In Silico Stark Effect: Determination of Excited-State Polarizabilities of Squaraine Dyes.

The static electric polarizabilities (α) of a quadrupolar squaraine dye are investigated in silico, either as the excess polarizability, i.e., the change from the ground to the lowest excited state, Δα, or as those of the two states separately, depending on the approach. The polarizabilities are worked out by making use of the energy and dipole moment Taylor expansions as a function of the electric field (E), in which α is represented by the quadratic and linear terms, respectively, and also by means of the linear response approach. Dipoles and energies are computed at a few values of the electric field, with different strategies that consider the geometry of the molecule either frozen in the ground state or relaxed at each E value. From a physical standpoint, the most appropriate approach to describing the molecular polarizability depends on the processes in which the molecule is involved: for example, fluorescence methods provide information about relaxed excited states, and absorption methods are used to determine the polarizability changes of excited states frozen in the ground-state conformation. We show that the excited-state polarizability does not strongly differ from the ground-state polarizability when the field is applied along the main axis of the squaraine. In contrast, remarkable differences are found when the field is applied perpendicular to the molecular plane due to a large geometrical distortion of the molecular backbone.

Interplay between diphenyl diselenide and copper: Impact on D. melanogaster survival, behavior, and biochemical parameters.

Copper (Cu2+) is a biologically essential element that participates in numerous physiological processes. However, elevated concentrations of copper have been associated with cellular oxidative stress and neurodegenerative diseases. Organo­selenium compounds such as diphenyl diselenide (DPDS) have in vitro and in vivo antioxidant properties. Hence, we hypothesized that DPDS may modulate the toxicity of Cu2+ in Drosophila melanogaster. The acute effects (4 days of exposure) caused by a high concentration of Cu2+ (3 mM) were studied using endpoints of toxicity such as survival and behavior in D. melanogaster. The potential protective effect of low concentration of DPDS (20 µM) against Cu2+ was also investigated. Adult flies aged 1-5 days post-eclosion (both sexes) were divided into four groups: Control, DPDS (20 µM), CuSO4 (3 mM), and the combined exposure of DPDS (20 µM) and CuSO4 (3 mM). Survival, biochemical, and behavioral parameters were determined. Co-exposure of DPDS and CuSO4 increased acetylcholinesterase (AChE) activity and the generation of reactive oxygen species (ROS as determined by DFCH oxidation). Contrary to our expectation, the co-exposure reduced survival, body weight, locomotion, catalase activity, and cell viability in relation to control group. Taken together, DPDS potentiated the Cu2+ toxicity.

A dual attack on the peroxide bond. The common principle of peroxidatic cysteine or selenocysteine residues.

The (seleno)cysteine residues in some protein families react with hydroperoxides with rate constants far beyond those of fully dissociated low molecular weight thiol or selenol compounds. In case of the glutathione peroxidases, we could demonstrate that high rate constants are achieved by a proton transfer from the chalcogenol to a residue of the active site [Orian et al. Free Radic. Biol. Med. 87 (2015)]. We extended this study to three more protein families (OxyR, GAPDH and Prx). According to DFT calculations, a proton transfer from the active site chalcogenol to a residue within the active site is a prerequisite for both, creating a chalcogenolate that attacks one oxygen of the hydroperoxide substrate and combining the delocalized proton with the remaining OH or OR, respectively, to create an ideal leaving group. The "parking postions" of the delocalized proton differ between the protein families. It is the ring nitrogen of tryptophan in GPx, a histidine in GAPDH and OxyR and a threonine in Prx. The basic principle, however, is common to all four families of proteins. We, thus, conclude that the principle outlined in this investigation offers a convincing explanation for how a cysteine residue can become peroxidatic.

Simulations of flow-induced director structures in nematic liquid crystals through leslie-ericksen equations. I. Computational methodology in two dimensions

A computational treatment of the constitutive equations of nematodynamics, based on the Leslie-Ericksen approach, is presented and discussed for a rotating planar nematic sample subjected to a constant magnetic field. The dynamics of the velocity v and director n fields is taken into account exactly. Coupled partial differential equations suitable to be solved numerically are worked out, in terms of derived functionals of v and n and of their spatial and time derivatives. Time-dependent patterns of the director are obtained using a finite-difference scheme in a spatial polar grid. Several experimental situations are analyzed, corresponding to common experimental setups: continuously rotating samples for different values of the rotational speed; 30 degrees and 90 degrees step-rotation experiments. A comparison is made to existing approximate treatments. Dependence upon the sample dimension is also discussed.

Simulations of flow-induced director structures in nematic liquid crystals through leslie-ericksen equations. II. Interpretation Of NMR experiments in liquid crystal polymers

Computationally exact and approximate solutions of the Leslie-Ericksen equations for nematic liquid crystals in two dimensions are employed to calculate director distributions in cylindrical samples, rotating under the influence of a magnetic field. In particular, the time evolution of systems prepared initially in metastable states with respect to the magnetic field is investigated, and calculated director distributions are used to interpret rheo-NMR experiments in nematic liquid crystal polymers.