PHOTOCHEMICAL-LIKE BEHAVIOUR OF RIBOFLAVIN IN THE DARK PROMOTED BY ENZYME-GENERATED TRIPLET ACETONE

Abstract— Triplet acetone obtained enzymically by the isobutyraldehyde/peroxidase/O₂ system transfers its energy to the riboflavin, which changes into a decomposition product of lumichrome type and produces aggregates. When riboflavin and tryptophan are added to this system, further products—formylkynurenine and an adduct between the riboflavin and the tryptophan—are formed. The adduct can be separated by gel filtration and ion exchange chromatography and is similar to that prepared by irradiating riboflavin in the presence of tryptophan.     

Anomalous top magnetic couplings

The real and imaginary parts of the one-loop electroweak contributions to the left and right tensorial anomalous couplings of the tbW vertex in the Standard Model (SM) are computed.     

On the Local and Superlinear Convergence of Quasi-Newton Methods

This paper presents a local convergence analysis for severalwell-known quasi-Newton methods when used, without line searches,in an iteration of the form to solve for x^* such that Fx^* = 0. The basic idea behind theproofs is that under certain reasonable conditions on x_o, Fand x_o, the errors in the sequence of approximations {H_k} toF'(x^*)^–1 can be shown to be of bounded deterioration inthat these errors, while not ensured to decrease, can increaseonly in a controlled way. Despite the fact that H_k is not shownto approach F'(x^*)^–1, the methods considered, includingthose based on the single-rank Broyden and double-rank Davidon-Fletcher-Powellformulae, generate locally Q-superlinearly convergent sequences{x_k}.     

Luminescence-Based Optical Fiber Chemical Sensors

Abstract

A scheme for the simultaneous determination of temperature and analyte concentration for application in luminescence-based chemical sensors is proposed. This scheme is applied to an optical oxygen sensor, which is based on the quenching of the fluorescence of a ruthenium complex. Temperature measurement is performed using the excitation radiation and an absorption long-pass filter. Preliminary results are presented that show the viability of an oxygen measurement that is independent of temperature and optical power level. The possibility of self-referenced temperature measurements with semiconductor nanoparticles is also investigated. In order to optimize the sensor design, several different optical fiber probe geometries for oxygen sensing are tested and compared, including different methods of coupling radiation into the optical fiber system. Polyvinyl alcohol (PVA) and polyacrylamide membranes are tested as supports for sensor immobilization in fiber-optical pH sensing devices in aqueous solution. Some results are presented that show the feasibility of using fiber-optical pH indicators for remote monitoring.     

Simulation of phase equilibria and interfacial properties of binary mixtures on the liquid-vapour interface using lattice sums

Molecular dynamics simulations of Lennard-Jones binary mixtures were performed to obtain phase equilibria and thermodynamic properties for the liquid—vapour interface. The dispersion interactions were handled using the lattice sum method where the full interaction is obtained and there is no requirement for any long range correction to the properties. The application of the method using the Lorentz—Berthelot combining rule for unlike interactions is discussed. The coexisting densities, adsorption of molecules at the interface and surface tension are the main results of this work. Coexisting properties were compared with Gibbs ensemble Monte Carlo results and with those of the grand canonical Monte Carlo method combined with the histogram reweighting technique, and good agreement was found. The lattice sum method results were compared with those of the spherically truncated and shifted potential to analyse the truncation effect. The adsorption of molecules at the interface and surface tension increase with interaction.     

PHOTOCHEMISTRY OF MEROCYANINE 540

The photophysical properties of merocyanine 540 have been determined in methanol solution over a modest temperature range. Triplet state population is inefficient (the limiting triplet quantum yield being 0.25) due to rapid isomerization of the central double bond from the first excited singlet state. Activation energies have been measured for isomerization from the excited singlet state (20 kJ mol-1) and for conversion of the resultant cis-isomer back to the original trans-form (63 kJ mol-1), both processes involving formation of a twisted species. The dye is easily oxidized to give an unstable adduct which decomposes on the sub-ms timescale. Reversible redox chemistry occurs upon excitation in the presence of electron acceptors. These various observations are discussed in terms of the known chemotherapeutic activity of MC540 and it is concluded that the most probable mechanisms for cytotoxicity involve either local thermal disruption of cell membranes or in situ photogeneration of toxins derived from breakdown of the dye.     

Long-range interactions in Monte Carlo simulation of confined water

We investigate methods for the treatment of long-range interactions in the context of grand canonical Monte Carlo (GCMC) simulations of water adsorption in slit-shaped activated carbon pores. Several approaches, ranging from the simple minimum image convention to the more complex two-dimensional Ewald summations, are implemented and compared with respect to accuracy and speed of computation. The performance of some of these methods in GCMC is found to be significantly different from that in molecular dynamics applications. Of all the methods studied, one proposed by Heyes and van Swol was the most promising, providing the best balance between accuracy and speed. In our application, it was shown to be about 2 times faster than the fastest of the two-dimensional Ewald methods. We expect this conclusion to apply in general to systems that are periodic in two dimensions and finite in the third.     

Structure and thermodynamic properties of a polydisperse fluid in contact with a polydisperse matrix

We present expressions that allow the determination of the structural and thermodynamic properties of a polydisperse liquid mixture in contact with a polydisperse matrix; polydispersity of the fluid and of the matrix are described by distribution functions f ^f(σ) and f m (σ), where σ and σ characterize the size of the fluid and of the matrix particles. The formalism is based on the replica trick (to describe the properties of a fluid in contact with a matrix) and on the expansion of size-dependent functions in terms of orthogonal polynomials associated with the distribution functions f _f(σ) and f _m (σ). In our expressions structural and thermodynamic properties are calculated from coefficient functions (of these expansions) which are obtained from a numerical solution of the generalized replica Ornstein-Zernike equations, solved along with a suitable closure relation.     

Influence of chemical and physical surface heterogeneity on chemical reaction equilibria in carbon micropores

Recent simulation results are presented for the equilibrium yield of the ammonia synthesis reaction in various model microporous carbons. It is found that the reaction equilibria within the micropores is affected by many factors, including pore size, pore shape, connectivity, surface roughness, and surface chemical activation. In order to probe these effects, reactive Monte Carlo simulations of the reaction were performed in several microporous carbon models: smooth slit-shaped carbon pores, a realistic carbon model generated from experimental diffraction data, single-walled carbon nanotubes, and smooth slit-shaped pores activated by carboxyl surface groups. The simulations show that the ammonia conversion is most sensitive to the carbon pore width and to the amount of surface chemical activation. Effects of surface corrugation and pore connectivity on the equilibrium reaction yield are minimal.     

Heavy quarkonium properties from Cornell potential using variational method and supersymmetric quantum mechanics

Using the variational method and supersymmetric quantum mechanics we calculated, in an approximate way, the eigenvalues, eigenfunctions and wave functions at the origin of the Cornell potential. We compared results with numerical solutions for heavy quarkonia \(c\bar {c}\), \(b\bar {b}\) and \(b\bar {c}\).