ELECTRON TRANSFER IN PHOTOSENSITIZATION OF BIOMOLECULES BY ACRIDINE DYES

Abstract— The photosensitization of some biomolecules by proflavine was studied using the technique of electron paramagnetic resonance. It was shown that this process is directly influenced by the relative molar concentration of dye and substrate. For some values of these concentrations, a singlet associated with a radical form of proflavine is observed. The appearance of this signal is discussed in terms of electron transfer from an ionized proflavine molecule directly to another neutral dye molecule or via a molecule of substrate as an intermediate. It is shown that this transfer is facilitated by stacking. These results were compared with radiolysis of the same kind of complexes in frozen aqueous solution.     

An improvement of Davidson's iteration method: Applications to MRCI and MRCEPA calculations

Davidson's method is widely used for finding the lowest eigenvalues of large matrices. Recently, mathematicians have shown that Davidson's derivation could be improved. They have corrected the derivation yielding a new iteration method. In this article this new method is adapted for realistic MRCI and MRCEPA calculations. Results show that the new method converges significantly faster in H₂O and O₂ with moderately elongated bonds than Davidson's original method. The new method offers new insights into the rate of convergence of Davidson's original method. © 1996 by John Wiley & Sons, Inc.     

Thermal relaxation of poled non-linear optical sidechain polymers: A new,semi-empirical model

Electro-Optic relaxation of a poled, Non-Linear Optical sidechain polymer with T_g 140°C, containing 4-dimethylamino-4′-nitrostilbene (“DANS”) in the sidechains, has been studied at 120°C with and without annealing at the same temperature. The time-dependence of the decaying EO coefficients r(t) shows a strong departure from the classical single-exponential Debye model, especially in the unannealed samples. This departure is attributed to physical ageing, slowing down the orientational relaxation of the sidechains. The Debye model with r(t)-r(0). exp -t/τ] is modified semi-empirically by introducing a time-dependent characteristic Debye relaxation time τ(t). Of several trial expressions, one is selected which fits the relaxation data. This is τ(τ)-τ_i+C.t^b     

Structure and function of proteins in hydrated choline dihydrogen phosphate ionic liquid

Ionic liquids are being intensely studied as promising media for the stabilization of proteins and other biomolecules. Choline dihydrogen phosphate (CDHP) has been identified as one of the most promising candidates for this application. In this work we have probed in more detail the effects that CDHP may have on the thermodynamics, structure, and stability of proteins, including one of therapeutic interest. Microcalorimetry and circular dichroism spectropolarimetry (CD) were used to assess the thermal stability of protein solutions in CDHP/water mixtures at various concentrations. Increasing thermal stability of lysozyme and interleukin-2 in proportion to CDHP concentration was observed. Isothermal titration calorimetry (ITC) was used to quantify binding interactions, and indicate that the mechanism for stability does not appear to be dependent upon CDHP binding to protein. CD and small angle X-ray scattering (SAXS) analyses were used to probe for structural changes due to the presence of CDHP. SAXS indicates charge effects on the surface of the protein play a role in protein stability in ionic liquids, and no significant alteration of the overall tertiary conformation of lysozyme was observed at 25 °C. However, after incubation at 37 °C or at higher concentrations of CDHP, small changes in protein structure were seen. Effects on protein activity were monitored using turbidity assays, and CDHP decreases protein activity but does not eliminate it. Protein solubility was also monitored using a turbidity assay and was found to be inversely proportional to the concentration of CDHP in solution.     

QUANTUM YIELD OF SINGLET OXYGEN PRODUCTION BY XANTHENE DERIVATIVES

The singlet oxygen quantum yield (φ₁o₂) of 11 purified fluorescein derivatives was determined by reaction with singlet oxygen acceptors in aqueous and ethanolic solutions; in both solvents φ₁o₂ was enhanced with increasing halogenation. Tryptophan and 2,2,6,6-tetramethylpiperidone were found to be unadapted for the determination of φ₁o₂, in our systems; however, the use of 9.10-dipropionic anthracene acid andp-nitrosodimethylaniline in conjunction with imidazole derivatives was suitable for ¹O₂ detection in water. Both methods lead to results in excellent agreement. As in ethanol. φ₁o₂, was equal to the triplet state quantum yield (φ_T), the comparison between the two solvents showed that φ_T in water was greater than in ethanol. The comparison between our values obtained with polychromatic light with published data obtained with monochromatic light suggests that the triplet quantum yield of fluorescein derivatives is wavelength independent.     

Densification Behaviour of Silica Aerogels upon Isothermal Sintering

Isothermal sintering has been studied for a homologous series of supercritically dried base-catalyzed silica aerogels with different initial densities. The change in density and macroscopic viscosity has been recorded in situ using a dilatometer and a beam-bending viscosimeter. Changes of the nanostructural features of the aerogel network have been monitored by means of small angle X-ray scattering. The cylinder model introduced by Scherer is generally accepted to describe the sintering behaviour of bodies with moderate porosity, such as xerogels. In case of highly porous aerogels, however, preferential densification of large pores and little loss of specific surface area is observed, at least during the initial stages. For the samples under investigtion the increase in density and the corresponding variation of specific inner surface and viscosity are compared to the analytical predictions by Scherer, the scaling approach introduced by Sempéré et al. and the results obtained from numerical simulation of viscous flow.     

Experimental verification of the steady-state behavior of a beam system with discontinuous support

This article deals with the experimental verification of the long-term behavior of a periodically excited linear beam supported by a one-sided spring. Numerical analysis of the beam showed subharmonic, quasi-periodic, and chaotic behavior. Further, three different routes leading to chaos were found. Because of the relative simplicity of the beam system and the variety of calculated nonlinear phenomena, an experimental setup is made of this beam system to verify the numerical results. The experimental results correspond very well with the numerical results as far as the subharmonic behavior is concerned. Measured chaotic behavior is proved to be chaotic by calculating Lyapunov exponents of experimental data.     

Note on prime representations of convex polyhedral sets

Consider a convex polyhedral set represented by a system of linear inequalities. A prime representation of the polyhedron is one that contains no redundant constraints. We present a sharp upper bound on the difference between the cardinalities of any two primes.This research was supported by the Natural Sciences and Engineering Research Council of Canada under Grant Nos. A8807, A4625, and A7742.