Pulsed microwave induced bioeffects

High-power pulsed microwave radiation, when applied to solutions containing dissolved carbon dioxide (or bicarbonate), hydrogen peroxide, and the soluble organic semi-conductor diazoluminomelanin, generates sound, pulsed luminescence, and electrical discharge. Microbes exposed to these phenomena experienced damage comparable to short-time, high-temperature insults, even though the average and measurable localized temperatures were insufficient to cause the observed effects     

Enantioselective total synthesis and confirmation of the absolute and relative stereochemistry of streptorubin B

The enantioselective total synthesis of the pyrrolophane natural product streptorubin B is described. Key steps in the concise route include the application of a one-pot enantioselective aldol cyclization/Wittig reaction and an anionic oxy-Cope rearrangement to forge the crucial 10-membered ring. Comparisons between CD spectra of synthetic and natural samples of streptorubin B coupled with X-ray crystallography allowed for the determination of the absolute stereochemistry of this natural product for the first time. These studies also provided unambiguous proof of the relative configuration between the butyl side chain and the bispyrrole subunit. Additional studies revealed a novel atropstereoselective Paal-Knorr pyrrole condensation and provided fundamental experimental insight into the barrier for atropisomerization of the natural product.     

Coupled ultrafast lattice and polarization dynamics in ferroelectric nanolayers

We report the first analysis of the polarization and lattice dynamics in a metal/ferroelectric/metal nanolayer system by femtosecond x-ray diffraction. Two Bragg reflections provide information on the coupled dynamics of the two relevant phonon modes for ferroelectricity in perovskites, the tetragonal distortion and the soft mode. Optical excitation of the SrRuO(3) metal layers generates giant stress (>1 GPa) compressing the PbZr(0.2)Ti(0.8)O(3) layers by up to 2%. The resulting change of tetragonality reaches a maximum after 1.3 ps. As a result, the ferroelectric polarization P is reduced by up to 100% with a slight delay that is due to the anharmonic coupling of the two modes.     

Characterization of stimuli-sensitive polymers for biomedical applications

Stimuli-sensitive polymers were synthesized by copolymerizing varying ratios of N-isopropyl acrylamide(NIPAAm) and acrylic acid(AAc). The influence of polyelectrolytes on the lower critical solution temperatures(LCSTs) of these temperature/pH sensitive polymers was investigated in the pH range of 2-12. Polyelectrolyte complexes were prepared by mixing poly(NIPAAm-co-AAc) as anionic polyelectrolyte with poly(allyl amine)(PAA) or poly(L-lysine)(PLL) as cationic polyelectrolytes, respectively. Back titration was performed to determine the pK_a values of PAAc in poly(NIPAAm-co-AAc) and to study the effect of comonomer ionization on the cloud point temperature. The effect of polyelectrolyte complex formation on the conformation of PLL was studied as a function of temperature by means of circular dichroism(CD). The swelling ratio of poly(NIPAAm-co-AAc) hydrogels as a function of pH at various temperature was obtained by measuring the weight of the hydrogels in buffer solutions. The LCSTs of the poly(NIPAAm-co-AAc) were strongly affected by pH, polyelectrolyte solutes, AAc content, and charge density. The influence of more hydrophobic PLL as a polyelectrolyte on the cloud point of PNIPAAm/water in the copolymer was stronger than that of poly(allyl amine)(PAA). Indomethacin was loaded into these hydrogels, and controlled release of this molecule from the hydrogel was determined under various temperature and pH conditions using UV/Vis spectrophotometry.     

A new incorporation mechanism for trivalent actinides into bioapatite: a TRLFS and EXAFS study

One of the most toxic byproducts of nuclear power and weapons production is the transuranics, which have a high radiotoxicity and long biological half-life due to their tendency to accumulate in the skeletal system. This accumulation is inhomogeneous and has been associated with the chemical properties and structure of the bone material rather than its location or function. This suggests a chemical driving force to incorporation and requires an atomic scale mechanistic understanding of the incorporation process. Here we propose a new incorporation mechanism for trivalent actinides and lanthanides into synthetic and biologically produced hydroxyapatite. Time-resolved laser fluorescence spectroscopy and extended X-ray absorption fine structure have been used to demonstrate that trivalent actinides and lanthanides incorporate into the amorphous grain boundaries of apatite. This incorporation site can be used to explain patterns in uptake and distribution of radionuclides in the mammalian skeletal system.     

Measuring the range of plasmonic interaction

When gold nanoparticles are covered with nanometric layers of transparent polyelectrolytes, the plasmon absorption spectrum A(λ) increases by a factor of approximately three and shifts to the red. These modifications of dissipative experimental observables stop when the cover layer thickness approaches the particle diameter. Spectral modifications of dispersive parameters like the reflection R, however, keep changing with increasing cover layer thickness. The shift of the plasmon resonance caused by two interacting particle layers is studied as a function of the separating distance between the two layers. We discuss these observations in the context of an effective medium theory and conclude that it can only be applied for a layer thickness on the order of the particle diameter.     

Unprecedented directing group ability of cyclophanes in arene fluorinations with diaryliodonium salts

For the first time it is shown that exceptionally electron-rich arene rings can be fluorinated exclusively during the reductive elimination reactions of diaryliodonium fluorides. The 5-methoxy[2.2]paracyclophan-4-yl directing group simultaneously reduces unproductive aryne chemistry and eliminates ligand exchange reactions by a combination of steric and electronic effects. Use of the cyclophane directing group permits an unprecedented degree of control in fluorination reactions of diaryliodonium salts.