Kinetic analysis of a slow electron transfer coupled with a father—son reaction

The electrode reaction mechanism involving interaction of the products of a slow, rate-determining electron transfer with the parent molecule (father—son reaction) has been examined theoretically under the conditions of cyclic voltammetry. The kinetic analysis indicates how to determine the correct values of the kinetic parameters for both the heterogeneous charge transfer and the homogeneous chemical step. The irreversible cathodic reaction of diphenylmethylphenylsulphide in anhydrous DMF provides a good example of this mechanism, since the diphenylmethyl carbanion resulting from the irreversible two-electron reduction undergoes proton transfer from the parent molecule (self-protonation mechanism).     

Thermodynamics of caffeine in aqueous denaturant solutions

The effects of urea, guanidinium chloride, and potassium chloride on the solution properties of caffeine have been investigated by solubility and partition experiments and by measuring heats of dilution, solution, and mixing. Thermodynamic transfer properties are used to discuss the possible mechanism of action of the two denaturants. The ability of caffeine to form stacked aggregates is taken as possible evidence for a direct interaction of this solute with urea and guanidinium ion. The data are discussed both in terms of an ideally associating system and in terms of the virial expansion following the McMillan-Mayer solution theory.Part of this paper was presented at the Journées de Calorimetrie et d'Analyse Thermique, Torino, Italy, June 28–30, 1978.     

Measurement of the effective focal shift in an optical trap

The focus of an oil-immersion microscope objective is shifted because of the refractive-index mismatch between the cover glass and the aqueous sample. We present a procedure with which to determine the focal shift by use of an inverted microscope equipped with optical tweezers. As the position of the sample chamber is scanned vertically, we measure the axial displacement of an optically trapped bead; the relative motion of the bead with respect to the surface supplies the effective focal shift. Measurements of this quantity deviate from electromagnetic calculations of the focal shift, a discrepancy attributable to the depth-dependent decrease in axial trap stiffness that arises from spherical aberration.     

Passive all-optical force clamp for high-resolution laser trapping

Optical traps are useful for studying the effects of forces on single molecules. Feedback-based force clamps are often used to maintain a constant load, but the response time of the feedback limits bandwidth and can introduce instability. We developed a novel force clamp that operates without feedback, taking advantage of the anharmonic region of the trapping potential where the differential stiffness vanishes. We demonstrate the utility of such a force clamp by measuring the unfolding of DNA hairpins and the effect of trap stiffness on opening distance and transition rates.     

Photocycloaddition of chalcones to yield cyclobutyl ditopic cyclophanes

The intramolecular photocycloaddition of chalcones to give cyclobutanes has proved to be a fast and convenient method to shrink a cyclophane ring to a tricyclic system, in order to prepare potential ditopic receptors. X-Ray results confirm the previously indicated structure for the cyclobutanes 2a (n=1, m=1), in which the cyclization occurs by a head-to-head syn ring closure. NMR results indicate that the same process occurs for the cyclobutanes 2b (n=2, m=2) and 2c (n=1, m=3).     

A route toward the generation of thermally stable Au cluster anions supported on the MgO surface

On the basis of experimental evidence and DFT calculations, we propose a simple yet viable way to stabilize and chemically activate gold nanoclusters on MgO. First the MgO surface is functionalized by creation of trapped electrons, (H (+))(e (-)) centers (exposure to atomic H or to H 2 under UV light, deposition of low amounts of alkali metals on partially hydroxylated surfaces, etc.); the second step consists in the self-aggregation of gold clusters deposited from the gas phase. The calculations show that the (H (+))(e (-)) centers act both as nucleation and activation sites. The process can lead to thermally stable gold cluster anions whose catalytic activity is enhanced by the presence of an excess electron.     

Independent component model of the default-mode brain function: combining individual-level and population-level analyses in resting-state fMRI

Resting-state functional magnetic resonance imaging (RS-fMRI) is a technique used to investigate the spontaneous correlations of blood-oxygen-level-dependent signals across different regions of the brain. Using functional connectivity tools, it is possible to investigate a specific RS-fMRI network, referred to as "default-mode" (DM) network, that involves cortical regions deactivated in fMRI experiments with cognitive tasks. Previous works have reported a significant effect of aging on DM regions activity. Independent component analysis (ICA) is often used for generating spatially distributed DM functional connectivity patterns from RS-fMRI data without the need for a reference region. This aspect and the relatively easy setup of an RS-fMRI experiment even in clinical trials have boosted the combined use of RS-fMRI and ICA-based DM analysis for noninvasive research of brain disorders. In this work, we considered different strategies for combining ICA results from individual-level and population-level analyses and used them to evaluate and predict the effect of aging on the DM component. Using RS-fMRI data from 20 normal subjects and a previously developed group-level ICA methodology, we generated group DM maps and showed that the overall ICA-DM connectivity is negatively correlated with age. A negative correlation of the ICA voxel weights with age existed in all DM regions at a variable degree. As an alternative approach, we generated a distributed DM spatial template and evaluated the correlation of each individual DM component fit to this template with age. Using a "leave-one-out" procedure, we discuss the importance of removing the bias from the DM template-generation process.     

Cosserat model for periodic masonry deduced by nonlinear homogenization

The paper deals with the problem of the determination of the in-plane behavior of periodic masonry material. The macromechanical equivalent Cosserat medium, which naturally accounts for the absolute size of the constituents, is derived by a rational homogenization procedure based on the Transformation Field Analysis. The micromechanical analysis is developed considering a Cauchy model for masonry components. In particular, a linear elastic constitutive relationship is considered for the blocks, while a nonlinear constitutive law is adopted for the mortar joints, accounting for the damage and friction phenomena occurring during the loading history. Some numerical applications are performed on a Representative Volume Element characterized by a selected commonly used texture, without performing at this stage structural analyses. A comparison between the results obtained adopting the proposed procedure and a nonlinear micromechanical Finite Element Analysis is presented. Moreover, the substantial differences in the nonlinear behavior of the homogenized Cosserat material model with respect to the classical Cauchy one, are illustrated.