Stiffer optical tweezers through real-time feedback control

Using real-time re-programmable signal processing we connect acousto-optic steering and back-focal-plane interferometric position detection in optical tweezers to create a fast feedback controlled instrument. When trapping 3 µm latex beads in water we find that proportional-gain position-clamping increases the effective lateral trap stiffness ~13-fold. A theoretical power spectrum for bead fluctuations during position-clamped trapping is derived and agrees with the experimental data. The loop delay, ~19 µs in our experiment, limits the maximum achievable effective trap stiffness

RNA packaging motor: From structure to quantum mechanical modelling and sequential-stochastic mechanism

The bacteriophages of the Cystoviridae family package their single stranded RNA genomic precursors into empty capsid (procapsids) using a hexameric packaging ATPase motor (P4). This molecular motor shares sequence and structural similarity with RecA-like hexameric helicases. A concerted structural, mutational and kinetic analysis helped to define the mechanical reaction coordinate, i.e. the conformational changes associated with RNA translocation. The results also allowed us to propose a possible scheme of coupling between ATP hydrolysis and translocation which requires the cooperative action of three consecutive subunits. Here, we first test this model by preparing hexamers with defined proportions of wild type and mutant subunits and measuring their activity. Then, we develop a stochastic kinetic model which accounts for the catalytic cooperativity of the P4 hexamer. Finally, we use the available structural information to construct a quantum-chemical model of the chemical reaction coordinate and obtain a detailed description of the electron density changes during ATP hydrolysis. The model explains the results of the mutational analyses and yields new insights into the role of several conserved residues within the ATP binding pocket. These hypotheses will guide future experimental work

First derivative potentiometric and spectrophotometric determinations of cimetidine in tablets

Cimetidine is an imidazole derivative, which acts as H2-receptor antagonist to inhibit gastric acid secretion and is thereby useful in the treatment of ulcers and other hyperacidity stomach disorders. This study sets out to provide simple, inexpensive and sensitive analytical techniques for the assay of cimetidine in tablet dosage forms using first derivative potentiometric and UV-spectrophotometric techniques, with a view to providing simple, sensitive and cost-effective analytical methods. Ten brands of cimetidine tablets were purchased from pharmacies in Yenagoa and Port Harcourt, Niger Delta region of Nigeria. The samples were assayed by Potentiometric and UV spectrophotometric methods after a Thin Layer Chromatographic (TLC) fingerprint. The potentiometric assay results of the samples ranged from 92-100% of stated amount for 70% of the samples. The coefficient of variation for In-between run, Intra-day run and accuracy of the UV spectrophotometric method was within 3%. The percentage purity of cimetidine from UV determination at 260 nm ranged from 86 - 97%, showing that 50% of the samples conformed to the stated standard – ie: contain 95- 105% of stated amount. The TLC fingerprints of the samples were similar in many respects, including R<sub>f</sub> values which ranged from 0.8 to 0.9 compared to the reference sample with R<sub>f</sub> value of 0.9. The results suggest that either of the two methods can be fine-tuned for assay of cimetidine in tablet formulations.Keywords: Potentiometry, ultraviolet (UV) spectrophotometry, analytical methods, cimetidine, Tablet Formulations, volumetric standard (VS

Physicochemical and heavy metal analysis of well water obtained from selected settlements around Dangote cement factory in Gboko, Nigeria

Drinking water in developing countries especially Nigeria is susceptible to toxins as a result of effluents and pollutants. This study investigated the physicochemical characteristics and heavy metal contents of water samples collected from six different hand-dug wells from some settlements around Dangote Cement Plant Gboko. The concentration of all the physicochemical parameter studied were generally below maximum limits prescribed by WHO except for turbidity which was found in two of the samples opposite the factory (OF) and Tarhembe (T) to be 51.8 NTU and 17.5 NTU respectively which were higher than the maximum limit of (5.0 NTU) set by WHO. Total metal digestion of water samples was carried out using concentrated nitric acid (HNO3). The digests were analyzed for heavy metals (Cd, Cr, Cu, Fe, Mn, Pb, and Zn.) using AAS. Results showed that Cd, Cr and Pb in all the water samples were not detected, the concentrations of Cu and Zn ranged from 0.01-0.05 mgL-1 and 0.08-0.13 mgL-1 respectively and were below the permissible limit set by WHO. The concentration of Fe and Mn ranged from 0.25-1.16 mgL-1 and 0.33-0.92 mgL-1 respectively and were found to be above the permissible limits recommended by WHO for drinking water. In this study, contaminants level were generally below the WHO maximum recommended limits except Fe and Mn contents which showed elevated values above the recommended levels, hence the need to put mechanisms in place to control the levels of these heavy metals in the well water sources around the area under study.Keywords: Atomic Absorption Spectrophotometer, Heavy metals, Ground water, WH

Contextualisation of Data Flow Diagrams for security analysis

Data flow diagrams (DFDs) are popular for sketching systems for subsequent threat modelling. Their limited semantics make reasoning about them difficult, but enriching them endangers their simplicity and subsequent ease of take up. We present an approach for reasoning about tainted data flows in design-level DFDs by putting them in context with other complementary usability and requirements models. We illustrate our approach using a pilot study, where tainted data flows were identified without any augmentations to either the DFD or its complementary models

Detection of intermediates and kinetic control during assembly of bacteriophage P22 procapsid

Bacteriophage P22 serves as a model for the assembly and maturation of other icosahedral double-stranded DNA viruses. P22 coat and scaffolding proteins assemble in vitro into an icosahedral procapsid, which then expands during DNA packaging (maturation). Efficient in vitro assembly makes this system suitable for design and production of monodisperse spherical nanoparticles (diameter ≈50 nm). In this work we explore the possibility of controlling the outcome of assembly by scaffolding protein engineering. The scaffolding protein exists in monomer-dimer-tetramer equilibrium. We address the role of monomers and dimers in assembly by using three different scaffolding proteins with altered monomer-dimer equilibrium (weak dimer, covalent dimer, monomer). The progress and outcome of assembly was monitored by time-resolved X-ray scattering which allowed us to distinguish between closed shells and incomplete assembly intermediates. Binding of scaffolding monomer activates the coat protein for assembly. Excess dimeric scaffolding protein resulted in rapid nucleation and kinetic trapping yielding incomplete shells. Addition of monomeric wild type scaffold with excess coat protein completed these metastable shells. Thus, the monomeric scaffolding protein plays an essential role in the elongation phase by activating the coat and effectively lowering its critical concentration for assembly

Organizational Mortality of Small Firms: The Effects of Entrepreneurial Age and Human Capital

This paper addresses the issue of internal determination of organizational outcomes. It is argued that in small and simply structured organizations a considerable proportion of the variance in organizational activities and outcomes is associated with individuals. In particular, the paper uses human capital theory to derive hypotheses about individual determinants of organizational mortality. These hypotheses are tested with event-history data of firm registrations and de-registrations in a West German region. The hypotheses are corroborated by the data, but the effects may nonetheless be due to processes linking individual characteristics with organizational performance other than those suggested by the human capital approach