Fluorescence depolarization in a scattering medium: effect of size parameter of a scatterer

Fluorescence anisotropy imaging in drug discovery

Non-invasive measurement of drug-target engagement can provide critical insights in the molecular pharmacology of small molecule drugs. Fluorescence polarization/fluorescence anisotropy measurements are commonly employed in protein/cell screening assays. However, the expansion of such measurements to the in vivo setting has proven difficult until recently. With the advent of high-resolution fluorescence anisotropy microscopy it is now possible to perform kinetic measurements of intracellular drug distribution and target engagement in commonly used mouse models. In this review we discuss the background, current advances and future perspectives in intravital fluorescence anisotropy measurements to derive pharmacokinetic and pharmacodynamic measurements in single cells and whole organs.

Linear Extrapolation of the Analyte-Specific Light Scattering and Fluorescence Depolarization in Turbid Samples

Anisotropy and depolarization are two interconvertible parameters in fluorescence and light scattering spectroscopy that describe the polarization distribution of emitted and scattered photons generated with linearly polarized excitation light. Whereas anisotropy is more frequently used in fluorescence literature for studying association/dissociation of fluorophore-bearing reagents, depolarization is more popular in the light-scattering literature for investigating the effect of scatterers' geometries and chemical compositions. Presented herein is a combined computational and experimental study of the scattering and fluorescence depolarization enhancement induced by light scattering in turbid samples. The most important finding is that sample light scattering and fluorescence depolarization increases linearly with sample light-scattering extinction. Therefore, one can extrapolate the analyte-specific scattering and fluorescence depolarization through linear curve fitting of the sample light scattering and fluorescence depolarization as a function of the sample concentration or the path length of the sampling cuvettes. An example application of this linear extrapolation method is demonstrated for quantifying the fluorophore-specific fluorescence depolarization and consequently its anisotropy for an aggregation-induced-emission sample. This work should be important for a wide range of macromolecular, supramolecular, and nanoscale fluorescent materials that are often strong light scatterers due to their large sizes.

Fluorescent Lipid Nanoparticles as Biomembrane Models for Exploring Emerging Contaminant Bioavailability Supported by Density Functional Theory Calculations

Experimental sensing schemes and thermodynamic in-silico studies are combined holistically in this manuscript so as to give new insights into the bioavailability of environmental contaminants via permeation across lipid nanoparticles (liposomes) as a mimicry of biological membranes. Using Prodan and Laurdan as fluorescent membrane probes, phosphatidylcholine-based unilamellar liposomes are harnessed to investigate membranotropic effects of alkyl esters of p-hydroxybenzoic acid and triclosan in vitro on the basis of steady-state fluorescence anisotropy, light scattering, and generalized polarization measurements. The feasibility of the analytical responses to ascertain differences in temperature-dependent contaminant bioavailability is investigated in detail. High level density functional theory (DFT) calculations (RI-BP86-D3/def2-SVP) have been resorted to investigate noncovalent 1:1 complexes of the fluorescent probes and emerging contaminants with dipalmitoylphosphatidylcholine, as a minimalist model of a lipid nanoparticle, to evaluate both the interaction energies and the geometries of the complexes. This information can be related to the degree of penetration of the guest across the lipid bilayer. Our experimental results supported by in-silico DFT calculations and ecotoxicological data let us to conclude that simple analytical measurements of liposomal changes in lipid packaging, permeability, and fluidity are appropriate to foresee the potential bioavailability and toxicity of emerging contaminants.

Two-photon Fluorescence Anisotropy Microscopy for Imaging and Direct Measurement of Intracellular Drug Target Engagement

Small molecule therapeutic drugs must reach their intended cellular targets (pharmacokinetics) and engage them to modulate therapeutic effects (pharmacodynamics). These processes are often difficult to measure in vivo due to their complexities and occurrence within single cells. It has been particularly difficult to directly measure cellular drug target binding. Fluorescence polarization is commonly used in pharmacological screening assays to measure drug-protein or protein-protein interactions. We hypothesized that fluorescence polarization imaging could be adapted and used with fluorescently labeled drugs to measure drug target engagement in vivo. Here we summarize recent results using two photon fluorescence anisotropy microscopy. Our imaging technique offers quantitative pharmacological binding information of diverse molecular interactions at the microscopic level, differentiating between bound and unbound states. Used in combination with other recent advances in the development of novel fluorescently labeled drugs, we expect that the described imaging modality will provide a window into the distribution and efficacy of drugs in real time and in vivo at the cellular and subcellular level.

Depolarization coefficients of light in multiply scattering media

The depolarization coefficients are calculated for multiply scattered linearly and circularly polarized light. For a number of media (aqueous suspension of polystyrene particles, water droplets in air), the calculations are carried out both numerically, with solving the vector radiative transfer equation and analytically, within the polarization mode approximation. In the latter case the depolarization coefficients are expressed explicitly in terms of the scattering and absorption coefficients, and the scattering matrix elements of the medium. The range of applicability of the polarization mode approximation is established. For most practically important cases, this method is shown to provide a satisfactory degree of accuracy. We also find the fundamental values of the depolarization coefficients for a Rayleigh medium.

In vivo imaging of specific drug-target binding at subcellular resolution

The possibility of measuring binding of small-molecule drugs to desired targets in live cells could provide a better understanding of drug action. However, current approaches mostly yield static data, require lysis or rely on indirect assays and thus often provide an incomplete understanding of drug action. Here, we present a multiphoton fluorescence anisotropy microscopy live cell imaging technique to measure and map drug-target interaction in real time at subcellular resolution. This approach is generally applicable using any fluorescently labelled drug and enables high-resolution spatial and temporal mapping of bound and unbound drug distribution. To illustrate our approach we measure intracellular target engagement of the chemotherapeutic Olaparib, a poly(ADP-ribose) polymerase inhibitor, in live cells and within a tumour in vivo. These results are the first generalizable approach to directly measure drug-target binding in vivo and present a promising tool to enhance understanding of drug activity.

Penetration of light into multiple scattering media: model calculations and reflectance experiments. Part II: The radial transfer

In continuation of our contribution to "The Axial Transfer" (Appl. Spectr. 2012. 66(8): 934-943), this paper describes the distribution of localized incident radiation in multiple scattering layers of arbitrary thickness and analyzes the lateral intensity profiles of radiation leaving the sample from its illuminated and non-illuminated surfaces. The theoretical profiles are calculated with different approximations of the equation of transfer. We derive for both non-absorbing and absorbing layers simple analytical expressions and verify their accuracy and range of applicability by comparison with Monte Carlo simulations. Particular emphasis is given to the analysis of the radial absorption, an under-theorized and under-investigated feature that can help to identify weak or hidden absorbers. In addition, we contribute to the description of how the radial reflectance is affected by anisotropy or by error sources like multiple surface reflection for samples in glass cells or deflectance (sideway loss) of radiation in small samples. Finally, the theoretical results are compared with experimental data of radial reflectance for quasi non-absorbing and absorbing powder layers.

Characterizing breast cancer tissues through the spectral correlation properties of polarized fluorescence

We study the spectral correlation properties of the polarized fluorescence spectra of normal and cancerous human breast tissues, corresponding to patients belonging to diverse age groups and socioeconomic backgrounds. The emission range in the visible wavelength regime of 500 to 700 nm is analyzed, with the excitation wavelength at 488 nm, where flavin is one of the active fluorophores. The correlation matrices for parallel and perpendicularly polarized fluorescence spectra reveal correlated domains, differing significantly in normal and cancerous tissues. These domains can be ascribed to different fluorophores and absorbers in the tissue medium. The spectral fluctuations in the perpendicular component of the cancerous tissue clearly reveal randomization not present in the normal channel. Random matrix-based predictions for the spectral correlations match quite well with the observed behavior. The eigenvectors of the correlation matrices corresponding to large eigenvalues clearly separate out different tissue types and identify the dominant wavelengths, which are active in cancerous tissues.

Depth-resolved fluorescence measurement in a layered turbid medium by polarized fluorescence spectroscopy

We show that, when a turbid medium with a layered fluorophore distribution is excited by linearly polarized light, measurement of angle-resolved polarized fluorescence can provide depth-resolved fluorescence measurements.

Depolarization of light in a multiply scattering medium: effect of the refractive index of a scatterer

We report the results of a study carried out to investigate the influence of the refractive index and size parameter of a scatterer on the depolarization of linearly and circularly polarized light in a turbid medium. The results show that for a given refractive index of the surrounding medium, the influence of the refractive index of the scatterer on the depolarization of both linearly and circularly polarized light is rather weak for samples with smaller-sized scatterers (Rayleigh scatterers, radius a<<lambda , anisotropy parameter g < or =0.2 ). For a given value of optical thickness (tau= mu(s) xd , mu(s) being the scattering coefficient, d the physical thickness), the depolarization of circularly polarized light was observed to be higher than that of linearly polarized light for these samples. In contrast, for samples prepared using larger-sized scatterers (Mie scatterers, a > or =lambda , g > or =0.7), linearly polarized light was observed to depolarize much faster than circularly polarized light when the refractive index of scatterers was large (n=1.59) but no appreciable difference in depolarization of linearly and circularly polarized light was observed when the refractive index of scatterers had a lower value (n=1.37) . Further, for scattering samples having Mie scatterers, for comparable values of tau and g , depolarization of polarized light was much higher for samples with scatterers of lower refractive index.