy, and microscopy of agarose microbeads, respectively. The results suggest the reduction of heterogeneity is due to both electrostatic screening between the protein and ligand and tuning the steric availability within the agarose support. Overall, we have shown that single molecule spectroscopy can aid in understanding the influence of ionic strength on the population of functional adsorbent sites participating in the ion-exchange chromatographic separation of proteins.

Troika of single particle tracking programing: SNR enhancement, particle identification, and mapping

Single particle tracking (SPT) techniques provide a microscopic approach to probe in vivo and in vitro structure and reactions. Automatic analysis of SPT data with high efficiency and accuracy spurs the development of SPT algorithms. In this perspective, we review a range of available techniques used in SPT analysis programs. In addition, we present an example SPT program step-by-step to provide a guide so that researchers can use, modify, and/or write a SPT program for their own purposes.