Time-resolved evanescent wave-induced fluorescence anisotropy for the determination of molecular conformational changes of proteins at an interface

Detailed Study of BSA Adsorption on Micro- and Nanocrystalline Diamond/β-SiC Composite Gradient Films by Time-Resolved Fluorescence Microscopy

The adsorption of bovine serum albumin (BSA) on micro- and nanocrystalline diamond/β-SiC composite films synthesized using the hot filament chemical vapor deposition (HFCVD) technique has been investigated by confocal fluorescence lifetime imaging microscopy. BSA labeled with fluorescein isothiocyanate (FITC) was employed as a probe. The BSA^FITC conjugate was found to preferentially adsorb on both O-/OH-terminated microcrystalline and nanocrystalline diamond compared to the OH-terminated β-SiC, resulting in an increasing amount of BSA adsorbed to the gradient surfaces with an increasing diamond/β-SiC ratio. The different strength of adsorption (>30 times for diamond with a grain size of 570 nm) coincides with different surface energy parameters and differing conformational changes upon adsorption. Fluorescence data of the adsorbed BSA^FITC on the gradient film with different diamond coverage show a four-exponential decay with decay times of 3.71, 2.54, 0.66, and 0.13 ns for a grain size of 570 nm. The different decay times are attributed to the fluorescence of thiourea fluorescein residuals of linked FITC distributed in BSA with different dye-dye and dye-surface distances. The longest decay time was found to correlate linearly with the diamond grain size. The fluorescence of BSA^FITC undergoes external dynamic fluorescence quenching on the diamond surface by H- and/or sp²-defects and/or by amorphous carbon or graphite phases. An acceleration of the internal fluorescence concentration quenching in BSA^FITC because of structural changes of albumin due to adsorption, is concluded to be a secondary contributor. These results suggest that the micro- and nanocrystalline diamond/β-SiC composite gradient films can be utilized to spatially control protein adsorption and diamond crystallite size, which facilitates systematic studies at these interesting (bio)interfaces.

Aptamers in analytics

Nucleic acid aptamers are promising alternatives to antibodies in analytics. They are generally obtained through an iterative SELEX protocol that enriches a population of synthetic oligonucleotides to a subset that can recognize the chosen target molecule specifically and avidly. A wide range of targets is recognized by aptamers. Once identified and optimized for performance, aptamers can be reproducibly synthesized and offer other key features, like small size, low cost, sensitivity, specificity, rapid response, stability, and reusability. This makes them excellent options for sensory units in a variety of analytical platforms including those with electrochemical, optical, and mass sensitive transduction detection. Many novel sensing strategies have been developed by rational design to take advantage of the tendency of aptamers to undergo conformational changes upon target/analyte binding and employing the principles of base complementarity that can drive the nucleic acid structure. Despite their many advantages over antibodies, surprisingly few aptamers have yet been integrated into commercially available analytical devices. In this review, we discuss how to select and engineer aptamers for their identified application(s), some of the challenges faced in developing aptamers for analytics and many examples of their reported successful performance as sensors in a variety of analytical platforms.

Steady-state acceptor fluorescence anisotropy imaging under evanescent excitation for visualisation of FRET at the plasma membrane

We present a novel imaging system combining total internal reflection fluorescence (TIRF) microscopy with measurement of steady-state acceptor fluorescence anisotropy in order to perform live cell Förster Resonance Energy Transfer (FRET) imaging at the plasma membrane. We compare directly the imaging performance of fluorescence anisotropy resolved TIRF with epifluorescence illumination. The use of high numerical aperture objective for TIRF required correction for induced depolarization factors. This arrangement enabled visualisation of conformational changes of a Raichu-Cdc42 FRET biosensor by measurement of intramolecular FRET between eGFP and mRFP1. Higher activity of the probe was found at the cell plasma membrane compared to intracellularly. Imaging fluorescence anisotropy in TIRF allowed clear differentiation of the Raichu-Cdc42 biosensor from negative control mutants. Finally, inhibition of Cdc42 was imaged dynamically in live cells, where we show temporal changes of the activity of the Raichu-Cdc42 biosensor.

Time-resolved fluorescence anisotropy imaging

Fluorescence can be characterized by its intensity, position, wavelength, lifetime, and polarization. The more of these features are acquired in a single measurement, the more can be learned about the sample, i.e., the microenvironment of the fluorescence probe. Polarization-resolved fluorescence lifetime imaging-time-resolved fluorescence anisotropy imaging, TR-FAIM-allows mapping of viscosity or binding or of homo-FRET which can indicate dimerization or generally oligomerization.

Structural dynamics of a lytic peptide interacting with a supported lipid bilayer

The interaction of a melittin mutant with a 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC)-supported lipid bilayer was studied with the use of time-resolved evanescent wave-induced fluorescence spectroscopy (TREWIFS) and evanescent wave-induced time-resolved fluorescence anisotropy measurements (EW-TRAMs). The mutant peptide was labeled at position K14 with AlexaFluor 430 and retained the lytic activity characteristic of native melittin. The fluorescence decay kinetics of the conjugate was found to be biexponential with a short-lived component, τ(1), due to photoinduced electron transfer between AlexaFluor 430 and proximal side chains within or between the peptides. The longer-lived component, τ(2), was sensitive to the polarity of the microenvironment at or near the K14 position of the peptide. Upon interaction with a DPPC-supported bilayer, the proportional contribution of τ(1) increased, indicating a conformational change of the peptide. The values of τ(1) and τ(2) indicate that the AlexaFluor 430 probe experienced an environment with an equivalent polarity no less than that of methanol. EW-TRAMs data from the melittin mutant revealed hindered rotational motions of the AlexaFluor 430 probe both in the plane and perpendicular to the plane of the supported lipid bilayer. The data indicate a highly ordered and polar environment near the center of the melittin helix consistent with the formation of a toroidal pore.

Time-gated total internal reflection fluorescence microscopy with a supercontinuum excitation source

We present the instrumental development of a versatile total internal reflection fluorescence lifetime imaging microscopy setup illuminated by a supercontinuum laser source. It enables performing wide-field fluorescence lifetime imaging with subwavelength axial resolution for a large range of fluorophores. The short overall acquisition time and the axial resolution are well suited for dynamic neurobiological applications.

Total internal reflection fluorescence lifetime and anisotropy screening of cell membrane dynamics

A high content screening (HCS) system for fluorescence measurements at surfaces, in particular the plasma membrane of living cells, is described. The method is based on multiple total internal reflections (TIRs) of an incident laser beam within the glass bottom of a microtiter plate such that up to 96 individual samples could be illuminated by an evanescent electromagnetic field. Fluorescence lifetimes and time-resolved fluorescence anisotropies of these samples were assessed. While fluorescence lifetime represents a general measure for the interaction of a marker molecule with its microenvironment, the rotational diffusion time corresponds to the relaxation time of a molecule from a position with a defined orientation into a position with an arbitrary orientation. Thus, time-resolved fluorescence anisotropy reflects the viscosity of the microenvironment, i.e., membrane fluidity in the case of living cells. For all measurements in this study, either human glioblastoma cells incubated with the fluorescent membrane marker NBD-cholesterol or human breast cancer cells expressing a membrane-associating fluorescent protein were used.

Effect of refractive index on the fluorescence lifetime of green fluorescent protein

The average fluorescence lifetime of the green fluorescent protein (GFP) in solution is a function of the refractive index of its environment. We report that this is also the case for GFP-tagged proteins in cells. Using time-correlated single-photon counting (TCSPC)-based fluorescence lifetime imaging (FLIM) with a confocal scanning microscope, images of GFP-tagged proteins in cells suspended in different refractive index media are obtained. It is found that the average fluorescence lifetime of GFP decreases on addition of glycerol or sucrose to the media in which the fixed cells are suspended. The inverse GFP lifetime is proportional to the refractive index squared. This is the case for GFP-tagged major histocompatibility complex (MHC) proteins with the GFP located inside the cytoplasm, and also for GPI-anchored GFP that is located outside the cell membrane. The implications of these findings are discussed with regard to total internal reflection fluorescence (TIRF) techniques where the change in refractive index is crucial in producing an evanescent wave to excite fluorophores near a glass interface. Our findings show that the GFP fluorescence lifetime is shortened in TIRF microscopy in comparison to confocal microscopy.

Direct determination of the experimentally observed penetration depth of the evanescent field via near-infrared absorptions enhanced by the off-resonance of surface plasmons

Recently, absorption-sensitive surface plasmon resonance (SPR) techniques have attracted much attention. SPR near-infrared spectroscopy (SPR-NIRS) based on the Kretschmann configuration is one of the techniques for absorption enhancement. The enhanced spectrum obtained by SPR-NIRS basically corresponds to the measurement of an NIR absorption spectrum with a very short path length. However, the path length cannot be applied for Lambert's law due to the enhanced evanescent field. A direct determination of the penetration depth of the evanescent field is carried out via NIR absorptions enhanced by the off-resonance of surface plasmons, which is a principle of SPR-NIRS. The signal intensities of the enhanced NIR spectra of micrometer-thick polymer films having various thicknesses are compared with the classic theory of penetration depth. It is confirmed that the effective depth of the SPR-NIRS measurement can be expressed by the classic theory of penetration depth of the evanescent field proposed by Harrick.

Scanning-less wide-field single-photon counting device for fluorescence intensity, lifetime and time-resolved anisotropy imaging microscopy

A scanning-less single-photon counting system for FLIM and fluorescence anisotropy wide-field imaging is described and characterized in this paper. The two polarizations of the fluorescence are divided by a Glan prism and acquired at the same time by the Q(A) detector. Fluorescence decay profiles can be reconstructed for any desired area up to each pixel and used to calculate time-resolved fluorescence anisotropy decays.

Fluorescence lifetime imaging study of a thin protein layer on solid surfaces

Understanding the fundamental interactions between proteins and solid surfaces is essential in the area of implantable medical devices. Fluorescence methods offer the sensitivity required to study the formation of the initial thin protein layers that mediate biocompatibility of materials. Thin protein layers (bovine serum albumin labelled with 1-anilino-8-naphthalenesulfonate, BSA-ANS) deposited on several surfaces (glass, silicon, stainless steel, polystyrene, and silver island film) were studied using confocal frequency domain Fluorescence Lifetime Imaging Microscopy (FLIM) and single-point multifrequency lifetime analysis techniques. FLIM provides spatial information about both fluorophores located on the surface and physicochemical parameters of the surface microenvironment. The average fluorescence lifetimes (tau(av)) of the adsorbed BSA-ANS generated by the contact between a protein solution and the material surface were measured by the multifrequency modulation and phase shift. Results indicate that tau(av) values of the albumin complexes on the surfaces (approximately 12 ns) are, in general, shorter than tau(av) found in the bulk solution (approximately 14 ns). For some surfaces, like polystyrene and silver island film the differences in tau(av) of the adsorbed BSA-ANS were found to be much greater. The differences in fluorescence lifetimes may indicate structural changes in the BSA protein induced by contact with the surface.

Mathematical Model of Binding of Albumin—Bilirubin Complex to the Surface of Carbon Pyropolymer

We proposed a mathematical model and estimated the parameters of adsorption of albumin-bilirubin complex to the surface of carbon pyropolymer. Design data corresponded to the results of experimental studies. Our findings indicate that modeling of this process should take into account fractal properties of the surface of carbon pyropolymer.

Motion of Single DNA Molecules at a Liquid−Solid Interface As Revealed by Variable-Angle Evanescent-Field Microscopy

A variable-angle total-internal-reflection fluorescence microscope (VATIRFM) capable of providing a large range of incident angles was constructed for imaging single DNA molecule dynamics at a solid/liquid interface. An algorithm using a public-domain image-processing program, ImageJ, was developed for single-molecule counting. The experimental counts at various incident angles with different evanescent-field layer (EFL) thicknesses are affected by molecular diffusion. The dynamics of molecules near the surface and the observed counts in the VATIRFM are elucidated using a limited one-dimensional random-walk diffusion model. The simulation fits well with the experimental counting results. Further analysis using the simulation reveals the details of single-molecule motion. One implication is that the measured intensities cannot be used directly to determine the distances of molecules from the surface, though the majority of fluorescence does come from the EFL. Another implication is that rather than providing molecular concentrations within EFL the experimental counting results depict the distance-dependent dynamics of molecules near the surface. Thus, the VATIRFM could be a powerful technique to study the surface repulsion/attraction of molecules within a few hundred nanometers of the surface. Further studies show that molecules at low ionic strengths experience electrostatic repulsion at distances much further away from the surface than the calculated thickness of the electrical double layer.