The onset of coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.     

Uniform molecular analysis using femtosecond laser mass spectrometry

The potential of femtosecond laser time-of-flight mass spectrometry (FLMS) for uniform quantitative analysis of molecules has been investigated. Various samples of molecular gases and vapours have been studied, using ultra-fast ( approximately 50 fs) laser pulses with very high intensity (up to 1.6 x 10(16) Wcm(-2)) for non-resonant multiphoton ionisation/tunnel ionisation. Some of these molecules have high ionisation potentials, requiring up to ten photons for non-resonant ionisation. The relative sensitivity factors (RSF) have been determined as a function of the laser intensity and it has been demonstrated that for molecules with very different masses and ionisation potentials, uniform ionisation has been achieved at the highest laser intensities. Quantitative laser mass spectrometry of molecules is therefore a distinct possibility. Copyright 1999 John Wiley & Sons, Ltd.     

Theoretical modelling of frequency dependent elastic loss in composite piezoelectric transducers

The large number of degrees of freedom in the design of piezoelectric transducers requires a theoretical model that is computationally efficient so that a large number of iterations can be performed in the design optimisation. The materials used are often lossy, and indeed loss can be used to enhance the operational characteristics of these designs. Motivated by these needs, this paper extends the one-dimensional linear systems model to incorporate frequency dependent elastic loss. The reception sensitivity, electrical impedance and electromechanical coupling coefficient of a 1–3 composite transducer, with frequency dependent loss in the polymer filler, are investigated. By plotting these operating characteristics as a function of the volume fraction of piezoelectric ceramic an optimum design is obtained. A device with a non-standard, high shear attenuation polymer is also simulated and this leads to an increase in the electromechanical coupling coefficient. A comparison with finite element simulations is then performed. This shows that the two methods are in reasonable agreement in their electrical impedance profiles in all the cases considered. The plots are almost identical away from the main resonant peak where the frequency location of the peaks are comparable but there is in some cases a 20% discrepancy in the magnitude of the peak value and in its bandwidth. The finite element model also shows that the use of a high shear attenuation polymer filler damps out the unwanted, low frequency modes whilst maintaining a reasonable impedance magnitude.     

QM/MM modeling of compound I active species in cytochrome P450, cytochrome C peroxidase, and ascorbate peroxidase

QM/MM calculations provide a means for predicting the electronic structure of the metal center in metalloproteins. Two heme peroxidases, Cytochrome c Peroxidase (CcP) and Ascorbate Peroxidase (APX), have a structurally very similar active site, yet have active intermediates with very different electronic structures. We review our recent QM/MM calculations on these systems, and present new computational data. Our results are in good agreement with experiment, and suggest that the difference in electronic structure is due to a large number of small differences in structure from one protein to another. We also discuss recent QM/MM calculations on the active species of cytochrome P450, in which a similar sensitivity of the electronic structure to the environment is found. However, this does not appear to explain different catalytic profiles of the different drug-metabolizing isoforms of this class of enzyme.     

A concise total synthesis of salinosporamide A

A concise and straightforward 14-step total synthesis of (+/-)-salinosporamide A, based on a diastereoselective acid-catalysed intramolecular cyclisation of to the pyrrolidinone , and a regioselective reduction of the malonate derivative 8b to the aldehyde 9, is described.     

Diffusion and convection of gaseous and fine particulate from a chimney

^** Email: ajm{at}maths.strath.ac.uk Particle dispersion from a high chimney is considered and anexpression for the subsequent concentration of the particulatedeposited on the ground is derived. We consider the generalcase wherein the effects of both diffusion and convection onthe steady state ground concentration of particulate are incorporated.Two key parameters emerge from this analysis: the ratio of diffusionto convection and the nondimensionalised surface mass transferrate. We also solve the inverse problem of recovering thesetwo parameters given the boundary concentration profile andprovide an estimate of the concentration flux above the chimneystack.     

Isomerization Study of Aryltropane Analogs of Cocaine

Isomerization of brain imaging agents aryltropane cocaine analogs have been examined by HPLC under chemical conditions similar to those encountered during radiolabeling.     

Voltage-pulsed and laser-pulsed atom probe tomography of a multiphase high-strength low-carbon steel

The differences in artifacts associated with voltage-pulsed and laser-pulsed (wavelength = 532 or 355 nm) atom-probe tomographic (APT) analyses of nanoscale precipitation in a high-strength low-carbon steel are assessed using a local-electrode atom-probe tomograph. It is found that the interfacial width of nanoscale Cu precipitates increases with increasing specimen apex temperatures induced by higher laser pulse energies (0.6-2 nJ pulse(-1) at a wavelength of 532 nm). This effect is probably due to surface diffusion of Cu atoms. Increasing the specimen apex temperature by using pulse energies up to 2 nJ pulse(-1) at a wavelength of 532 nm is also found to increase the severity of the local magnification effect for nanoscale M2C metal carbide precipitates, which is indicated by a decrease of the local atomic density inside the carbides from 68 ± 6 nm(-3) (voltage pulsing) to as small as 3.5 ± 0.8 nm(-3). Methods are proposed to solve these problems based on comparisons with the results obtained from voltage-pulsed APT experiments. Essentially, application of the Cu precipitate compositions and local atomic density of M2C metal carbide precipitates measured by voltage-pulsed APT to 532 or 355 nm wavelength laser-pulsed data permits correct quantification of precipitation.     

Triterpenoid acids and lactones from the leaves of Fadogia tetraquetra var. tetraquetra (Rubiaceae)

Four triterpenoids isolated from the leaves of Fadogia tetraquetra var. tetraquetra, 3beta-hydroxy-11alpha, 12alpha-epoxyoleanan-28,13beta-olide (1), 3beta-hydroxyurs-11-en-28,13beta-olide (2), oleanolic acid (3), and ursolic acid (4), were evaluated for their antiviral and antibacterial properties. Compound 4 showed potent activity against the Semliki Forest virus with an IC50 of 14.7 microM, but was also found to be significantly cytotoxic (68% reduction in cell viability after 24 hours exposure at 50 microM) towards baby hamster kidney (BHK21) host cells. A viability assay on the mammalian human hepatocellular carcinoma (Huh-7) cell line showed no significant effects on intracellular ATP content after 48 hours exposure to compounds 1-4 at this concentration. Compound 4 also inhibited Staphylococcus aureus (MIC 12.5 microM), but was inactive against Enterobacter aerogenes, Escherichia coli, and Pseudomonas aeruginosa. Compounds 1-3 were inactive against all tested bacterial strains at 50 microM concentration.