Molecular Dynamics of Polyrotaxane in Solution Investigated by Quasi-Elastic Neutron Scattering and Molecular Dynamics Simulation: Sliding Motion of Rings on Polymer.

In this study, we investigated the molecular dynamics of polyrotaxane (PR), composed of α-cyclodextrins (CDs) and a poly(ethylene glycol) (PEG) axial chain, in solution by means of quasi-elastic neutron scattering (QENS) measurements and full-atomistic molecular dynamics (MD) simulations. From QENS experiments, we estimated the diffusion coefficients of CD and PEG monomers in PR, which are in quantitative agreement with those obtained by MD simulations. By analyzing the simulation results, we succeeded, for the first time, in observing and quantifying the sliding motion of CD along a PEG chain. The diffusion coefficient for the sliding motion is almost 6 times lower than that of the translational diffusion of CD in PR at room temperature. The retardation of the sliding motion is caused by the energy barrier on PEG produced by molecular interactions between CD and PEG. We propose a simple equation to describe the diffusion coefficient of the sliding dynamics in PR by combining the Einstein-Stokes diffusion model and a one-dimensional jump diffusion model. This work provides a general strategy for the molecular designs to control the sliding motion in PR.

Measurement of dipole potential in bilayer lipid membranes by dielectric spectroscopy.

Planar bilayer lipid membranes formed from egg phosphatidylcholine in aqueous media containing the lipophilic anion, dipicrylamine (DPA), were studied by dielectric spectroscopy over a frequency range of 10 Hz-10 MHz. The membranes showed dielectric relaxation due to the translocation of DPA between the membrane interfaces. Incorporating either cholesterol or 6-ketocholestanol into the membranes increased the characteristic frequency of the relaxation, which is proportional to the translocation rate constant of DPA. The results suggested that the sterol dipoles induced positive potential changes within the membrane interior. The changes of the dipole potential were 70 mV for cholesterol and 150 mV for 6-ketocholestanol when the sterol mole fraction was 0.67. The opposite effect was caused by phloretin added to the aqueous media, and the maximum dipole potential change was -90 mV at 100 µM.

Deciphering DNA-based asymmetric catalysis through intramolecular Friedel-Crafts alkylations.

We describe asymmetric intramolecular Friedel-Crafts alkylations with a DNA-based hybrid catalyst and propose a plausible binding model. This study shows promise for studying relationships between the helical chirality of DNA and enantioselectivity of the chemical reaction.