Orientational distribution of linear dye molecules in bilayer membranes

Photobleaching kinetics and time-integrated emission of fluorescent probes in cellular membranes

Since the pioneering work of Hirschfeld, it is known that time-integrated emission (TiEm) of a fluorophore is independent of fluorescence quantum yield and illumination intensity. Practical implementation of this important result for determining exact probe distribution in living cells is often hampered by the presence of autofluorescence. Using kinetic modelling of photobleaching combined with pixel-wise bleach rate fitting of decay models with an updated plugin to the ImageJ program, it is shown that the TiEm of a fluorophore in living cells can be determined exactly from the product of bleaching amplitude and time constant. This applies to mono-exponential bleaching from the first excited singlet and/or triplet state and to multi-exponential combinations of such processes. The TiEm can be used to correct for illumination shading and background autofluorescence without the need for fluorescent test layers or separate imaging of non-stained cells. We apply the method to simulated images and to images of cells, whose membranes were labelled with fluorescent sterols and sphingolipids. Our bleaching model can be extended to include a probability density function (PDF) of intrinsic bleach rate constants with a memory kernel. This approach results in a time-dependent bleach rate coefficient and is exemplified for fluorescent sterols in restricted intracellular environments, like lipid droplets. We show that for small deviations from the classical exponential bleaching, the TiEm of decay functions with rate coefficients remains largely independent of fluorescence lifetime and illumination, and thereby represents a faithful measure of probe distribution.

Self-alignment of dye molecules in micelles and lamellae for three-dimensional imaging of lyotropic liquid crystals

We report alignment of anisotropic amphiphilic dye molecules within oblate and prolate anisotropic micelles and lamellae, the basic building blocks of surfactant-based lyotropic liquid crystals. Absorption and fluorescence transition dipole moments of these dye molecules orient either parallel or orthogonal to the liquid crystal director. This alignment enables three-dimensional visualization of director structures and defects in different lyotropic mesophases by means of fluorescence confocal polarizing microscopy and two-photon excitation fluorescence polarizing microscopy. The studied structures include nematic tactoids, Schlieren texture with disclinations in the calamitic nematic phase, oily streaks in the lamellar phase, developable domains in the columnar hexagonal phase, and various types of line defects in the discotic cholesteric phase. Orientational three-dimensional imaging of structures in the lyotropic cholesterics reveals large Burgers vector dislocations in cholesteric layering with singular disclinations in the dislocation cores that are not common for their thermotropic counterparts.

Heterogeneity of fluorescence determined by the method of area-normalized time-resolved emission spectroscopy

Heterogeneity of fluorescence due to multiple fluorophores or emitter species is a common problem in using fluorescent molecules to probe the structure, dynamics, and properties of microheterogeneous media (micelles, membranes, proteins, etc.) and biological media. Time-resolved emission spectra (TRES) and area-normalized TRES (TRANES) are useful to identify unambiguously emission from a single species (homogeneity) and emission from two species (dual emission). The features in TRANES spectra that are characteristic of solvation dynamics associated with single species and two species, and many other cases are also described.

Characterisation of membrane mimetics on a dual polarisation interferometer

Dual polarisation interferometry (DPI) has been used to characterise the formation of hybrid bilayer membranes (HBM) on a silicon-oxynitride surface. This technique allows the simultaneous determination of multiple physical properties of an HBM, as the HBM is being formed in a single experiment: mass, thickness in the z-direction (normal to the surface), tilt angle of the first layer and refractive index. Decanoic acid was covalently attached to an amine modified silicon-oxynitride sensor chip surface via 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride condensation reaction. The decanoic acid layer was 0.92+/-0.12 nm thick, indicating a tilt angle of 57 degrees from surface normal, and possessed a mass of 1.05+/-0.10 ng mm(-2) and a refractive index (RI) of 1.450+/-0.020. Phospholipid vesicles made from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) were injected over the fatty acid surface to form an HBM. The DPPC HBM was 4.32+/-0.68 nm thick, with a total mass of 3.18+/-0.60 ng mm(-2) and a RI of 1.404+/-0.007. The DMPC HBM was 2.12+/-0.34 nm thick, with a total mass of 2.25+/-0.51 ng mm(-2), and a RI of 1.435+/-0.007. DPI thus provides an insight into HBM formation and differences between the structural organisation of HBMs of different composition.

Radiative decay engineering 5: metal-enhanced fluorescence and plasmon emission

Metallic particles and surfaces display diverse and complex optical properties. Examples include the intense colors of noble metal colloids, surface plasmon resonance absorption by thin metal films, and quenching of excited fluorophores near the metal surfaces. Recently, the interactions of fluorophores with metallic particles and surfaces (metals) have been used to obtain increased fluorescence intensities, to develop assays based on fluorescence quenching by gold colloids, and to obtain directional radiation from fluorophores near thin metal films. For metal-enhanced fluorescence it is difficult to predict whether a particular metal structure, such as a colloid, fractal, or continuous surface, will quench or enhance fluorescence. In the present report we suggest how the effects of metals on fluorescence can be explained using a simple concept, based on radiating plasmons (RPs). The underlying physics may be complex but the concept is simple to understand. According to the RP model, the emission or quenching of a fluorophore near the metal can be predicted from the optical properties of the metal structures as calculated from electrodynamics, Mie theory, and/or Maxwell's equations. For example, according to Mie theory and the size and shape of the particle, the extinction of metal colloids can be due to either absorption or scattering. Incident energy is dissipated by absorption. Far-field radiation is created by scattering. Based on our model small colloids are expected to quench fluorescence because absorption is dominant over scattering. Larger colloids are expected to enhance fluorescence because the scattering component is dominant over absorption. The ability of a metal's surface to absorb or reflect light is due to wavenumber matching requirements at the metal-sample interface. Wavenumber matching considerations can also be used to predict whether fluorophores at a given distance from a continuous planar surface will be emitted or quenched. These considerations suggest that the so called "lossy surface waves" which quench fluorescence are due to induced electron oscillations which cannot radiate to the far-field because wavevector matching is not possible. We suggest that the energy from the fluorophores thought to be lost by lossy surface waves can be recovered as emission by adjustment of the sample to allow wavevector matching. The RP model provides a rational approach for designing fluorophore-metal configurations with the desired emissive properties and a basis for nanophotonic fluorophore technology.

Fluorescence imaging of two-photon linear dichroism: cholesterol depletion disrupts molecular orientation in cell membranes

The plasma membrane of cells is an ordered environment, giving rise to anisotropic orientation and restricted motion of molecules and proteins residing in the membrane. At the same time as being an organized matrix of defined structure, the cell membrane is heterogeneous and dynamic. Here we present a method where we use fluorescence imaging of linear dichroism to measure the orientation of molecules relative to the cell membrane. By detecting linear dichroism as well as fluorescence anisotropy, the orientation parameters are separated from dynamic properties such as rotational diffusion and homo energy transfer (energy migration). The sensitivity of the technique is enhanced by using two-photon excitation for higher photo-selection compared to single photon excitation. We show here that we can accurately image lipid organization in whole cell membranes and in delicate structures such as membrane nanotubes connecting two cells. The speed of our wide-field imaging system makes it possible to image changes in orientation and anisotropy occurring on a subsecond timescale. This is demonstrated by time-lapse studies showing that cholesterol depletion rapidly disrupts the orientation of a fluorophore located within the hydrophobic region of the cell membrane but not of a surface bound probe. This is consistent with cholesterol having an important role in stabilizing and ordering the lipid tails within the plasma membrane.

Location and orientation of DODCI in lipid bilayer membranes: effects of lipid chain length and unsaturation

The location and orientation of a linear dye molecule, DODCI, in lipid bilayer membrane were determined by the effect of viscosity and refractive index of the aqueous medium on the fluorescence properties of the dye bound to the membrane. The membrane-bound dye is solubilized in two sites, one near the surface (short fluorescence lifetime) and another in the interior of the membrane (long lifetime). The ratio of the dye in the two locations and the orientation of the dye (parallel or perpendicular to the membrane) are sensitive to the lipid chain length and unsaturation in the alkyl chain. The fraction of the dye in the interior region is higher for short alkyl chains (C12>C14>C16>>C18C20) and in unsaturated lipids (C14:1>C14:0, C16:1>C16:0). These experimental results are consistent with the general principle that the penetration of an amphiphilic organic molecule in the interior region of the membrane is more when the structure of th bilayer is more fluid-like.