Diffusion of Transient Radicals in Alcohols and Cyclohexane from Ambient to Supercritical Conditions Studied by the Transient Grating Method

Membrane fluidity and lipid order in ternary giant unilamellar vesicles using a new bodipy-cholesterol derivative

Cholesterol-rich, liquid-ordered (L(o)) domains are believed to be biologically relevant, and yet detailed knowledge about them, especially in live cells under physiological conditions, is elusive. Although these domains have been observed in model membranes, understanding cholesterol-lipid interactions at the molecular level, under controlled lipid mixing, remains a challenge. Further, although there are a number of fluorescent lipid analogs that partition into liquid-disordered (L(d)) domains, the number of such analogs with a high affinity for biologically relevant L(o) domains is limited. Here, we use a new Bodipy-labeled cholesterol (Bdp-Chol) derivative to investigate membrane fluidity, lipid order, and partitioning in various lipid phases in giant unilamellar vesicles (GUVs) as a model system. GUVs were prepared from mixtures of various molar fractions of dioleoylphosphatidylcholine, cholesterol, and egg sphingomyelin. The L(d) phase domains were also labeled with 1,1'-didodecyl-3,3,3',3'-tetramethylindocarbocyanine (DiI-C(12)) for comparison. Two-photon fluorescence lifetime and anisotropy imaging of Bdp-Chol are sensitive to lipid phase domains in GUVs. The fluorescence lifetime of Bdp-Chol in liquid-disordered, single-phase GUVs is 5.50 +/- 0.08 ns, compared with 4.1 +/- 0.4 ns in the presence of DiI-C(12). The observed reduction of fluorescence lifetime is attributed to Förster resonance energy transfer between Bdp-Chol (a donor) and DiI-C(12) (an acceptor) with an estimated efficiency of 0.25 and donor-acceptor distance of 2.6 +/- 0.2 nm. These results also indicate preferential partitioning (K(p) = 1.88) of Bdp-Chol into the L(o) phase. One-photon, time-resolved fluorescence anisotropy of Bdp-Chol decays as a triexponential in the lipid bilayer with an average rotational diffusion coefficient, lipid order parameter, and membrane fluidity that are sensitive to phase domains. The translational diffusion coefficient of Bdp-Chol, as measured using fluorescence correlation spectroscopy, is (7.4 +/- 0.3) x 10(-8) cm(2)/s and (5.0 +/- 0.2) x 10(-8) cm(2)/s in the L(d) and L(o) phases, respectively. Experimental translational/rotational diffusion coefficient ratios are compared with theoretical predictions using the hydrodynamic model (Saffman-Delbrück). The results suggest that Bdp-Chol is likely to form a complex with other lipid molecules during its macroscopic diffusion in GUV lipid bilayers at room temperature. Our integrated, multiscale results demonstrate the potential of this cholesterol analog for studying lipid-lipid interactions, lipid order, and membrane fluidity of biologically relevant L(o) domains.

Application of Fokker-Planck-Kramers equation treatment for short-time dynamics of diffusion-controlled reaction in supercritical Lennard-Jones fluids over a wide density range

The validity of a Fokker-Planck-Kramers equation (FPKE) treatment of the rate of diffusion-controlled reaction at short times [K. Ibuki and M. Ueno, J. Chem. Phys. 119, 7054 (2003)] is tested in a supercritical Lennard-Jones fluid over a wide density range by comparing it with the Langevin dynamics and molecular dynamics simulations and other theories. The density n range studied is 0.323n(c)< or =n< or =2.58n(c) and the temperature 1.52T(c), where n(c) and T(c) are the critical density and temperature, respectively. For the rate of bimolecular reactions, the transition between the collision-limited and diffusion-limited regimes is expected to take place in this density range. The simulations show that the rate constant decays with time extensively at high densities, and that the magnitude of decay decreases gradually with decreasing density. The decay profiles of the rate constants obtained by the simulations are reproduced reasonably well by the FPKE treatment in the whole density range studied if a continuous velocity distribution is used in solving the FPKE approximately. If a discontinuous velocity distribution is used instead of the continuous one, the FPKE treatment leads to a rate constant much larger than the simulation results at medium and low densities. The rate constants calculated from the Smoluchowski-Collins-Kimball (SCK) theory based on the diffusion equation are somewhat smaller than the simulation results in medium and low densities when the intrinsic rate constant is chosen to adjust the steady state rate constant in the low density limit to that derived by the kinetic collision theory. The discrepancy is relatively small, so that the SCK theory provides a useful guideline for a qualitative discussion of the density effect on the rate constant.

Polar Solvation and Solvation Dynamics in Supercritical CHF3: Results from Experiment and Simulation

Solvation dynamics of the probe trans-4-(dimethylamino)-4'-cyanostilbene (DCS) have been measured in supercritical fluoroform at 310 K (1.04 Tc) and solvent densities over the range 1.4-2.0 rho(c) using optical Kerr-gated emission spectroscopy. Steady-state measurements and computer simulations of this and the related system coumarin 153 (C153) in fluoroform are used to help interpret the observed dynamics. The solvent contribution to the Stokes shift of DCS is estimated to be 2300 +/- 400 cm(-1) and nearly density independent over the range (0.7-2.0)rho(c). Spectral response functions are bimodal and can be fit to biexponential functions having time constants of approximately 0.5 ps (85%) and 3-10 ps (15%) over the observable range ((1.4-2.0)rho(c)). Computer simulations based on a 2-site model of fluoroform and assuming an electrostatic solvation mechanism appear to properly account for the magnitude and weak density dependence of the Stokes shifts but predict much faster solvation than is observed. Possible reasons for the discrepancy are discussed.

Diffusion coefficients as a monitor of reaction kinetics of biological molecules

For revealing spectrally silent dynamics in chemical reactions, a new method, the time-dependent diffusion coefficient, is presented. Principles and typical examples of this method, in particular applications to biologically related reactions, are reviewed. The pulsed laser induced transient grating signal of the photo-decomposition reaction of caged ATP showed that the diffusion coefficient increases gradually with time reflecting the molecular size decrease by the dissociation. Hence, this rate should be a direct measurement of the photo-dissociation rate of ATP from the caged state. In an application to a protein folding reaction, the time-development of the diffusion coefficient was observed during the folding reaction. This time dependence was interpreted in terms of the intermolecular interaction change; i.e., conversion from the intermolecular hydrogen bonding to intramolecular one. It was found that the change of the hydrogen bonding network occurred by the two state manner in entire refolding process of cytochrome c. The unique feature of this time-dependent diffusion coefficient method is discussed.