REPAIR OF PSORALEN PLUS NEAR ULTRAVIOLET LIGHT DAMAGE IN BACTERIOPHAGES T3 AND T4

Abstract— Repair of T3 and T4 DNA damaged by treatment with 8-methoxypsoralen plus near UV (PNUV) has been investigated. It is shown that the excision repair mechanisms of the host cell can repair a substantial fraction of the psoralen-DNA mono-adducts in T3 DNA, but cannot by themselves repair crosslinks. In contrast neither the excision repair system of the host nor the phage coded v gene endonuclease is involved in the repair of psoralen adducts in T4 DNA. Multiplicity reactivation is effective in the recovery of T4 DNA containing psoralen-DNA mono-adducts, but is ineffective in the recovery of crosslinked phages. Comparisons of the lethality of PNUV treatment and the number of crosslinks induced in T4 DNA show clearly that mono-adducts are lethal to this phage. Both T3 and T4, however, appear to effectively repair many mono-adducts by postreplicational repair.     

The central role of the metal ion for photoactivity: Zn– vs. Ni–Mabiq

Photoredox catalysts are integral components of artificial photosystems, and have recently emerged as powerful tools for catalysing numerous organic reactions. However, the development of inexpensive and efficient earth-abundant photoredox catalysts remains a challenge. We here present the photochemical and photophysical properties of a Ni–Mabiq catalyst ([Ni^II(Mabiq)]OTf (1); Mabiq = 2-4:6-8-bis(3,3,4,4-tetramethyldihydropyrrolo)-10-15-(2,2-biquinazolino)-[15]-1,3,5,8,10,14-hexaene1,3,7,9,11,14-N₆)—and of a Zn-containing analogue ([Zn^II(Mabiq)OTf] (2))—using steady state and time resolved optical spectroscopy, time-dependent density functional theory (TDDFT) calculations, and reactivity studies. The Ni and Zn complexes exhibit similar absorption spectra, but markedly different photochemical properties. These differences arise because the excited states of 2 are ligand-localized, whereas metal-centered states account for the photoactivity of 1. The distinct properties of the Ni and Zn complexes are manifest in their behavior in the photo-driven aza-Henry reaction and oxidative coupling of methoxybenzylamine.

The development of earth-abundant photoredox catalysts remains a challenge. Studies of Ni- and Zn-Mabiq complexes demonstrate how the coordinating metal ion influences the photochemistry, photodynamics and reactivity of photocatalysts.     

Probing Charge-Transfer Processes in a Covalently Linked [Ge9]-Cluster Imine Dyad

C₆₀ donor dyads in which the carbon cage is covalently linked to an electron-donating unit have been discussed as one possibility for an electron-transfer system, and it has been shown that spherical [Ge₉] cluster anions show a close relation to fullerenes with respect to their electronic structure. However, the optical properties of these clusters and of functionalized cluster derivatives are almost unknown. We now report on the synthesis of the intensely red [Ge₉] cluster linked to an extended π-electron system. [Ge₉{Si(TMS)₃}₂{CH₃C=N}-DAB(II)^Dipp]⁻ ( 1⁻ ) is formed upon the reaction of [Ge₉{Si(TMS)₃}₂]²⁻ with bromo-diazaborole DAB(II)^Dipp-Br in CH₃CN (TMS=trimethylsilyl; DAB(II)=1,3,2-diazaborole with an unsaturated backbone; Dipp=2,6-di-iso-propylphenyl). Reversible protonation of the imine entity in 1⁻ yields the deep green, zwitterionic cluster [Ge₉{Si(TMS)₃}₂{CH₃C=N(H)}-DAB(II)^Dipp] ( 1-H ) and vice versa. Optical spectroscopy combined with time-dependent density functional theory suggests a charge-transfer excitation between the cluster and the antibonding π* orbital of the imine moiety as the cause of the intense coloration. An absorption maximum of 1-H in the red region of the electromagnetic spectrum and the corresponding lowest-energy excited state at λ=669 nm make the compound an interesting starting point for further investigations targeting the design of photo-active cluster compounds.     

1D Simulations for Microbial Enhanced Oil Recovery with Metabolite Partitioning

We have developed a mathematical model describing the process of microbial enhanced oil recovery (MEOR). The one-dimensional isothermal model comprises displacement of oil by water containing bacteria and substrate for their feeding. The bacterial products are both bacteria and metabolites. In the context of MEOR modeling, a novel approach is partitioning of metabolites between the oil and the water phases. The partitioning is determined by a distribution coefficient. The transfer part of the metabolite to oil phase is equivalent to its ”disappearance,” so that the total effect from of metabolite in the water phase is reduced. The metabolite produced is surfactant reducing oil–water interfacial tension, which results in oil mobilization. The reduction of interfacial tension is implemented through relative permeability curve modifications primarily by lowering residual oil saturation. The characteristics for the water phase saturation profiles and the oil recovery curves are elucidated. However, the effect from the surfactant is not necessarily restricted to influence only interfacial tension, but it can also be an approach for changing, e.g., wettability. The distribution coefficient determines the time lag, until residual oil mobilization is initialized. It has also been found that the final recovery depends on the distance from the inlet before the surfactant effect takes place. The surfactant effect position is sensitive to changes in maximum growth rate, and injection concentrations of bacteria and substrate, thus determining the final recovery. Different methods for incorporating surfactant-induced reduction of interfacial tension into models are investigated. We have suggested one method, where several parameters can be estimated in order to obtain a better fit with experimental data. For all the methods, the incremental recovery is very similar, only coming from small differences in water phase saturation profiles. Overall, a significant incremental oil recovery can be achieved, when the sensitive parameters in the context of MEOR are carefully dealt with.     

Relative reactivity enhancement of (Me5Cp)2ZrCl2 in mixture with (1,2,4-Me3Cp)2ZrCl2 during ethene/1-hexene copolymerization

Dual-site ethene/1-hexene copolymerizations with MAO-activated (1,2,4-Me₃Cp)₂ZrCl₂ and (Me₅Cp)₂ZrCl₂ catalysts were performed. Copolymers with narrow molecular weight distributions and bimodal short chain branching distributions could be produced. The combined catalyst system demonstrates a number of discrepancies from an expected average behavior of the individual sites. Dual-site (1,2,4-Me₃Cp)₂ZrCl₂/(Me₅Cp)₂ZrCl₂ systems produce copolymers with lower incorporation than expected. Clear evidences for relative activity enhancement of the (Me₅Cp)₂ZrCl₂ catalyst in the mixture were observed in melting endotherms and Crystaf profiles. Molecular weights obtained by the mixture were higher than for any of the individual catalysts. A similar effect is observed for a dual-site system of the (1,2,4-Me₃Cp)₂ZrCl₂ catalyst together with the Me₄Si₂(Me₄Cp)₂ZrCl₂ catalyst as an alternative to (Me₅Cp)₂ZrCl₂.     

Supported metallocene catalysts prepared by impregnation of MAO modified silica by a metallocene/monomer solution

Silica supported (butylcyclopentadienyl)₂ZrCl₂/MAO catalysts were synthesized according to the “incipient wetness” method from a solution of metallocene in a liquid monomer. The monomer was allowed to polymerize yielding a catalyst containing polyhexene (PH), polystyrene (PS) or polyoctadiene (PO). One catalyst containing no polymer was also synthesized. The catalysts were used to polymerize ethene at 70°C and 4 bar total pressure. The measured average activities were 5 300 kg PE/(mol Zr · h) for (BuCp)₂ZrCl₂/MAO/PH/SiO₂, 8 600 kg PE/(mol Zr · h) for (BuCp)₂ZrCl₂/MAO/PS/SiO₂, 3 400 kg PE/(mol Zr · h) for (BuCp)₂ZrCl₂/MAO/PO/SiO₂ and 5 700 kg PE/(mol Zr · h) for (BuCp)₂ZrCl₂/MAO/SiO₂. The polyhexene, polystyrene or polyoctadiene in the catalyst forms a protective layer around the active sites. Even after exposure to air for five hours these catalysts retain some polymerization activity.     

Pressure effects on termination mechanisms during ethene polymerization catalyzed by dicyclopentadienylzirconium dichloride/methylaluminoxane

Polymerization of ethene catalyzed by dicyclopentadienylzirconium dichloride/methylaluminoxane was performed in toluene at 50°C under 0.38–8.95 bar ethene. Both Fourier-transform infrared spectroscopy and gel-permeation chromatography showed that the molecular weight is independent of the ethene pressure (concentration). The trans-vinylene unsaturation content increased as the pressure decreased. Termination by β-H transfer to a coordinated monomer and a kinetically controlled isomerization reaction are suggested, in order to explain the observations.     

Sterically demanding bis(2-silylindenyl)zirconium(IV) dichlorides as polymerisation catalyst precursors

A set of different 1- and 2-silyl-substituted zirconocene dichloride/MAO catalyst systems was investigated with respect to their performance in ethene/1-hexene copolymerisations. In-depth studies of bis(2-dimethylsilylindenyl) zirconium(IV) dichloride ( 1 ) revealed a multi-site behaviour, illustrating sensitivity to the reaction temperature and the comonomer mole fraction. Surprisingly, an upper limit is observed for the latter, leading to complete catalyst inhibition. Analysis of the chain termination processes implies the possibility of a predominant, although in general less favourable, β-hydride elimination route under certain polymerisation conditions.     

Experimental and theoretical investigations of the structure of methylaluminoxane (MAO) cocatalysts for olefin polymerization

The structure of methylaluminoxane (MAO), used as a cocatalyst for olefin polymerization, has been investigated by Raman and in situ IR spectroscopy, polymerization experiments, and density functional calculations. From experimental results, a number of quantum chemical calculations, and bonding properties of related compounds, we have suggested a few Me₁₈Al₁₂O₉ cage structures, including a highly regular one with C_3h symmetry, which may serve as models for methylaluminoxane solutions. The cages themselves are rigid but may contain up to three bridging methyl groups on the cage surfaces that are labile and reactive. Bridging methyls were substituted with Cl atoms to form a compound otherwise similar to MAO. Chlorinated MAO is unable to activate a metallocene catalyst, even in the presence of trimethylaluminum (TMA), but allows subsequent activation by regular MAO. With bis(pentamethylcyclopentadienyl)zirconium dichloride, MAO and TMA seem to influence chain termination independently. Several findings previously poorly explained are rationalized with the new model, including the observed lack of reaction products with excess TMA. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3106–3127, 2000