Reaction Fields in the Environment of Fluorescent Probes: Polarity Profiles in Membranes

Abstract

Fluorescent probes in biological systems are sensitive to environmental polarity by virtue of their response to the reaction field created by polarization of the dielectric medium. Classically, fluorophore solvatochromism is analyzed in terms of the Lippert equation and later variants, all of which rely upon the original reaction field of Onsager. A recent survey of the solvent dependence of EPR spin-label probes, which are responsive solely to the reaction field in the ground state without the complication of excited states, shows that the reaction field of Block and Walker performs best in describing the polarity dependence. In this model, the step-function transition to the bulk dielectric medium used by Onsager is replaced by a graded transition. Analysis of the Stokes shifts for representative fluorescent membrane probes, such as PRODAN, DANSYL, and anthroyl fatty acid, reveals that, of several different reaction fields (including that of Onsager), the Block-Walker model best describes the dependence on solvent dielectric constant and refractive index for the different probes simultaneously. This is after full allowance is made for all contributions involving polarizability of the fluorophore, a point that is frequently neglected or treated incorrectly in studies using biological fluorescent probes. By using the full range of polar and apolar solvents, it is then possible to establish a common reference for the polarity dependence of different fluorophores and to relate this also to the polarity dependence of biologically relevant spin-label EPR probes. An important application is calibration of the transmembrane polarity profile recorded by fluorescent probes in terms of the high-resolution profile obtained from site-specifically spin-labeled lipid chains.

Spin-Label EPR for Determining Polarity and Proticity in Biomolecular Assemblies: Transmembrane Profiles

“…For very large dielectric constants \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ f(\varepsilon_{\text{r}} ) \to 1, $$\end{document} but far less rapidly than in the Onsager model. Recently, the Block–Walker model has also been applied to analyze the polarity dependence of fluorescent probes, which allows transfer of data on environmental polarity between these and spin labels [ 20 ]. Interestingly, the Block–Walker model was used to represent the effects of solvation in EPR simulations with the Gaussian quantum chemical package [ 21 ], whereas other packages still use the Onsager approach.…”

Electromagnetic Model of a SPR Sensor Coupled to Array of Nanoparticles by Periodic Green’s Function

“…with = 3 /( + ) 3 being the fraction of the total volume of the particle occupied by the core, and the radius of the core and the thickness of the nanoparticle shell, respectively, and , , and the permittivities of the core, shell, and medium where the nanoparticles are inserted. Similarly, the spectral emission of the fluorescence for the excitation wavelength can be modeled by the polarizability of the fluorophore [15]. Due to the dimensions of the Core-Shell AuNps and the fluorophores, we can characterize the target particle excited by an effective dipole moment:…”

A New Interpretation of the Absorption and the Dual Fluorescence of Prodan in Solution

“…Prodan (2-dimethylamino-6-propionylnaphthalene, Fig. 1) and its derivatives, such as Laurdan, are widely used in biologically relevant systems [1][2][3][4][5][6][7] as fluorescent probes. It is very sensitive to the environment with its remarkable emission spectrum shifting by about 120 nm (6.4x10 3 cm -1 = 0.8 eV) from cyclohexane (λ max = 400 nm = 25.5x10 3 cm -1 ) to water (λ max = 520 nm = 19.1x10 3 cm -1 ).…”