Time-resolved fluorescence depolarization studies of rotational relaxation in viscous media

Physical properties of the cytoplasm modulate the rates of microtubule polymerization and depolymerization

The cytoplasm is a crowded, visco-elastic environment whose physical properties change according to physiological or developmental states. How the physical properties of the cytoplasm impact cellular functions in vivo remains poorly understood. Here, we probe the effects of cytoplasmic concentration on microtubules by applying osmotic shifts to fission yeast, moss, and mammalian cells. We show that the rates of both microtubule polymerization and depolymerization scale linearly and inversely with cytoplasmic concentration; an increase in cytoplasmic concentration decreases the rates of microtubule polymerization and depolymerization proportionally, whereas a decrease in cytoplasmic concentration leads to the opposite. Numerous lines of evidence indicate that these effects are due to changes in cytoplasmic viscosity rather than cellular stress responses or macromolecular crowding per se. We reconstituted these effects on microtubules in vitro by tuning viscosity. Our findings indicate that, even in normal conditions, the viscosity of the cytoplasm modulates the reactions that underlie microtubule dynamic behaviors.

Computational study of the DPAP molecular rotor in various environments: from force field development to molecular dynamics simulations and spectroscopic calculations

Fluorescent molecular rotors (FMRs) belong to an important class of environment-sensitive dyes capable of acting as nanoprobes in the measurement of viscosity and polarity of their micro-environment. FMRs have found widespread applications in various research fields, ranging from analytical to biochemical sciences, for example in intracellular imaging studies or in volatile organic compound detection. Here, a computational investigation of a recently proposed FMR, namely 4-(diphenylamino)phthalonitrile (DPAP), in various chemical environments is presented. A purposely developed molecular mechanics force field is proposed and then applied to simulate the rotor in a high- and low-polar solvent (i.e., acetonitrile, tetrahydrofuran, o-xylene and cyclohexane), a polymer matrix and a lipid membrane. Subtle effects of the molecular interactions with the embedding medium, the structural fluctuations of the rotor and its rotational dynamics are analyzed in some detail. The results correlate with a previous work, thus supporting the reliability of the model, and provide further insights into the environment-specific properties of the dye. In particular, it is shown how molecular diffusion and rotational correlation times of the FMR are affected by the surrounding medium and how the molecular orientation of the dye becomes anisotropic once immersed in the lipid bilayer. Moreover, a qualitative correlation between the FMR rotational dynamics and the fluorescence lifetime is detected, a result in line with the observed viscosity dependence of its emission. Finally, optical absorption spectra are computed and successfully compared with their experimental counterparts.

Rotational Diffusion of a New Large Non Polar Dye Molecule in Alkanes

Rotational reorientation times of a newly synthesized 2,5-bis(phenylethynyl)1,4-bis(dodecyloxy) benzene (DDPE) are experimentally determined in series of n-alkanes by employing steady state and time resolved fluorescence depolarization technique with a view to understand rotational dynamics of large non-polar solute molecule in non-polar solvents and few general solvents of different sizes and varying viscosity. It is observed that rotational reorientation times vary linearly as function of viscosity. The hydrodynamic stick condition describes the experimental results at low viscosities while the results tend to deviate significantly from it at higher viscosities. This is attributed to the possibility of long chains in solvents hosting a variety of chain defects (end-gauche, double-gauche, all-trans, kink, etc.) thereby reducing the effective length of the molecule, leading to a slightly reduced friction. The experimental results are compared with the predictions of Stokes-Einstein-Debye (SED) hydrodynamic theory as well as the quasi-hydrodynamic theories of Gierer-Wirtz (GW) and Dote-Kivelson-Shwartz (DKS). The predictions from these theories underestimate τr in the solvents employed in the study.

Glycosidic moiety changes the spectroscopic properties of dl-alpha-tocopherol in DMSO/water solution and in organic solvents

In this study we estimated how conjugation with a sugar moiety influences the spectral properties of tocopherol and relate the spectroscopic properties of glycosides to solvent properties such as viscosity and polarity. Spectroscopic properties (absorption, fluorescence, fluorescence anisotropy and fluorescence lifetime) of three dl-alpha-tocopheryl glycosides (dl-alpha-tocopheryl orthoacetate derivative and glycosides of dl-alpha-tocopherol model compounds: 2,2,5,7,8-pentamethyl-6-chromanol and Trolox) were studied in DMSO/water solution. In all investigated compounds dissolved in DMSO/water mixture the absorption and emission maxima were blue-shifted. The fluorescence lifetimes were longer compared with those obtained for the parent compounds, except for the Trolox glucoside, in which it was shorter. The observed effect is connected with an increase in the electronic energy in the ground state due to electron rearrangement in the chromanol system caused by interaction with the sugar moiety. The extent of the spectral shift is related to the sugar moiety substituted at the phenolic oxygen rather than to substitution at the 2a position in the chromanol ring. The fluorescent properties of dl-alpha-tocopheryl glucoside in organic solvents were measured. The Stokes shift was related to the orientational polarizability of the solvents. The study of viscosity suggested two different mechanisms explaining the results observed in a low- and high-viscosity environment. The results indicated the fundamental role of interactions between the chromophore and sugar moiety in a low-viscosity environment. The results obtained at high values of viscosity are discussed in terms of a frictional boundary solvent-solute interaction model.

Time-resolved fluorescence anisotropy imaging applied to live cells

We have developed a wide-field time-resolved imaging system to image quantitatively both the fluorescence lifetime and the rotational correlation time of a fluorophore. Using a polarization-resolved imager, we simultaneously image orthogonal polarization components of the fluorescence emission onto a time-gated intensified CCD. We demonstrate imaging of solvent viscosity variations through the rotational correlation time of fluorescein in a multiwell plate and apply this technique to probe the microviscosity in live cells.

Trehalose influence on beta-lactoglobulin stability and hydration by time resolved fluorescence

The stabilizing role of the disaccharide trehalose on beta-lactoglobulin (BLG) against its chemical denaturation both at native and acidic pH has been explored by means of time-resolved fluorescence of the probe acrylodan covalently bound to the unique free cysteine of BLG. The changes in acrylodan fluorescence lifetime with guanidinium chloride concentration reveal BLG sigmoidal denaturation profiles which depend upon the amount of trehalose in solution. When adding trehalose the transition midpoint shifts towards higher denaturant concentration. This effect has been measured by fitting the data with a two-state model whose parameters indicate that an almost 60% increase in the denaturation free energy is induced independently of trehalose concentrations and pH values. Fluorescence anisotropy measurements performed in the same conditions reveal that the internal dynamics are largely affected by the sugar, which makes the acrylodan environment more rigid, and by the denaturant that acts in the opposite way. The overall rotational diffusion of BLG suggests that trehalose affects the hydrodynamic properties of the solution in the proximity of the protein; tentative mechanisms are discussed.

Single-molecule identification by spectrally and time-resolved fluorescence detection

A method to identify single molecules rapidly and with high efficiency based on simple probability considerations is proposed. In principle, any property of a detected photon in a single-molecule fluorescence experiment, e.g., emission wavelength, arrival time after pulsed excitation, and polarization, can be analyzed within the framework of the outlined methodology. Monte Carlo simulations show that less than 500 photons are needed to assign an observed single molecule to one out of four species with a confidence level higher than 99.9%. We show that single dye molecules of four different dyes embedded in a polymer film can be identified with time-correlated single-photon counting spectrally resolved in two channels.

Efficient high repetition rate synchronous amplification of a passively mode-locked femtosecond dye laser

We demonstrate reliable operation of a stable synchronously pumped dye amplifier for femtosecond pulses from a passively mode-locked dye laser. Conversion efficiencies of 8% are obtained with output powers of 40 mW and 50-fs pulse widths at repetition rates of up to 10 kHz with pulse energy stability of 3% rms. Synchronization is achieved by driving the frequency-modulated mode-locked seed oscillator for the regenerative amplifier pump laser directly from the dye laser oscillator. Low timing jitter between the dye oscillator and seed laser of less than 1 ps leads to efficient amplification and low amplified spontaneous emission (1%) from the amplifier.

Comparison of approaches to the instrumental response function in fluorescence decay measurements

Deconvolution of pulse fluorometry data requires knowledge of the instrumental response, which is not directly observable in some circumstances. Various procedures for approaching the instrumental response function were evaluated for nanosecond fluorescence decay data analyzed by nonlinear least squares, including the commonly used time shift correction and several reference fluorophore methods. A new reference fluorophore technique using a Monte Carlo convolution is introduced and tested. The correction for scattered light in several reference techniques is also presented. The random convolution and one other reference fluorophore method consistently gave superior results over a wide range of experimental conditions.

Fluorescence studies of structural fluctuations in macromolecules as observed by fluorescence spectroscopy in the time, lifetime, and frequency domains

Fluorescence spectroscopy provides numerous opportunities for determination of the dynamic properties of macromolecules. Measurements may be performed in the time, lifetime, or the frequency domain. Presently, the highest resolution is provided by the frequency-domain measurements, using the newly available instruments.