Theory, measurements, and modeling of OH and HO2 formation in the reaction of cyclohexyl radicals with O2

The production of OH and HO(2) in Cl-initiated oxidation of cyclohexane has been measured using pulsed-laser photolytic initiation and continuous-laser absorption detection. The experimental data are modeled by master equation calculations that employ new G2(MP2)-like ab initio characterizations of important stationary points on the cyclo-C(6)H(11)O(2) surface. These ab initio calculations are a substantial expansion on previously published characterizations, including explicit consideration of conformational changes (chair-boat, axial-equatorial) and torsional potentials. The rate constants for the decomposition and ring-opening of cyclohexyl radical are also computed with ab initio based transition state theory calculations. Comparison of kinetic simulations based on the master equation results with the present experimental data and with literature determinations of branching fractions suggests adjustment of several transition state energies below their ab initio values. Simulations with the adjusted values agree well with the body of experimental data. The results once again emphasize the importance of both direct and indirect components of the kinetics for the production of both HO(2) and OH in radical + O(2) reactions.     

Matrix-dependent adhesion of vascular and valvular endothelial cells in microfluidic channels

The interactions between endothelial cells and the underlying extracellular matrix regulate adhesion and cellular responses to microenvironmental stimuli, including flow-induced shear stress. In this study, we investigated the adhesion properties of primary porcine aortic endothelial cells (PAECs) and valve endothelial cells (PAVECs) in a microfluidic network. Taking advantage of the parallel arrangement of the microchannels, we compared adhesion of PAECs and PAVECs to fibronectin and type I collagen, two prominent extracellular matrix proteins, over a broad range of concentrations. Cell spreading was measured morphologically, based on cytoplasmic staining with a vital dye, while adhesion strength was characterized by the number of cells attached after application of shear stresses of 11, 110, and 220 dyn cm(-2). Results showed that PAVECs were more well spread on fibronectin than on type I collagen (P < 0.0001), particularly for coating concentrations of 100, 200, and 500 microg mL(-1). PAVECs also withstood shear significantly better on fibronectin than on collagen for 500 microg mL(-1). PAECs were more well spread on collagen compared to PAVECs (P < 0.0001), but did not have significantly better adhesion strength. These results demonstrate that cell adhesion is both cell-type and matrix dependent. Furthermore, they reveal important phenotypic differences between vascular and valvular endothelium, with implications for endothelial mechanobiology and the design of microdevices and engineered tissues.     

Asymmetric wetting hysteresis on chemical defects

Using the Wilhelmy plate technique, the role of chemical defects in hysteretic wetting behavior was investigated. The wetting and dewetting work differ significantly, depending on the defect energy (i.e., high or low energy with respect to the matrix). For one, or an array of high-energy defects, advancing measurements departed from equilibrium theory, while the receding data were in close agreement. Conversely for low-energy defects, only the receding measurements showed significant departure from theory. We propose that distinct wetting mechanisms for high- and low-energy defects explain the phenomenon of asymmetric hysteresis, where the advancing or receding contact angle deviates more strongly from the equilibrium angle.     

On the regularity of the Neumann problem for free surfaces with surface tension

In 1952 H. Lewy established that a hydrodynamic free surface which is at least in a neighborhood of a point situated on the free surface is automatically , possibly in a smaller neighborhood of . This local result is an example which preceeds the theory developed by D. Kinderlehrer, L. Nirenberg and J. Spruck (1977-79), proving that in many cases free surfaces cannot have an arbitrary regularity; in particular, there exist such that if the surface in question is , then automatically is . In this paper we extend their methods to Neumann type problems for free surfaces with surface tension.

     

Vitamin D and Skin Cancer

Vitamin D signaling plays a key role in many important processes, including cellular proliferation, differentiation and apoptosis, immune regulation, hormone secretion and skeletal health. Furthermore, vitamin D production and supplementation have been shown to exert protective effects via an unknown signaling mechanism involving the vitamin D receptor (VDR) in several diseases and cancer types, including skin cancer. With over 3.5 million new diagnoses in 2 million patients annually, skin cancer is the most common cancer type in the United States. While ultraviolet B (UVB) radiation is the main etiologic factor for nonmelanoma skin cancer (NMSC), UVB also induces cutaneous vitamin D production. This paradox has been the subject of contradictory findings in the literature in regards to amount of sun exposure necessary for appropriate vitamin D production, as well as any beneficial or detrimental effects of vitamin D supplementation for disease prevention. Further clinical and epidemiological studies are necessary to elucidate the role of vitamin D in skin carcinogenesis.     

Dipolar Cycloaddition Reactions of Azomethine Ylides Generated by endocyclic‐exocyclic 1,3‐dipole Rearrangement

Dipolar cycloaddition of polycyclic azomethine ylides, in which the central nitrogen atom is part of a pyrrolidine ring and bears a methoxycarbonyl group with norbornenes has been shown to produce two main types of products featuring pyrrolizidine rings. In conjuction with results of quantum chemical calculations (B3LYP), mechanistic rationale was postulated. The key reaction step is unprecedented endocyclic to exocyclic azomethine ylide rearrangement by an intermolecular prototropic migration (formal [1,3] H‐shift).     

Synthesis of a Neutral Mixed‐Valence Diferrocenyl Carborane for Molecular Quantum‐Dot Cellular Automata Applications

The preparation of 7‐Fc⁺‐8‐Fc‐7,8‐nido‐[C₂B₉H₁₀]^? (Fc⁺FcC₂B₉^?) demonstrates the successful incorporation of a carborane cage as an internal counteranion bridging between ferrocene and ferrocenium units. This neutral mixed‐valence Fe^II/Fe^III complex overcomes the proximal electronic bias imposed by external counterions, a practical limitation in the use of molecular switches. A combination of UV/Vis‐NIR spectroscopic and TD‐DFT computational studies indicate that electron transfer within Fc⁺FcC₂B₉^? is achieved through a bridge‐mediated mechanism. This electronic framework therefore provides the possibility of an all‐neutral null state, a key requirement for the implementation of quantum‐dot cellular automata (QCA) molecular computing. The adhesion, ordering, and characterization of Fc⁺FcC₂B₉^? on Au(111) has been observed by scanning tunneling microscopy.     

Predicting Trigger Bonds in Explosive Materials through Wiberg Bond Index Analysis

Understanding the explosive decomposition pathways of high‐energy‐density materials (HEDMs) is important for developing compounds with improved properties. Rapid reaction rates make the detonation mechanisms of HEDMs difficult to understand, so computational tools are used to predict trigger bonds—weak bonds that break, leading to detonation. Wiberg bond indices (WBIs) have been used to compare bond densities in HEDMs to reference molecules to provide a relative scale for the bond strength to predict the activated bonds most likely to break to trigger an explosion. This analysis confirms that X?NO₂ (X=N,C,O) bonds are trigger linkages in common HEDMs such as TNT, RDX and PETN, consistent with previous experimental and theoretical studies. Calculations on a small test set of substituted tetrazoles show that the assignment of the trigger bond depends upon the functionality of the material and that the relative weakening of the bond correlates with experimental impact sensitivities.