Optical second harmonic spectroscopy of boron-reconstructed Si(001)

Optical second harmonic generation (SHG) spectroscopy is used to probe Si(001) following thermal decomposition of diborane at the surface. Incorporation of boron (B) at second layer substitutional sites at H-free Si(001) intensifies and redshifts the E1 SHG spectral peak, while subsequent H termination further intensifies and blueshifts E1, in sharp contrast to the effect of bulk B doping or nonsubstitutional B. Ab initio pseudopotential and semiempirical tight binding calculations independently reproduce these unique trends, and attribute them to the surface electric field associated with charge transfer to electrically active B acceptors, and rehybridization of atomic bonds.     

New stereoselective intramolecular

Efficient 1,4-asymmetric induction has been achieved in the highly stereocontrolled intramolecular [2 + 2] cycloadditions between ketenimines and imines, leading to 1,2-dihydroazeto[2, 1-b]quinazolines. The chiral methine carbon adjacent to the iminic nitrogen controls the exclusive formation of the cycloadducts with relative trans configuration at C2 and C8. The stepwise mechanistic model, based on theoretical calculations, fully supports the stereochemical outcome of these cycloadditions.     

Femtosecond growth dynamics of an underdense ionization frontmeasured by spectral blueshifting

Time-resolved spectral blue shifts of 100-fs laser pulses caused by ionization of atmospheric density N₂ and noble gases subjected to high (10¹⁴ W/cm² to 10¹⁶ W/cm²) light intensities are examined. Included are data for two experiments: self-shifting of the ionizing laser pulses for varying peak intensities, pressures (1-5 atm), and gas species; and time-resolved blue shifts of a weak copropagating probe pulse for the same range of ionization conditions. The self-shift data reveal a universal, reproducible pattern in the shape of the blueshifted spectra: as laser intensity, gas pressure, or atomic number increase, the self-blueshifted spectra develop from a near replica of the incident pulse spectrum into a complex structure consisting of two spectral peaks. The time-resolved data reveal different temporal dependence for each of these two features. A quantitative model for a simplified cylindrical focal geometry is presented     

Rapid estimation of catecholamines, octopamine and 5-hydroxytryptamine in biological tissues using high-performance liquid chromatography with coulometric detection

A rapid, convenient procedure is described for the simultaneous determination of catecholamines, monohydroxyphenolamines and 5-hydroxytryptamine in biological tissues. The procedure involves homogenization of tissue in perchloric acid, addition of heparin and centrifugation followed by direct injection of the supernatant onto a C18 reversed-phase high-performance liquid chromatographic column. The mobile phase employed sodium dodecyl sulfate as ion pair reagent with 20% acetonitrile and 10-12% methanol as organic modifier. Eluted fractions were detected electrochemically using dual coulometric electrodes operated in screen mode. The procedure has been applied to the analysis of norepinephrine, epinephrine, dopamine, octopamine, tyramine, 5-hydroxytryptamine and tryptophan in a variety of tissues including mammalian heart and brain and insect nerve cord.     

Microassay for N-acetyltransferase activity using high-performance liquid chromatography with electrochemical detection

A rapid, sensitive procedure has been developed for determination of N-acetyltransferase activity against octopamine, dopamine and 5-hydroxytryptamine. The assay, which is performed in a volume of 10 microliters, is based upon the separation and detection of monoamine substrates and their N-acetylated derivatives using high-performance liquid chromatography with electrochemical detection. The method has been used to measure N-acetyltransferase activity against octopamine, dopamine and 5-hydroxytryptamine in the cerebral ganglion of the American cockroach, Periplaneta americana and to study bi-substrate kinetics of the enzyme.