Cucurbit[8]uril-stabilized charge transfer complexes with diquat driven by pH: a SERS study

In this work, we have studied the pH-dependence of the formation of DQCB[8] complexes by surface-enhanced Raman scattering (SERS) spectroscopy. The SERS spectra suggest that at acidic pH CB[8] can form a binary complex with the dication DQ(+2) while at higher pH ternary complexes with the radical cation dimer (DQ(+)˙)(2) and the radical cation-dication dimer (DQ(+)˙DQ(+2)) are formed. The pH-enhanced diquat (DQ) dimerization inside the cucurbit[8]uril cavity has not been reported until now. In addition, this study provides very valuable information regarding the use of CB[8] functionalized silver nanoparticles as SERS substrate for sensing applications.     

Application of field portable EDXRF spectrometry to analysis of pigments of Levantine rock art

The results of the analyses of elemental composition of red and black pigments of Levantine rock art from La Saltadora rock shelters (Valltorta gorge, Castellón, Spain) are presented in this paper. Nondestructive analyses were carried out using a portable energy dispersive X‐ray fluorescence (EDXRF) spectrometer developed for in situ analysis. The results revealed the strong presence of calcium in all the analyzed locations due to the contribution of the underlying calcareous bedrock and the overlying crust. Iron is the main element detected in red pigments and manganese in black pigments. Iron and calcium ratios have been found indicative of the degree of preservation of the pictorial layer. Trace elements detected in the pigment composition confirm the use of different raw materials. Therefore, this work illustrates the potential of the portable EDXRF spectrometers for in situ analysis of rock art paintings. Copyright © 2010 John Wiley & Sons, Ltd.     

Erdős–Szekeres Theorem for Lines

According to the Erd?s–Szekeres theorem, for every n, a sufficiently large set of points in general position in the plane contains n in convex position. In this note we investigate the line version of this result, that is, we want to find n lines in convex position in a sufficiently large set of lines that are in general position. We prove almost matching upper and lower bounds for the minimum size of the set of lines in general position that always contains n in convex position. This is quite unexpected, since in the case of points, the best known bounds are very far from each other. We also establish the dual versions of many variants and generalizations of the Erd?s–Szekeres theorem.     

Escherichia coli β‐Galactosidase Inhibitors through Modifications at the Aglyconic Moiety: Experimental Evidence of Conformational Distortion in the Molecular Recognition Process

Herein, we describe the use of thioglycosides as glycosidase inhibitors by employing novel modifications at the reducing end of these glycomimetics. The inhibitors display a basic galactopyranosyl unit (1→4)‐bonded to a 3‐deoxy‐4‐thiopentopyranose moiety. The molecular basis of the observed inhibition has been studied by using a combination of NMR spectroscopy and molecular modeling techniques. It is demonstrated that these molecules are not recognized by Escherichia coli β‐galactosidase in their ground‐state conformation, with a conformational selection process taking place. In fact, the observed conformational distortion depends on the chemical nature of the compounds and results from the rotation around the glycosidic linkage (variation of Φ or Ψ) or from the deformation of the six‐membered ring of the pentopyranose. The bound conformations of the ligand are adapted in the enzymatic pocket with a variety of hydrogen‐bond, van der Waals, and stacking interactions.     

Origin of the size dependence of Au nanoparticles toward molecular oxygen dissociation

Density functional theory calculations using a plane-wave basis set and a generalized gradient approach exchange-correlation potential have been carried out to study the dissociation of molecular oxygen by Au nanoparticles and its dependence with particle size. The analysis of the energy related data shows that the reactivity is dominated by the energy barrier height from adsorbed O₂ to the dissociated state and by the stability of molecular oxygen on the nanoparticle. The energy barrier is found to be only slightly dependent on the particles size where large variations are found for the adsorption energy of the O₂ molecule on the different nanoparticles. A careful analysis of the electronic structure shows that the driving force for O₂ adsorption by these nanoparticles is the existence of a clear gap between occupied and unoccupied states for the naked particle. This allows accommodating bonding states with O₂ below the Fermi level resulting in a strong interaction. On the contrary, the Au nanoparticles with a more metallic electronic structure have necessarily to accommodate bonding and antibonding states below the Fermi level with a concomitant weaker interaction with O₂.     

Brownian Particle in a Poisson-Shot-Noise Active Bath: Exact Statistics,Effective Temperature,and Inference

The dynamics of an overdamped Brownian particle in a thermal bath that contains a dilute solution of active particles is studied. The particle moves in a harmonic potential and experiences Poisson shot-noise kicks with specified amplitude distribution due to moving active particles in the bath. From the Fokker–Planck equation for the particle dynamics, the stationary solution for the displacement distribution is derived along with the moments characterizing mean, variance, skewness, and kurtosis, as well as finite-time first and second moments. An effective temperature is also computed through the fluctuation–dissipation theorem and show that equipartition theorem holds for all zero-mean kick distributions, including those leading to non-Gaussian stationary statistics. For the case of Gaussian-distributed active kicks, a re-entrant behavior from non-Gaussian to Gaussian stationary states and a heavy-tailed leptokurtic distribution across a wide range of parameters are found as seen in recent experimental studies. Further analysis reveals statistical signatures of the irreversible dynamics of the particle displacement in terms of the time asymmetry of cross-correlation functions. Fruits of the work is the development of an compact inference scheme that may allow experimentalists to extract the rate and moments of underlying shot-noise solely from the statistics the particle position.     

Flag-dipole and flagpole spinor fluid flows in Kerr spacetimes

Flagpole and flag-dipole spinors are particular classes of spinor fields that has been recently used in different branches of theoretical physics. In this paper, we study the possibility and consequences of these spinor fields to induce an underlying fluid flow structure in the background of Kerr spacetimes. We show that flag-dipole spinor fields are solutions of the equations of motion in this context. To our knowledge, this is the second time that this class of spinor field appears as a physical solution, the first one occurring as a solution of the Dirac equation in ESK gravities.     

Bistable phase locking of a nonlinear optical cavity via rocking: Transmuting vortices into phase patterns

We report the experimental observation of the conversion of a phase-invariant nonlinear system into a bistable phase-locked one via rocking [G. J. de Valcárcel and K. Staliunas, Phys. Rev. E 67, 026604 (2003)10.1103/PhysRevE.67.026604]. This conversion results in vortices of the phase-invariant system being replaced by phase patterns such as domain walls. A model for the experimental device, a photorefractive oscillator, is given that reproduces the observed behavior.