Corrado Bacchiocchi

Ca2+-Induced Movement of Tropomyosin in Skeletal Muscle Thin Filaments Observed by Multi-Site FRET

To obtain information on Ca(2+)-induced tropomyosin (Tm) movement in Ca(2+)-regulated muscle thin filaments, frequency-domain fluorescence energy transfer data were collected between 5-(2-iodoacetyl-amino-ethyl-amino)naphthalene-1-sulfonic acid at Cys-190 of Tm and phalloidin-tetramethylrhodamine B isothiocyanate bound to F-actin. Two models were used to fit the experimental data: an atomic coordinate (AC) model coupled with a search algorithm that varies the position and orientation of Tm on F-actin, and a double Gaussian distance distribution (DD) model. The AC model showed that little or no change in transfer efficiency is to be expected between different sites on F-actin and Tm if Ca(2+) causes azimuthal movement of Tm of the magnitude suggested by structural data (C. Xu, R. Craig, L. Tobacman, R. Horowitz, and W. Lehman. 1999. Biophys. J. 77:985-992). However, Ca(2+) produced a small but significant change in our phase/modulation versus frequency data, showing that changes in lifetime decay can be detected even when a change of the steady-state transfer efficiency is very small. A change in Tm azimuthal position of 17 on the actin filament obtained with the AC model indicates that solution data are in reasonable agreement with EM image reconstruction data. In addition, the data indicate that Tm also appears to rotate about its axis, resulting in a rolling motion over the F-actin surface. The DD model showed that the distance from one of the two chains of Tm to F-actin was mainly affected, further verifying that Ca(2+) causes Tm to roll over the F-actin surface. The width of the distance distributions indicated that the position of Tm in absence and in presence of Ca(2+) is well defined with appreciable local flexibility.

Myosin-Induced Movement of αα, αβ, and ββ Smooth Muscle Tropomyosin on Actin Observed by Multisite FRET

The interaction of the αα, ββ, and αβ smooth muscle tropomyosin (Tm) isoforms with F-actin was systematically studied in the absence and in the presence of myosin subfragment 1 (S1) using multifrequency phase/modulation Forster resonance energy transfer (FRET). A Gaussian double distance distribution model was adopted to fit FRET data between a 5-(2-iodoacetyl-amino-ethyl-amino)naphthalene-1-sulfonic acid donor at either Cys-36 of the β-chain or Cys-190 of the α-chain and a 4-dimethylaminophenylazophenyl 4′-maleimide acceptor at Cys-374 of F-actin. Experimental data were obtained for singly and doubly labeled αβ Tm (donor only at α, only at β, or both) and for doubly labeled αα or ββ Tm. Data for singly labeled αβTm were combined in a global analysis with doubly labeled αβTm. In all doubly labeled isoforms, upon S1 binding, one donor-acceptor “apparent” distance increased slightly by 0.5–2 A, whereas the other decreased by 6–9 A. These changes are consistent with a uniform “rolling” motion of Tm over the F-actin surface. The analysis indicates that Tm occupies relatively well-defined positions, with some flexibility, in both the predominantly closed (−S1) and open (+S1) thin-filament states. The results for the αβTm heterodimer indicate that the local twofold symmetry of αα or ββ Tm is effectively broken in αβTm bound to F-actin, which implies a difference between the α- and β-chains in terms of their interaction with F-actin.

Order and Dynamics Inside H-PDLC Nanodroplets: An ESR Spin Probe Study

We have performed a detailed study of the order and dynamics of the commercially available BL038 liquid crystal (LC) inside nanosized (50-300 nm) droplets of a reflection-mode holographic-polymer dispersed liquid crystal (H-PDLC) device where LC nanodroplet layers and polymer layers are alternately arranged, forming a diffraction grating. We have determined the configuration of the LC local director and derived a model of the nanodroplet organization inside the layers. To achieve this, we have taken advantage of the high sensitivity of the ESR spin probe technique to study a series of temperatures ranging from the nematic to the isotropic phase of the LC. Using also additional information on the nanodroplet size and shape distribution provided by SEM images of the H-PDLC cross section, the observed director configuration has been modeled as a bidimensional distribution of elongated nanodroplets whose long axis is, on the average, parallel to the layers and whose internal director configuration is a uniaxial quasi-monodomain aligned along the nanodroplet long axis. Interestingly, at room temperature the molecules tend to keep their average orientation even when the layers are perpendicular to the magnetic field, suggesting that the molecular organization is dictated mainly by the confinement. This result might explain, at least in part, (i) the need for switching voltages significantly higher and (ii) the observed faster turn-off times in H-PDLCs compared to standard PDLC devices.

Energy transfer in condensed systems The effect of phase organization

We investigate the effects that a phase transformation, from isotropic to orientationally (nematic) and positionally (smectic) ordered mesophases, has on the energy transfer process between solute molecules. We combine Metropolis Monte Carlo simulations and a master equation approach to study radiationless energy transfer processes in three-dimensional ordered systems of identical uniaxial particles interacting via the Gay-Beme potential. The time-dependent excitation probability and the fluorescence anisotropy decay curves show an enhanced energy transfer in systems with a higher degree of order. We also find a non-isotropic evolution of the time-dependent excitation probability in systems with positional as well as orientational order, with a faster energy transfer in the smectic planes. @ 1997 Elsevier Science B.V.

Reduced distributed monopole model for the efficient prediction of energy transfer in condensed phases.

We propose a methodology for the realistic simulation and prediction of resonance energy transfer in condensed phases based on a combination of computer simulations of phase morphologies and of a distributed monopole model for the radiationless transfer. The heavy computational demands of the method are moderated by the introduction of a transition charges reduction scheme, originally developed for ground state interactions [Berardi, R. et al. Chem. Phys. Lett. 2004, 389, 373]. We demonstrate the scheme for a condensed glass phase formed by perylene monoimide end-capped 9,9-(di n,n)octylfluorene trimers, recently studied as light-harvesting materials, where we couple a coarse-grained Monte Carlo simulation of the molecular organization and a master equation approach modeling the energy diffusion process.

A Non-Standard Temperature Dependence of the Order Parameter of the 5CB Liquid Crystal Doped with an Azo-Derivative

The effects of the cis and trans form of the 4-OMe-phenylazobenzene at 1% and 7% mole fraction on order, mesophase stability and dynamics of the 5CB liquid crystal were studied using the ESR spin probe technique. The presence of the trans azo-derivative induced a downshift of the nematic-isotropic transition temperature T NI which increased with the concentration but did not change the dependence of the order parameter on the reduced temperature. The cis isomer caused even larger TNI shifts, determining a marked decrease of the order parameter with a peculiar dependence on the reduced temperature, quite different from the standard Haller-type behavior. The spin probe dynamics appeared instead to be essentially unchanged by the presence of the azo-derivative.

Director configuration in the twist-bend nematic phase of CB11CB

The director distribution in the nematic phases exhibited by the 1′′,11′′-bis(4-cyanobiphenyl-4′-yl)undecane (CB11CB) liquid crystal has been studied in the bulk using the EPR spin probe technique. EPR spectra confirmed the presence of a higher temperature uniaxial nematic phase and of a lower temperature nematic phase in which the director distribution is not uniform. Spectra recorded in the lower temperature nematic phase were not fully compatible with theoretical EPR spectra calculated according to the recently proposed model for the twist-bend phase in which the local domain director twists around an axis with a fixed tilt angle, θ0, but were well described by a “distributed-tilt” model in which the director has a relatively narrow distribution, centred at θ0.

EPR Study of Order and Dynamics of the 5CB Liquid Crystal in an H-PDLC Device

We have studied the effects of confinement on the order and dynamics of the 5CB liquid crystal (LC) inside nanosized droplets of a reflection-mode holographic-polymer dispersed LC (H-PDLC) device, consisting of alternating LC nanodroplets and polymer layers, forming a diffraction grating. Here we have investigated, taking advantage of the high sensitivity of the EPR spin probe technique, a series of temperatures spanning the nematic and isotropic phase of the LC. The occurrence of phase separation and the consequent formation of a diffraction grating was revealed by SEM images of the H-PDLC cross section and by the presence of a reflection peak around 565 nm. Differently from the case of BL038 LC based H-PDLCs, the results here indicate the absence of an ordered fraction of mesogens throughout the analysed temperature range. Taking into account a previous model of LC molecules arrangement in the same kind of device [Bacchiocchi, C., et al. (2009). J. Phys. Chem. B 113, 5391], we postulate the presence of very small droplets in which the surface anchoring constraints represent the dominant effect. This results in the hindrance of the LC uniform macroscopic alignment, providing a plausible explanation for the malfunctioning of these devices.

Energy transfer and orientational dynamics in isotropic and nematic phases. A computer simulation approach

We present a computer simulation study of the effect of molecular reorientation on the Forster-type energy transfer (ET) process in a nematic and isotropic phase. Monte Carlo (MC) equilibrium configurations and a stepwise diffusion algorithm are employed to model the ET process and the molecular rotational dynamics in the case of arbitrary time scales. We find faster fluorescence depolarization at higher rotational dynamic rates, but the transfer efficiency and directionality observed for fixed molecules is also maintained in the presence of molecular reorientation.

Glass-like Behavior at Molecular Level Induced in a Nematic by a Dispersion of Aerosil Nanoparticles

ABSTRACT Random disorder effects of an hydrophobic aerosil on the order and dynamics of the 3β-DOXYL-5α-cholestane spin probe in the liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) were studied using the ESR technique. Increasing the aerosil concentration up to 10 wt% does not change the nematic–isotropic transition temperature but decreases the probe order parameter. The dynamics in the isotropic phase is of Arrhenius type and essentially independent of the concentration. Deviations from this behavior are clearly observed in the nematic phase even at 1 wt% of aerosil and are well represented by a Vogel-Fulcher-Tammann type law, suggesting that a glass-like behavior can be induced by a very small amount of random disorder. PACS numbers: 61.30. Gd, 76.30.-v, 82.70. Gg