A Spirocyclohexyl Nitroxide Amino Acid Spin Label for Pulsed EPR Spectroscopy Distance Measurements

Site‐directed spin labeling and EPR spectroscopy offer accurate, sensitive tools for the characterization of structure and function of macromolecules and their assemblies. A new rigid spin label, spirocyclohexyl nitroxide α‐amino acid and its N‐(9‐fluorenylmethoxycarbonyl) derivative, have been synthesized, which exhibit slow enough spin‐echo dephasing to permit accurate distance measurements by pulsed EPR spectroscopy at temperatures up to 125 K in 1:1 water/glycerol and at higher temperatures in matrices with higher glass transition temperatures. Distance measurements in the liquid nitrogen temperature range are less expensive than those that require liquid helium, which will greatly facilitate applications of pulsed EPR spectroscopy to the study of structure and conformation of peptides and proteins.     

Fabrication of Large‐Scale Hierarchical ZnO Hollow Spheroids for Hydrophobicity and Photocatalysis

We report here the preparation of a crystalline, pure hexagonal phase of ZnO as hollow 500–800 nm spheroids in the presence of organic bases, such as pyridine, using zinc acetate as the precursor salt. The spheroids exhibit unique 3D hierarchical architectures, like cocoons, and demonstrate improved superhydrophobic (water contact angle, 150°) character due to the inherited air‐trapped capillarity within the cocoon structure. The simple synthetic strategy used in this process is modified hydrothermolysis (MHT), which represents a general approach and may contribute to the formation mechanism of the hollow nanostructures with highly improved porosity. Depending on the concentration of the precursor salt, it has been possible to cover glass plates or the inner wall of a reaction vessel with ZnO nanocrystals. A low salt concentration (<0.01 M ) allows the easy preparation of a superhydrophobic glass surface, whereas a high salt concentration (>0.01 M ) results in the precipitation of cocoons at the bottom of the reaction vessel as a solid mass together with a deposited thin film of ZnO nanocrystals covering the inner wall of the glass vessel. The thickness of the film successively grows through repetitive hydrothermolysis processes for which a low salt concentration (<0.01 M ) was employed. Because of the hollow cocoonlike morphology, the surface area of the film is greatly increased, which makes it accessible for functionalization by incoming substrates from both sides (internally and externally) and helps to drive a competent photocatalytic dye degradation pathway. The heterocyclic base pyridine exclusively develops cocoons. Thus, the mechanism of self‐aggregation of ZnO nanocrystals under MHT reaction conditions has been studied and the characterization of the compounds has been supported with physical measurements.     

Synthesis,Crystal Structure,and Rotational Energy Profile of 3-Cyclopropyl-1,2,4-benzotriazine 1,4-Di-<Emphasis Type="Italic">N</Emphasis>-oxide

Abstract  

1,2,4-Benzotriazine 1,4-di-N-oxides are potent antitumor drug candidates that undergo in vivo bioreduction leading to selective DNA damage in the low oxygen (hypoxic) cells found in tumors. Tirapazamine (TPZ) is the lead compound in this family. Here we report on the synthesis, crystal structure, and conformational analysis of a new analog, 3-cyclopropyl-1,2,4-benzotriazine 1,4-di-N-oxide (3). Compound 3 (C₁₀H₁₀N₃O₂) crystallized in the monoclinic space group C2/c. Unit cell parameters for 3: a = 16.6306 (12), b = 7.799 (5), c = 16.0113 (11) ?, α = 90, β = 119.0440 (10), γ = 90, and z = 8.     

A modified thermal peak model with the source function dependent on ion velocity for materials bombarded by heavy ions

A modified thermal spike model is proposed. In contrast to the previous model, this scheme involves a source function dependent on ion energy losses determined by the penetration depth of a target and the characteristic time of ion penetration until comes to a complete stop. Such an approach is especially significant for confirming its applicability for track formation processes observed in materials because, as is shown, characteristic times appreciably exceed the periods of phonon oscillations with frequencies of 10¹³ Hz or more. The performed calculations make it possible to infer that the source function of the modified model describes thermal processes differently than does the commonly used source function. It is established that the temperature of a material exceeds the solid-liquid transition temperature in the cylindrical regions with sizes that are smaller than those calculated without allowance for ion motion.     

Seasonal variation of daily total column ozone (TCO) and role of its depletion and formation rate on surface temperature over Dumdum at Kolkata, India

The purpose of the paper is to present the variation of total ozone concentration over Dumdum at Kolkata, India during different seasons. It is concluded that the trend of ozone concentration during different seasons are different. A critical analysis is done and following important results are obtained.

Fermi gamma-ray "bubbles" from stochastic acceleration of electrons

Gamma-ray data from Fermi Large Area Telescope reveal a bilobular structure extending up to ～50° above and below the Galactic Center. It has been argued that the gamma rays arise from hadronic interactions of high-energy cosmic rays which are advected out by a strong wind, or from inverse-Compton scattering of relativistic electrons accelerated at plasma shocks present in the bubbles. We explore the alternative possibility that the relativistic electrons are undergoing stochastic 2nd-order Fermi acceleration by plasma wave turbulence through the entire volume of the bubbles. The observed gamma-ray spectral shape is then explained naturally by the resulting hard electron spectrum modulated by inverse-Compton energy losses. Rather than a constant volume emissivity as in other models, we predict a nearly constant surface brightness, and reproduce the observed sharp edges of the bubbles.     

An NMR thermometer for cryogenic magic-angle spinning NMR: the spin-lattice relaxation of (127)I in cesium iodide

The accurate temperature measurement of solid samples under magic-angle spinning (MAS) is difficult in the cryogenic regime. It has been demonstrated by Thurber et al. (J. Magn. Reson., 196 (2009) 84-87) [10] that the temperature dependent spin-lattice relaxation time constant of ⁷⁹Br in KBr powder can be useful for measuring sample temperature under MAS over a wide temperature range (20–296 K). However the value of T₁ exceeds 3 min at temperatures below 20 K, which is inconveniently long. In this communication, we show that the spin-lattice relaxation time constant of ¹²⁷I in CsI powder can be used to accurately measure sample temperature under MAS within a reasonable experimental time down to 10 K.     

Synthesis and photo physical properties of Au @ Ag (core @ shell) nanoparticles disperse in poly vinyl alcohol matrix

Synthesis of core @ shell (Au @ Ag) nanoparticle with varying silver composition has been carried out in aqueous poly vinyl alcohol (PVA) matrix. Core gold nanoparticle (~15 nm) has been synthesized through seed-mediated growth process. Synthesis of silver shell with increasing thickness (~1–5 nm) has been done by reducing Ag⁺ over the gold sol in the presence of mild reducing ascorbic acid. Characterization of Au @ Ag nanoparticles has been done by UV–Vis, High resolution transmission electron microscope (HRTEM) and energy dispersive X-ray (EDX) spectroscopic study. The blue shift of surface plasmon resonance (SPR) band with increasing mole fraction of silver has been interpreted due to dampening of core, i.e. Au SPR by Ag. The dependence of nonlinear optical response of spherical core @ shell nanoparticles has been investigated as a function of relative composition of each metal. Simulation of SPR extinction spectra based on quasi-static theory is done. A comparison of our experimental and the simulated extinction spectra using quasi-static theory of nanoshell suggests that our synthesized bimetallic particles have core @ shell structure rather than bimetallic alloy particles.     

Observation of enhanced two-photon absorption in ZnS quantum dots at an ultraviolet wavelength

Two-photon absorption(2PA) in zinc sulphide(ZnS) and Mn2+-doped ZnS quantum dots is reported by the z-scan technique,with nanosecond pulsed laser radiation at 355 nm.The observed values of the 2PA cross section of all the samples are 105 times larger than that of bulk ZnS.     

Synthesis of silver nanoparticle in imidazolium and pyrolidium based ionic liquid reverse micelles: A step forward in nanostructure inorganic material in room temperature ionic liquid field

Two silver salts, silver tetrafluoroborate and silver trifluoromethanesulfonate were dissolved in two different room temperature ionic liquids (RTILs), 1-Butyl-3 methyl imidazolium tetrafluoroborate ([Bmim][BF₄]) and 1-Butyl-1 methyl pyrrolidinium trifluoromethanesulfonate ([Bmpy][Tfms]). Triton x-100 (TX-100) surfactant and cyclohexane as nonpolar medium are used to dissolve these RTILs to create reverse micellar system. Pure reverse micellar system is characterized by dynamic light scattering (DLS) measurement. These pure RTIL reverse micellar systems are used to prepare stable silver nanoparticle solution without using any other auxiliary solvent in the whole process.