Spatial symmetry breaking in the Belousov-Zhabotinsky reaction with light-induced remote communication

Domains containing spiral waves form on a stationary background in a photosensitive Belousov-Zhabotinsky reaction with light-induced alternating nonlocal feedback. Complex behavior of colliding and splitting wave fragments is found with feedback radii comparable to the spiral wavelength. A linear stability analysis of the uniform stationary states in an Oregonator model reveals a spatial symmetry breaking instability. Numerical simulations show behavior in agreement with that found experimentally and also predict a variety of other new patterns.     

Studies related to the total synthesis of pentostatin: An efficient,regiospecific glycosylation of 6,7-dihydroimidazo[4,5-d][1,2]diazepin-8(3H)-one and related homologs.

A much improved glycosylation procedure for fhe synthesis of pentostatin-like nucleosides is described.     

Anticancer anthrapyrazoles. Improved syntheses of clinical agents CI-937, CI-941, and piroxantrone hydrochloride

Improved processes for the synthesis of bulk quantities of the anthrapyrazole clinical agents CI-937, CI-941, and piroxantrone hydrochloride are reported. Reported also are detailed analytical and spectroscopic data for these agents and intermediates of the synthetic sequences.     

Transverse coupling of chemical waves

The transverse coupling of chemical waves is investigated using a model scheme for excitable media. Chemical waves supported on the surfaces of a semipermeable membrane couple via diffusion through the membrane, resulting in new types of spatiotemporal behavior. The model studies show that spontaneous wave sources may develop from interacting planar waves, giving rise to a complex sequence of patterns accessible only by perturbation. Coupled circular waves result in the spontaneous formation of spiral waves, which subsequently develop patterns in distinct domains with characteristic features. The long time entrainment behavior of coupled spiral waves reveals regions of 1:2 phase locking.     

Bridgehead carbocations via carbene fragmentation: erasing a 10(10) kinetic preference

1-Norbornyloxychlorocarbene (1-NorOCCl), 1-bicyclo[2.2.2]octyloxychlorocarbene (1-BcoOCCl), and 1-adamantyloxychlorocarbene (1-AdOCCl) were generated in dichloroethane (DCE) by photolysis of the appropriate diazirines. The exclusive product in each case was the bridgehead alkyl chloride formed by fragmentation of the carbene to [R(+) OC Cl(-)] ion pairs, loss of CO, and cation-anion collapse. In mixtures of methanol and DCE, each carbene gave three products: RCl, ROH, and ROMe. RCl and ROMe resulted from competition between ion pair collapse and methanol capture of the cation. ROH resulted from methanol capture of the carbene (before fragmentation), followed by eventual methanolysis and hydrolysis of ROCH(Cl)OMe. The ratios of carbene capture to carbene fragmentation fell in the order 1-NorOCCl > BcoOCCl > 1-AdOCCl; 1-Nor(+) was the least stable cation and the slowest to form by fragmentation, so that this carbene was the most readily captured. This trend was accentuated in methanol-pentane mixtures, where ionic fragmentation was further slowed in the less polar solvent. Laser flash photolysis with either UV or time-resolved infrared (TRIR) monitoring permitted the determination of the absolute rate constants for fragmentations of the carbenes in DCE at 25 degrees C. The rate constants (s(-1)) were: 1-NorOCCl (3.3 x 10(4)), 1-BcoOCCl (1.5 x 10(5)), and 1-AdOCCl (5.9 x 10(5)). The rate constants decreased in the order of increasing strain in the resulting bridgehead carbocation, but the range of rate constants was compressed to a factor of only approximately 18. This constrasts with the factor of 10(10) by which the acetolysis of 1-AdOTs at 70 degrees C exceeded that of 1-NorOTs. The fragmentation of 1-NorOCCl to the ion pair was 3 x 10(15) times faster than the acetolysis of 1-NorOTs. The activation energies were measured as 9.0 kcal/mol (log A = 11.2 s(-1)) for the fragmentation of 1-NorOCCl and 4.4 kcal/mol (log A = 8.44 s(-1)) for that of 1-BcoOCCl both in DCE. B3LYP/6-31G computed activation energies in simulated DCE were 14.6 and 2.7 kcal/mol, respectively.     

Synthesis of azide congeners of preQ1 as potential substrates for tRNA guanine transglycosylase

PreQ₁ ( 2 ) is a precursor of queuine ( 1 ) that in eubacteria is incorporated into transfer RNA (tRNA) by tRNA guanine transglycosylase (TGT) before being further elaborated into queuine. The queuine modification is unusual and occurs across all eukaryotes and eubacteria with few exceptions, but its function remains unclear. As the modified nucleotide occurs through incorporation of a specially synthesized nucleotide instead of via modification of a genetically encoded base, a study of the sites of modification by prepared probes is possible. We report the synthesis of two novel azide congeners ( 3 , 4 ) of preQ₁ for this purpose. The evaluation of their interaction with TGT shows that both probes act as weak competitive inhibitors of guanine exchange of guanine(34) tRNA^Tyr with a K_i of ~70 μM. However, we could not show that these are substrates for TGT-catalyzed incorporation into tRNA. We believe the reason for this is a marked loss of binding due to the azide functionality of 3 and 4 abrogating the possibility of two hydrogen bonds to the carbonyl group of Leu231 and Met260 of TGT, previously observed for the terminal methylene amine of preQ₁ by x-ray crystallography.     

Electrochemical investigation of magnetite-carbon nanocomposite in situ grown on nickel foam as a high-performance binderless pseudocapacitor

Journal of Solid State Electrochemistry - Magnetite-carbon nanocomposite was grown in situ on nickel foam by a novel auto-combustion method, and it was investigated for application as a...     

Single Phase Flow in Partially Fissured Media

Totally fissured media in which the individual cells are isolated by the fissure system are effectively described by double porosity models with microstructure. Such models contain the geometry of the individual cells in the medium and the flux across their interface with the fissure system which surrounds them. We extend these results to a dual-permeability model which accounts for the secondary flux arising from direct cell-to-cell diffusion within the solid matrix. Homogenization techniques are used to construct a new macroscopic model for the flow of a single phase compressible fluid through a partially fissured medium from an exact but highly singular microscopic model, and it is shown that this macroscopic model is mathematically well posed. Preliminary numerical experiments illustrate differences in the behaviour of solutions to the partially fissured from that of the totally fissured case.