PHOTOCHEMICAL INACTIVATION OF SINGLE-STRANDED VIRAL DNA IN THE PRESENCE OF UROCANIC ACID*

Abstract— Urocanic acid (UA) has previously been shown to react photochemically in vitro with N,N-dimethylthymine. In this study, mixtures of UA and phage G4 single-stranded DNA have been irradiated with UV light (λ≥ 254 nm) and the DNA assayed for infectivity. At the concentrations of UA employed (typically 5.4 × 10^-3 M ) there is extensive absorption of the incident light by the UA. The DNA is inactivated at rates greater than that predicted from the calculated shielding by UA, indicating that photosensitization is occurring. Photosensitization is also indicated by the fact that at high UA concentrations the inactivation rate does not decrease to zero but approaches a residual value. Furthermore, the ability to photoreactivate DNA that has been photolyzed in the presence of UA is much reduced relative to that observed upon photolysis of the DNA alone. UA is therefore responsible for the production of UV-induced DNA lesions, which are resistant to photoreactivation.
A general analysis of the effects of photosensitization on the kinetics of UV inactivation is presented in an appendix.     

Heterogeneous catalysts for cyclic carbonate synthesis from carbon dioxide and epoxides

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Metallonitrosyl fragment as electron acceptor: intramolecular charge transfer, long range electronic coupling, and electrophilic reactivity in the trans-[NCRu(py)(4)(CN)Ru(py)(4)NO](3+) ion

The new complex trans-[NCRu(py)(4)(CN)Ru(py)(4)NO](PF(6))(3) (I) was synthesized. In acetonitrile solution, I shows an intense visible band (555 nm, epsilon = 5800 M(-1) cm(-1)) and other absorptions below 350 nm, associated with d(pi) --> pi(py) and pi(py) --> pi(py) transitions. The visible band is presently assigned as a donor-acceptor charge transfer (DACT) transition from the remote Ru(II) to the delocalized [Ru(II)-NO(+)] moiety. Photoinduced release of NO is observed upon irradiation at the DACT band. Application of the Hush model reveals strong electronic coupling, with H(DA) = approximately 2000 cm(-1). The difference between the optical absorption energy and redox potentials for the donor and acceptor sites (Ru(III,II), 1.40 V, and NO(+)/NO, 0.50 V, vs Ag/AgCl, 3 M KCl, respectively) (hnu - DeltaE(red)) is 1.33 eV, a large value which probably relates to the significant changes in distances and angles for the Ru-N-O moiety upon reduction. UV-vis absorptions, IR frequencies, and redox potentials are solvent-dependent. Controlled potential reduction (of NO(+)) and oxidation (of Ru(II) associated with the dicyano-chromophore) of I afford stable species, [NCRu(II)(py)(4)(CN)Ru(py)(4)NO](2+) (I(red)) and [NCRu(III)(py)(4)(CN)Ru(py)(4)NO](4+) (I(ox)), respectively, which are characterized by UV-vis and IR spectroscopies. I(red) shows an EPR spectrum characteristic of [Ru(II)-NO(*)] complexes. Compound I is electrophilically reactive in aqueous solution above pH 5: values of the equilibrium constant for the reaction [NCRu(py)(4)(CN)Ru(py)(4)NO](3+)+ 2 OH(-) <--> [NCRu(py)(4)(CN)Ru(py)(4)NO(2)](+) + H(2)O, K = 3.2 +/- 1.4 x 10(15) M(-2), and of the rate constant for the nucleophilic addition of OH(-), k = 9.2 +/- 0.2 x 10(3) M(-1) s(-1)(25 degrees C, I = 1 M), are obtained, with DeltaH = 90.7 +/- 3.8 kJ mol(-1) and DeltaS = 135 +/- 13 J K(-1) mol(-1). The oxidized complex, I(ox), shows an enhanced electrophilic reactivity toward OH(-). This addition reaction is followed by irreversible processes, which most probably lead to disproportionation of bound nitrite and other products.     

USE OF SENSITIZED FLUORESCENCE FOR THE STUDY OF THE EXCHANGE OF SUBUNITS IN PROTEIN AGGREGATES*

The exchange of subunits between oligomer protein particles depends upon a cycle of dissociations and associations. To examine the dynamics of these cycles we have employed two methods based on the transfer of excitation energy between fluorochromes attached to different subunits of protein oligomers, at various temperatures and pressures. In the heterotransfer method, identical solutions independently labeled with two different fluorophores, donor D and acceptor A, are mixed. The fluorescence spectrum permits the determination of the subunit exchange by the increase in A and decrease in D fluorescence as mixed AD oligomers are formed. In the homotransfer method the aggregates are labeled with fluorescein to the extent that, ideally, each subunit carries a fluorophore. The emission is strongly depolarized because sufficiently often it takes place after a transfer to a fluorophore oriented differently from the one originally excited. Both dissociation and subunit exchange with unlabeled material result in an increase in polarization and can be independently determined by the homotransfer method. Both homo- and heterotransfer have been employed in the study of the effect of temperature on the stability of the aggregates and the relation between the rate of dissociation and the rate of exchange when dissociation of oligomers is induced by hydrostatic pressure.     

Computational strategies for a four-component Dirac-Kohn-Sham program: implementation and first applications

An implementation of the generalized gradient approximation within the four-component formulation of relativistic density-functional theory using G-spinor basis sets is presented. This approach is based on the direct evaluation of the relativistic density and its gradient from the G-spinor amplitudes and gradients without explicit reference to the total density matrix. This proves to be a particularly efficient scheme, with an intrinsic computational cost that scales linearly with the number of G-spinor basis functions. In order to validate this new implementation, incorporated in the parallel version of the program BERTHA, a detailed study of the diatomic system CsAu is also reported. The spectroscopic constants D(e),r(e),omega(e), and x(e)omega(e) and the dipole moment mu have been calculated and compared with the best available theoretical and experimental data. The sensitivity of our results to the details of the numerical schemes used to evaluate the matrix elements is analyzed in detail. Also presented is a comparative study of molecular properties in the alkali auride series which have been obtained using several standard non-relativistic density functionals.     

Semiempirical study of electronic and bonding properties of cobalt silicide clusters

The electronic structure of cobalt silicide clusters Co₇Si₇ and Si₇Co₇ was studied in comparison to that of Co₁₉ and Si₁₇ clusters under the scope of the MINDO/SR method. Clusters Co₇Si₇ and Si₇Co₇ represent the environment of a cobalt atom and that of a silicon atom in the cobalt monosilicide bulk, respectively. It is found that the Co? Si bond is essentially sp in character with an indirect participation (by electrostatic interaction) of the cobalt d orbitals. Our calculations show a charge transfer from silicon to the d orbitals of cobalt via sp–sp interaction with an internal sp–d hybridization. The theoretical density of states for cobalt silicide clusters are reported and compared with experimental results of surface spectroscopies. © 1992 by John Wiley & Sons, Inc.     

High temperature structures and orientational disorder in compressed solid nitrogen

An experimental study of the phase transitions at high temperature in compressed solid nitrogen has been performed using Raman spectroscopy. Knowledge of the equilibrium phase diagram in the region of the ordered epsilon phase and the two disordered delta and deltaloc phases, at pressures between 10 and 20 GPa, has been extended up to 500 K. The Raman scattering line shape and line width of the active vibrons has been measured accurately, along isobaric scans, across the phase transitions. Analysis of the width and of its different behavior with increasing temperature in the three phases led to more precise conclusions about the nature of the disorder in the different phases. Observation of an evident shoulder in the nu2 band of the deltaloc phase suggests the possibility that sites of two different symmetries may be occupied by the disk molecules in this structure.     

Use of cetyltrimethylammonium bromide as surfactant for the determination of copper and chromium in gasoline emulsions by electrothermal atomic absorption spectrometry

In this work, the use of cetyltrimethylammonium bromide as surfactant for the preparation of oil-in-water emulsions for the determination of Cu and Cr in gasoline by electrothermal atomic absorption spectrometry (ET AAS) was evaluated. The surfactant amount was tested in the range of 25 to 300 mg, added to 2 ml of gasoline, and completed to 10 mL with 0.1% (v/v) nitric acid solution. 150 mg of surfactant was found optimum, and a sonication time of 10 min sufficient to form an oil-in-water emulsion that was stable for several hours. The ET AAS temperature program was established based on pyrolysis and atomization curves. The pyrolysis temperatures were set at 700 and 1300 °C for Cu and Cr, respectively and the selected atomization temperatures were 2400 and 2500 °C. The time and temperature of the drying stage and the atomization time were experimentally tested to provide optimum conditions. The limits of detection were found to be 5 μg L^− 1 and 1.5 μg L^− 1 for Cu and Cr, respectively in the original gasoline samples. The relative standard deviation (RSD) ranged from 4 to 9% in oil-in-water emulsions spiked with 5 μg L^− 1 and 15 μg L^− 1 of each metal, respectively. Recoveries varied from 90 to 98%. The accuracy of the proposed method was tested by an alternate procedure using complete evaporation of the gasoline sample. The method was adequate for the determination of Cu and Cr in gasoline samples collected from different gas stations in Salvador, BA, Brazil.     

Kinetic and thermodynamic acidities of substituted 1-benzyl-1-methoxy-2-nitroethylenes. Strong reduction of the transition state imbalance compared to other nitroalkanes

Acidity constants of six substituted 1-benzyl-1-methoxy-2-nitroethylenes (2-Z with Z = m-NO(2), m-CF(3), m-Cl, H, p-Me, p-MeO) and their respective nitronic acids were determined in 50% DMSO-50% water (v/v) at 20 degrees C. Kinetic data were obtained on the reversible deprotonation of all six 2-Z by OH(-) and piperidine and on the reversible deprotonation of 2-NO(2)() by piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine in the same solvent. These data allowed a determination of the Br?nsted coefficients alpha (dependence on acidity of 2-Z) and beta (dependence on amine basicity). The fact that alpha > beta indicates the presence of a transition state imbalance which, however, is much smaller than that for the deprotonation of arylnitromethanes. The reasons for this reduction in the imbalance and their relevance to a recent study of the deprotonation of Fischer carbene complexes are discussed.     

Hyperlayer hollow-fiber flow field-flow fractionation of cells

Interest in low-cost, analytical-scale, highly efficient and sensitive separation methods for cells, among which bacteria, is increasing. Particle separation in hollow-fiber flow field-flow fractionation (HF FlFFF) has been recently improved by the optimization of the HF FIFFF channel design. The intrinsic simplicity and low cost of this HF FlFFF channel allows for its disposable usage. which is particularly appealing for analytical bio-applications. Here, for the first time, we present a feasibility study on high-performance, hyperlayer HF FIFFF of micrometer-sized bacteria (Escherichia coli) and of different types of cells (human red blood cells, wine-making yeast from Saccharomyces cerevisiae). Fractionation performance is shown to be at least comparable to that obtained with conventional, flat-channel hyperlayer FIFFF of cells, at superior size-based selectivity and reduced analysis time.