Microcionamides A and B, bioactive peptides from the philippine sponge Clathria (Thalysias) abietina

Microcionamides A (1) and B (2) have been isolated from the Philippine marine sponge Clathria (Thalysias) abietina. These new linear peptides are cyclized via a cystine moiety and have their C-terminus blocked by a 2-phenylethylenamine group. Their total structures, including absolute stereochemistry, were determined by a combination of spectral and chemical methods. Compound 1 was shown to slowly isomerize about the C-36/C-37 double bond when stored in DMSO. Microcionamides A (1) and B (2) exhibited significant cytotoxicity against the human breast tumor cells lines MCF-7 and SKBR-3 and displayed inhibitory activity against Mycobacterium tuberculosis H(37)Ra.     

A device for feline head positioning and stabilization during magnetic resonance imaging

Minimization of head movement and reproduction of standard head positions are essential for reliable brain functional magnetic resonance imaging. Devices for stabilization and alignment of feline preparations are not available currently. We describe a system that involves minimal surgery, allows for both acute and chronic atraumatic positioning, and has the potential to be used for unanesthetized animals. The device uses non-metallic materials and stabilizes the head by means of an apparatus that fixes the head with nylon screws and dental cement in the frontal sinuses. Application of the head-stabilizing device decreases head movements by more than a factor of ten. Anatomical images show that this device provides 3 dimensional head placement at a precision comparable to that of a stereotactic frame, i.e. within 1 mm.     

Origin of antimony segregation in GaInSb/InAs strained-layer superlattices

We show how cross-sectional scanning tunneling microscopy may be used to reconstruct the Sb segregation profiles in GaInSb /InAs strained-layer superlattices. These profiles are accurately described by a one-dimensional model parametrizing the spatial evolution of an Sb seed at the InAs-on-GaInSb interface in terms of two-anion-layer exchange. We argue that the segregation seed, which decreases from 2 / 3 to 1 / 2 monolayer when growth conditions are made less anion rich, has its origin in the Sb-bilayer reconstruction maintained during GaInSb epitaxy.     

选择正确的MOSFET:工程师所需要知道的

随着制造技术的发展和进步,系统设计人员必须跟上技术的发展步伐,才能为其设计挑选最合适的电子器件.MOSFET是电气系统中的基本部件,工程师需要深入了解它的关键特性及指标才能做出正确选择.本文将讨论如何根据RDS(ON)、热性能、雪崩击穿电压及开关性能指标来选择正确的MOSFET.     

Automated peak width measurements for targeted analysis of ion mobility unresolved species

Peak broadening in ion mobility (IM) is a relatively predictable process and abnormally broad peaks can be indicative of the presence of unresolved species. Here, we introduce a new ion mobility peak fitting (IM_FIT) software package for automated and systematic determination of traveling wave ion mobility (TWIM) unresolved species. To identify IM unresolved species, the IM_FIT software generates a trend line by plotting ions' mobility peak widths as a function of their arrival times. Utilizing user-defined thresholds, IM_FIT allows for automated and rapid detection of ions that deviate from the peak width trend line. To demonstrate the advantages of IM_FIT for automated detection of IM unresolved species, IM-mass spectrometry (IM-MS) data from a sample mixture containing polypropylene glycol and multiple peptides were analyzed. A total of 14 out of the 34 observed singly-charged IM peaks above 5% relative abundance (i.e., signal-to-noise ratios above ∼200) were tagged as potentially co-eluting ions by IM_FIT. Subsequently, the 14 IM peaks tagged as potentially unresolved (presumably, peaks corresponding to co-eluting compounds), were further analyzed by automated IM deconvolution (AIMD), liquid chromatography-IM-MS (LC-IM-MS), and/or ultra-high resolution mass spectrometry. Using the aforementioned techniques, more than 85% of the tagged IM peaks (12 out of 14) were confirmed to contain co-eluting ions. As an additional new finding, IM_FIT facilitated the discovery of an unexpected sequence-scrambled y-type fragment ion.     

Elemental mapping and quantitative analysis of Cu, Zn, and Fe in rat brain sections by laser ablation ICP-MS

This report details the application of laser ablation quadrupole ICP-MS for the (multi)elemental mapping of 100-μm-thick sections of rat brain. The laser spot size used was 60 μm, and the laser scan speed was 120 μm s⁻¹. The analysis was relatively rapid, allowing mapping of a whole brain thin section (≈1 cm²) in about 2 h. Furthermore, the method was amenable to multi-element data collection including the physiologically important elements P and S and afforded sub μg g⁻¹ detection limits for the important trace elements Cu and Zn. Calibrations were performed with pressed pellets of biological certified reference materials, and the elemental distributions and concentrations of Cu, Zn, and Fe were determined in whole rat brain sections. The distributions and concentration ranges for these elements were consistent with previous studies and demonstrate the utility of this technique for rapid mapping of brain thin sections.     

Cryptocin, a potent tetramic acid antimycotic from the endophytic fungus Cryptosporiopsis cf. quercina

[formula: see text] The endophytic fungus Cryptosporiopsis cf. quercina produces cryptocin in culture. Among other fungi, this unique tetramic acid displays antimycotic activity against Pyricularia oryzae, the causal agent of rice blast disease. Cryptocin also possesses activity against a wide variety of plant pathogenic but not human pathogenic fungi. The fine rhomboid-like crystals of cryptocin allowed structural elucidation by X-ray crystallography. The importance of cryptocin to the symbiotic relationship of C. quercina to its hosts is briefly discussed.     

George Gamow: Scientific Amateur and Polymath

George Gamow (1904-1968) was among the first of the many brilliant scientists who forsook Europe for the United States in the early 1930s. Although most were fleeing the fascist imperium of Hitler and Mussolini, Gamow was one of a few who managed to escape the burgeoning despotism of Stalin in the Soviet Union. His early application of quantum mechanics to the atomic nucleus and his subsequent insight into the role played by the physics of the atom and its nucleus in stars, galaxies, and the universe identifies him as a scientist of unusual genius. Gamow displayed a boisterous, infectious - almost Rutherfordian - interest in all aspects of pure science. His interests were broad and his industry prodigious. His scientific output covered areas as diverse as nuclear physics, astrophysics, cosmology, biological genetics, and the fascinating question of the relationship of the large-scale structure and development of the universe to the properties of elementary particles and fields. He also was an immensely imaginative and prolific author of popular expositions on scientific subjects. One who is as well-known for his authorship of the Mr. Tompkins series of science popularizations as for his contributions to the development of the physical consequences of the big-bang theory of the expanding universe and the prediction of the cosmic background radiation must be unique in the scientific pantheon.     

A Comparative Study of Electrochemical Reduction of 4‐Nitrophenyl Covalently Grafted on Gold and Carbon

4‐Nitrophenyl layers were grafted on gold and glassy carbon surfaces by electrochemical reductive adsorption of the corresponding diazonium salt. Electrochemical conversion efficiencies of 4‐nitrophenyl moieties to 4‐aminophenyl moieties on gold versus on glassy carbon in a protic medium were investigated using X‐ray photoelectron spectroscopy (XPS). In total contrast to all previous comparative studies showing greater electrochemical reactivity of aryl diazonium salt‐derived layers on gold than on glassy carbon, a much lower rate of conversion to 4‐aminophenyl was observed on gold than on glassy carbon by both cyclic voltammetry (CV) and chronoamperometry (CA) methods. The lower electron transfer rate during conversion observed on gold versus glassy carbon was proposed to be due to a mechanism related to the molecular structure rearrangement of 4‐nitrophenyl during the process on glassy carbon. However, whilst complete conversion of 4‐nitrophenyl to 4‐aminophenyl on gold by chronoamperometry was achieved, on glassy carbon complete reduction could not be achieved under the same conditions.