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.

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We have studied a system composed of a hydrophobic aerosil (R812) dispersed in the liquid crystal 4-noctyl-4′-cyanobiphenyl (8CB) using the spin probe electron spin resonance (ESR) technique, and in particular, we have determined, for different aerosil concentrations, the temperature dependence of the orientational order parameter, 〈P 2 〉, and the rotational diffusion coefficient, D ⊥ , of the probe 5-doxyl stearic acid in the ordered and isotropic phases of the system. We have found that increasing the silica concentration up to 10 wt % does not significantly change the transition temperatures of the system. The probe order parameter is instead depressed, and we found that the β exponent of an empirical Haller-type equation, used to fit its temperature dependence, changes roughly linearly with the aerosil concentration. The concentration effect on the probe dynamics is relatively small in the isotropic phase, where the D ⊥ temperature dependence is well fitted for all of the systems with an Arrhenius-type equation. In the nematic phase, the dynamical behavior is more complex: we found that, while local probe motion is still rather fast even when the macroscopic behavior is gellike, the temperature dependence of D ⊥ is still of Arrhenius-type up to 3 wt % aerosil concentration but it becomes of Vogel-Fulcher-Tammann-type for the 10 wt % R812 system.

We have explored the changes in the phase stability, orientational order, and dynamics of the nematic 4-cyano-4'-n-pentylbiphenyl (5CB) doped with either the trans or the cis form of different p-azobenzene derivatives using the ESR spin-probe technique. In particular, we have studied the effects induced by each of the seven nonmesogenic 4-R-phenylazobenzenes (R = H, F, Br, CH3, CF3, On-Bu, Ot-Bu) at 1% and 7% mole fraction on the order parameter and on the shift of the nematic-isotropic transition temperature (TNI), as reported by a nitroxide spin probe, and we have tried to relate them to the solute shape and charge distribution. In all the cases the presence of the azo-derivative causes a depression of T(NI), more pronounced for the cis isomers. The dependence of on the reduced temperature T* = T/T(NI) remains the same as that of pure 5CB in all trans-doped samples at 1% and 7% and decreases only slightly in the cis at 1%. However, we observe different and in some cases large variations (up to 25%) in for the cis at 7%, showing solute effects that go beyond the shift in T(NI). Surprisingly enough, even at the highest concentration, the probe dynamics appears to be essentially independent of the nature, the configuration, and the concentration of the different solutes and very similar to that observed in the pure 5CB.

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