Probing the dynamics of planar supported membranes by Nile red fluorescence lifetime distribution

Spectral Phasor Applied to Spectrally-Resolved Single Molecule Localization Microscopy

Spectrally-resolved single-molecule localization microscopy (srSMLM) has emerged as a powerful tool for exploring the spectral properties of single emitters in localization microscopy. By simultaneously capturing the spatial positions and spectroscopic signatures of individual fluorescent molecules, srSMLM opens up the possibility of investigating an additional dimension in super-resolution imaging. However, appropriate and dedicated tools are required to fully capitalize on the spectral dimension. Here, we propose the application of the spectral phasor analysis as an effective method for summarizing and analyzing the spectral information obtained from srSMLM experiments. The spectral phasor condenses the complete spectrum of a single emitter into a two-dimensional space, preserving key spectral characteristics for single-molecule spectral exploration. We demonstrate the effectiveness of spectral phasor in efficiently classifying single Nile Red fluorescence emissions from largely overlapping cyanine fluorescence signals in dual-color PAINT experiments. Additionally, we employed spectral phasor with srSMLM to reveal subtle alterations occurring in the membrane of Gram-positive Enterococcus hirae in response to gramicidin exposure, a membrane-perturbing antibiotic treatment. Spectral phasor provides a robust, model-free analytic tool for the detailed analysis of the spectral component of srSMLM, enhancing the capabilities of multi-color spectrally-resolved single-molecule imaging.

Live-cell imaging of the chloroplast outer envelope membrane using fluorescent dyes

Chloroplasts are organelles composed of sub-organellar compartments-stroma, thylakoids, and starch granules-and are surrounded by outer and inner envelope membranes (OEM and IEM, respectively). The chloroplast OEM and IEM play key roles not only as a barrier separating the chloroplast components from the cytosol but also in the interchange of numerous metabolites and proteins between the chloroplast interior and the cytosol. Fluorescent protein markers for the chloroplast OEM have been widely used to visualize the outermost border of chloroplasts. However, the use of marker proteins requires an established cellular genetic transformation method, which limits the plant species in which marker proteins can be used. Moreover, the high accumulation of OEM marker proteins often elicits abnormal morphological phenotypes of the OEM. Because the OEM can currently only be visualized using exogenous marker proteins, the behaviors of the chloroplast and/or its OEM remain unknown in wild-type cells of various plant species. Here, we visualized the OEM using live-cell staining with the fluorescent dyes rhodamine B and Nile red in several plant species, including crops. We propose rhodamine B and Nile red as new tools for visualizing the chloroplast OEM in living plant cells that do not require genetic transformation.

SIGNIFICANCE STATEMENT

We established a live-cell imaging method to visualize the chloroplast outer envelope membrane by staining living cells with fluorescent dyes. This method does not require genetic transformation and allows the observation of the chloroplast outer envelope membrane in various plant species.

Unveiling a Hidden Event in Fluorescence Correlative Microscopy by AFM Nanomechanical Analysis

Fluorescent imaging combined with atomic force microscopy (AFM), namely AFM-fluorescence correlative microscopy, is a popular technology in life science. However, the influence of involved fluorophores on obtained mechanical information is normally underestimated, and such subtle changes are still challenging to detect. Herein, we combined AFM with laser light excitation to perform a mechanical quantitative analysis of a model membrane system labeled with a commonly used fluorophore. Mechanical quantification was additionally validated by finite element simulations. Upon staining, we noticed fluorophores forming a diffuse weakly organized overlayer on phospholipid supported membrane, easily detected by AFM mechanics. The laser was found to cause a degradation of mechanical stability of the membrane synergically with presence of fluorophore. In particular, a 30 min laser irradiation, with intensity similar to that in typical confocal scanning microscopy experiment, was found to result in a ∼40% decrease in the breakthrough force of the stained phospholipid bilayer along with a ∼30% reduction in its apparent elastic modulus. The findings highlight the significance of analytical power provided by AFM, which will allow us to “see” the “unseen” in correlative microscopy, as well as the necessity to consider photothermal effects when using fluorescent dyes to investigate, for example, the deformability and permeability of phospholipid membranes.

Fluorescence microscopy of the pressure-dependent structure of lipid bilayers suspended across conical nanopores

Glass and fused-quartz nanopore membranes containing a single conically shaped pore are promising solid supports for lipid bilayer ion-channel recordings due to the high inherent stability of lipid bilayers suspended across the nanopore orifice, as well as the favorable electrical properties of glass and fused quartz. Fluorescence microscopy is used here to investigate the structure of the suspended lipid bilayer as a function of the pressure applied across a fused-quartz nanopore membrane. When a positive pressure is applied across the bilayer, from the nanopore interior relative to the exterior bulk solution, insertion or reconstitution of operative ion channels (e.g., α-hemolysin (α-HL) and gramicidin) in the bilayer is observed; conversely, reversing the direction of the applied pressure results in loss of all channel activity, although the bilayer remains intact. The dependence of the bilayer structure on pressure was explored by imaging the fluorescence intensity from Nile red dye doped into suspended 1,2-diphytanoyl-sn-glycero-3-phosphocholine bilayers, while simultaneously recording the activity of an α-HL channel. The fluorescence images suggest that a positive pressure results in compression of the bilayer leaflets and an increase in the bilayer curvature, making it suitable for ion-channel formation and activity. At negative pressure, the fluorescence images are consistent with separation of the lipid leaflets, resulting in the observed loss of the ion-channel activity. The fluorescence data indicate that the changes in the pressure-induced bilayer structure are reversible, consistent with the ability to repeatedly switch the ion-channel activity on and off by applying positive and negative pressures, respectively.

Glyco-acrylate copolymers for bilayer tethering on benzophenone-modified substrates

Model biological membranes are becoming increasingly important for studying fundamental biophysical phenomena and developing membrane-based devices. To address the anticipated problem of non-physiological interactions between membrane proteins and substrates seen in "solid-supported lipid bilayers" that are formed directly on hydrophilic substrates, we have developed a polymer-tethered lipid bilayer system based on a random copolymer with multiple lipid analogue anchors and a glyco-acrylate backbone. This system is targeted at applications that, most importantly, require stability and robustness since each copolymer has multiple lipid analogues that insert into the bilayer. We have combined this copolymer with a flexible photochemical coupling scheme that covalently attaches the copolymer to the substrate. The Langmuir isotherms of mixed copolymer/free lipid monolayers measured at the air-water interface indicate that the alkyl chains of the copolymer lipid analogues and the free lipids dominate the film behavior. In addition, no significant phase transitions are seen in the isotherms, while hysteresis experiments confirm that no irreversible states are formed during the monolayer compression. Isobaric creep experiments at the air-water interface and AFM experiments of the transferred monolayer are used to guide processing parameters for creating a fluid, homogeneous bilayer. Bilayer homogeneity and fluidity are monitored using fluorescence microscopy. Continuous bilayers with lateral diffusion coefficients of 0.6 microm(2)/s for both leaflets of the bilayer are observed for a 5% copolymer system.

Fluorescent dyes undergoing intramolecular proton transfer with improved sensitivity to surface charge in lipid bilayers

4'-(Dialkylamino)-3-hydroxyflavones are characterized by an excited-state proton transfer reaction between two tautomeric excited states, which results in two emission bands well separated on the wavelength scale. Due to the high sensitivity of the relative intensities of the two emission bands to solvent polarity, hydrogen bonding and local electric fields, these dyes found numerous applications in biomembrane studies. In order to further improve their fluorescence characteristics, we have synthesized new dyes where the 2-phenyl group is substituted with a 2-thienyl group. In organic solvents, the new dyes exhibit red shifted absorption and dual fluorescence. Although they show lower sensitivity to solvent polarity and H-bond donor ability (acidicity) than their parent 3-hydroxyflavone dyes, they exhibit a much higher sensitivity to solvent H-bond acceptor ability (basicity). Moreover, when tested in lipid vesicles of different surface charge, the new dyes show much better resolved dual emission and higher sensitivity to the surface charge of lipid bilayers than the parent dyes. The response of the new dyes to surface charge is probably connected with the H-bond basicity of the membrane surface, which is the highest for negatively charged surfaces. As a consequence, the new dyes appear as prospective fluorophores for the development of new fluorescent probes for biomembranes.

Interaction of 2-Hydroxy-substituted Nile Red Fluorescent Probe with Organic Nitrogen Compounds

The fluorescent properties of 2-hydroxy Nile red dye (HONR) proved to be highly sensitive to the basicity of hydrogen bond acceptors. Fluorescence quantum yields and fluorescence decay profiles were measured as the function of the concentration of organic nitrogen compounds in solvents of various polarities. The detailed mechanism and the kinetics of the fluorescence quenching were revealed with the combined analysis of the steady-state and time-resolved spectroscopic data. The relative contribution of the competing reaction steps was found to be very sensitive to the basicity of the additive and to solvent polarity. The most profound change appeared in the unimolecular deactivation pathways of the excited hydrogen-bonded HONR, whereas the formation rate of this species varied to a lesser extent. The dissociation into excited HONR and ground-state base was able to compete with the energy dissipation only when 2,4,6-trimethylpyridine was used as hydrogen bond acceptor in toluene. The bimolecular quenching of the excited hydrogen-bonded complex played significant role in apolar solvents. Proton displacement along the hydrogen bond in the excited complex led to excited ion pairs in polar media.

Photoinduced electron transfer studies of Nile red in the presence of TiO2 colloidal nanoparticles

Photoinduced electron transfers between Nile red (NR) with TiO2 colloidal nanoparticles are studied using picosecond transient absorption and time resolved fluorescence spectroscopy. The dynamics of electron transfer from the dye molecule to the semiconductor were understood from the transient, and also the formation of conduction band electron and Nile red cation radical were detected.