Dynamics of low temperature induced water shedding from AOT reverse micelles

The effects of low temperature and ionic strength on water encapsulated within reverse micelles were investigated by solution NMR. Reverse micelles composed of AOT and pentane and solutions with varying concentrations of NaCl were studied at temperatures ranging from 20 degrees C to -30 degrees C. One-dimensional (1)H solution NMR spectroscopy was used to monitor the quantity and structure of encapsulated water. At low temperatures, e.g., -30 degrees C, reverse micelles lose water at rates that are dependent on the ionic strength of the aqueous nanopool. The final water loading (w0 = [water]/[surfactant]) of the reverse micelles is likewise dependent on the ionic strength of the aqueous phase. Remarkably, water resonance(s) at temperatures between -20 degrees C and -30 degrees C displayed fine structure indicating the presence of multiple transient water populations. Results of this study demonstrate that reverse micelles are an excellent vehicle for studies of confined water across a broad range of conditions, including the temperature range that provides access to the supercooled state.     

Fast local backbone dynamics of encapsulated ubiquitin

Backbone dynamics of ubiquitin confined within AOT reverse micelles have been evaluated based on analysis of 15N NMR relaxation data. Results indicate that upon encapsulation the protein experiences a slight overall increase in the value of the order parameter, S2, indicating a restriction in the average amplitude of fast local N-H bond vector motion. The largest increases in S2 upon encapsulation were concentrated in the region of beta-sheet 2 and, additionally, at the transitions of secondary structure motifs and loop regions. In addition, statistical analysis of the residue average ratio of the 15N longitudinal and transverse NMR relaxation time constants indicates that chemical exchange contributions to relaxation are consistent with previous aqueous studies. Earlier studies have demonstrated that native protein structure can be maintained in the encapsulated state. These results presented here establish that the dynamical behavior of encapsulated ubiquitin is likewise nativelike and adds important new observations regarding the enhancement of protein stability under confinement.     

Significant parameters in the optimization of MALDI-TOF-MS for synthetic polymers

One of the most significant issues in any analytical practice is optimization. Optimization and calibration are key factors in quantitation. In matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), instrument optimization is a limitation restricting quantitation. An understanding of the parameters that are most influential and the effects of these parameters on the mass spectrum is required for optimization. This understanding is especially important when characterizing synthetic polymers by MALDI-TOF-MS, due to the breadth of the polymer molecular mass distribution (MMD). Two considerations are important in quantitation, additivity of signal and signal-to-noise (S/N). In this study, the effects of several instrument parameters were studied using an orthogonal experimental design to understand effects on the signal-to-noise (S/N) of a polystyrene distribution. The instrument parameters examined included detector voltage, laser energy, delay time, extraction voltage, and lens voltage. Other parameters considered were polymer concentration and matrix. The results showed detector voltage and delay time were the most influential of the instrument parameters for polystyrene using all trans-retinoic acid (RA) as the matrix. These parameters, as well as laser energy, were most influential for the polystyrene with dithranol as the matrix.     

Magnetic Relaxometry with an Atomic Magnetometer and SQUID Sensors on Targeted Cancer Cells

Magnetic relaxometry methods have been shown to be very sensitive in detecting cancer cells and other targeted diseases. Superconducting Quantum Interference Device (SQUID) sensors are one of the primary sensor systems used in this methodology because of their high sensitivity with demonstrated capabilities of detecting fewer than 100,000 magnetically-labeled cancer cells. The emerging technology of atomic magnetometers (AM) represents a new detection method for magnetic relaxometry with high sensitivity and without the requirement for cryogens. We report here on a study of magnetic relaxometry using both AM and SQUID sensors to detect cancer cells that are coated with superparamagnetic nanoparticles through antibody targeting. The AM studies conform closely to SQUID sensor results in the measurement of the magnetic decay characteristics following a magnetization pulse. The AM and SQUID sensor data are well described theoretically for superparamagnetic particles bound to cells and the results can be used to determine the number of cells in a cell culture or tumor. The observed fields and magnetic moments of cancer cells are linear with the number of cells over a very large range. The AM sensor demonstrates very high sensitivity for detecting magnetically labeled cells does not require cryogenic cooling and is relatively inexpensive.     

State and velocity distributions of Cl atoms produced in the photodissociation of ICI at 237 nm

Photofragment spectroscopy of ICI molecules photodissociated at 237 nm is studied by 2 + 1 resonance-enhanced multi-photon ionization and time of flight techniques. Doppler profiles of the chlorine atom fragments in two spin—orbit states show that chlorine atoms in the ground state, ²P_3/2, are produced from a perpendicular dissociative transition, and chlorine atoms in the excited state, ²P , arise from a parallel transition. The possible electronically excited states leading to dissociation in both the perpendicular and parallel cases are considered.     

A general differential technique for the determination of parameters for d(α)/dt=f(α)A exp (−E/RT)

A general differential method is developed and described which determines the Arrhenius parameters, energy of activation and the preexponential factor, as functions of degree of conversion from sets of two or more experiments with differing thermal programs. (These experiments may be performed at any combination of isothermal, constant heating rate or other temperature programs.) The method tests to see whether or not the kinetics follow the equation,f()=(1–)ⁿ, and calculates the correct reaction order, n, when such an equation is applicable. The correct energy of activation,E, is determined as a function of both temperature and conversion. The correct preexponential term,A, is calculated for all cases described by equation, d()/dt=f () A exp(–E/RT), except for the autocatalytic case in whichf()=0)=0. Calculation of parameters for equations involving other functions forf() will be described in a subsequent paper.

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Zusammenfassung Es wurde ein allgemeines Differentialverfahren entwickelt und beschrieben, das die Arrheniusschen Parameter, namentlich die Aktivierungsenergie und den präexponentiellen Faktor, als Funktion des Umsetzungsgrades aus Angaben über zwei oder mehrere Experimente mit verschiedenen Temperaturprogrammen beschreibt. (Diese Experimente können mit beliebigen Kombinationen aus isothermen Temperaturprogrammen, solchen mit konstanter Aufheizgeschwindigkeit oder anderen Temperaturprogrammen durchgeführt werden.) Das Verfahren überprüft, ob die Kinetik durch die Gleichungf()=(1–)ⁿ beschrieben werden kann und berechnet die richtige Reaktionsordnungn, wenn eine solche Gleichung anwendbar ist. Die richtige Aktivierungsenergie E wird als Funktion von Temperatur und Umsatz bestimmt. Der richtige präexponentielle Ausdruck wird für alle beschriebenen Fälle mittels der Gleichung d()/dt=f()A exp (-E/RT) berechnet. Eine Ausnahme bildet der Fall Autokatalyse, wobeif(=0)=0 gilt. Die Berechnung der Parameter für Gleichungen mit anderen Funktionen fürf() wird in einem späteren Manuskript beschrieben.

     

Erratum: transient pulsations of small gas bubbles in water [J. Acoust. Soc. Am. 84, 985-998 (1988)] [corrected and republished

Transient behavior of small gas bubbles in a liquid set into violent motion by ultrasonic pressure waves is of interest because of widespread use of microsecond pulses in diagnostic ultrasound. Such pulses contain only a few pressure cycles and the transient pulsations of bubbles set in motion by such pulses would determine the bubble-ultrasound interaction. A computer study has been made to obtain a global representation of the pulsation amplitudes R (t) of small gas bubbles (nuclei) in water during the first few cycles of a cw ultrasonic pressure. One objective was to obtain a better understanding of cavitation phenomena where many nuclei with initial radii Rn from 0.1-20 microns are set in motion at pressures ranging from 0.5-5 bars and at frequencies from 0.5-10 MHz. Results allowed construction of surfaces showing the relative bubble amplitude R/Rn as a function of Rn and of the time t/TA, where TA is the acoustic period. One finding is that, in the range of peak pressures found in diagnostic pulses, transient cavities would be generated during the first pressure cycle from nuclei with initial radii as small as a few microns (micron). Nuclei that grow into transient cavities in the first pressure cycle are here called "prompt" nuclei. At a specified pressure, the size range of radii Rn in which they occur decreases with increasing frequency. At 5 bars, the range of Rn for prompt nuclei is 0.166-11.35 microns at 0.5 MHz and vanishes at 10 MHz.     

Phase integral theory,coupled wave equations,and mode conversion

Phase integral or WKB theory is applied to multicomponent wave equations, i.e., wave equations in which the wave field is a vector, spinor, or tensor of some kind. Specific examples of physical interest often have special features that simplify their analysis, when compared with the general theory. The case of coupled channel equations in atomic or molecular scattering theory in the Born-Oppenheimer approximation is examined in this context. The problem of mode conversion, also called surface jumping or Landau-Zener-Stuckelberg transitions, is examined in the multidimensional case, and cast into normal form. The group theoretical principles of the normal form transformation are laid out, and shown to involve both the Lorentz group and the symplectic group.     

A rapid solvent extraction method for the determination of caesium-137 in environmental materials

A solvent extraction method for the quantitative determination of caesium-137 in environmental samples is described. Caesium carrier is added to the ashed sample, which is dissolved with hydrochloric acid. The solution is filtered to remove any carbon and the pH is adjusted to 2.0. Sodium dodecylbenzene sulphonate is added and the extraction is done with phenol-nitrobenzene mixture. The organic phase is washed with EDTA solution and the caesium is stripped from the washed solvent with 6 M hydrochloric acid. The caesium is precipitated as caesium chlorostannate to ensure absolute decontamination from potassium-40. Recovery from standard solution is about 85% and from vegetation, rainwater and effluent samples about 80%. The method is applicable in the presence of large quantities of interfering ions, and decontamination factors for many nuclides are greater than 10³.     

The influence of electrospray deposition in matrix-assisted laser desorption/ionization mass spectrometry sample preparation for synthetic polymers

Although electrospray sample deposition in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) sample preparation increases the repeatability of both the MALDI signal intensity and the measured molecular mass distribution (MMD), the electrospray sample deposition method may influence the apparent MMD of a synthetic polymer. The MMDs of three polymers of differing thermal stability, polystyrene (PS), poly(ethylene glycol) (PEG), and poly(propylene glycol) (PPG), were studied by MALDI time-of-flight (TOF) MS as the electrospray deposition voltage was varied. The MMDs obtained using the electrospray deposition method were compared with those obtained for hand-spotted samples. No change was observed in the measured polymer MMD when the electrospray deposition voltage was varied in the analysis of PS, but those of PEG and PPG changed at higher electrospray voltages due to increased ion fragmentation. It was also shown that the fragmentation in the hand-spotted samples is dependent on the matrix used in sample preparation.