Electromagnetic bubble generation by half-cycle pulses

Electromagnetic (EM) bubbles (EMB's), unipolar, super-short, and intense nonoscillating solitary pulses of EM radiation, can be generated in a gas of nonlinear atoms by available half-cycle pulses (HCP's). We investigate how EMB's characteristics (amplitude, length, formation distance, and total number) are controlled by the amplitude and length of originating HCP's. We also predict shocklike wave fronts in the multibubble regime.     

High-dynamic-range laser-pulse-contrast measurement with a plasma-shuttered streak camera

The dynamic range of a picosecond visible streak camera has been improved by the combination of a plasma shutter and multishot averaging performed with a photoconductive switch sweep circuit. We use this technique to measure the contrast of a 100-fs laser pulse over 2 ns with a dynamic range of 7 orders of magnitude.     

Photo-oxidation of polymers without light: Oxidation of polybutadiene and an ABS polyblend with singlet oxygen

In recent years much evidence has been accumulated to implicate electronically excited oxygen (¹Δ_g) molecules as the agent responsible in photosensitized oxidations for the formation of allylic hydroperoxides from olefins and of endoperoxides from 1,3-dienes. Little regarding the mechanistic aspects of the photo-oxidative degradation of polybutadiene (PBD) is known, however. To determine if electronically excited oxygen (¹Δ_g) molecules can oxidize PBD, the ABS polyblend and standard samples of PBD's containing high trans, high cis, and high vinyl content were treated in homogeneous solution at low temperature with chemically produced singlet oxygen in situ. The source of the singlet oxygen was the triphenylphosphite-ozone adduct. Studies by spectroscopy, elemental analysis, viscosity determinations, and gel measurements showed only the cis- and the trans-PBD were susceptible to oxidation; no chain scission was involved in the attack of cis- and trans-PBD by singlet oxygen; the oxidation of the cis PBD involved the initial formation of hydroperoxides which on thermal decomposition yielded gel. The trans-PBD was found to oxidize but apparently by a mechanism different from that of cis-PBD. Initial singlet oxygen attack of ABS proceeds by oxidation of the PBD portion of the polyblend. It was also observed that when only a small amount of the double bonds in the cis-PBD polymer had been oxidized to hydroperoxides, subsequent thermal treatment of this sample resulted in gross structural changes in the whole polymer.     

Facile generation of dialkylaminyl radicals from N-hydroxypyridine-2-thione carbamates. Application in kinetic studies of small ring cycloalkylaminyl radical ring openings

The preparation of N-hydroxypyridine-2-thione carbamates and the formation of dialkylaminyl radicals from these species are described and the application of this methodology in kinetic studies of aminyl radical rearrangements is demonstrated.     

The influence of external fields on magnetically ordered FeCl3·6H2O

Mössbauer resonance has been used to study a single crystal of FeCl₃·6H₂O at 1.16°K in external magnetic fields. The spectra exhibit pronounced splittings of the resonance lines at low field, followed by compression and then expansion of the resonance spectrum with increasing field. The results demonstrate two unique sets of ionic spins, and a gradual transformation from an antiferromagnetic state to magnetically induced paramagnetism.     

Probing the interaction of HTI-286 with tubulin using a stilbene analogue

HTI-286 is a synthetic analogue of the natural product hemiasterlin. HTI-286 is a potent antitumor agent that induces tubulin oligomerization. To investigate the binding stoichiometry and the binding site during this ligand-induced tubulin association, we synthesized an analogue of HTI-286 containing the chromophore stilbene. Using the distinct absorbance of the stilbene analogue, we determined the amounts of inhibitors bound to different tubulin oligomers by analytical ultracentrifugation. Herein we describe our findings based on these experiments. At the ratio of inhibitor to protein equal to or greater than 1, the stilbene analogue induces oligomerization of tubulin to a ring structure. The binding stoichiometry in the ring is one inhibitor per tubulin monomer (defined as an alpha/beta-heterodimer). At the ratio of inhibitor to protein less than 1, tubulin forms multiple intermediates, with the binding stoichiometry less than one inhibitor per tubulin monomer for all intermediates. The stable complex between the inhibitor and tubulin monomer was not detected under our experimental conditions. The binding site of the stilbene analogue does not overlap with the classic tubulin-binding agent, colchicine.