Wolfram Schnabel’s research while affiliated with Humboldt-Universität zu Berlin and other places

Upon UV-irradiation, various phosphine oxides of the general structure I were found to be fragmented readily into free radicals I′ and I″ Laser flash photolysis studies revealed that radicals of type I" with R

Hydroxyl radicals react with diphenyliodonium ions, Ph2I+, and triphenylsulphonium ions, Ph3S+, in aqueous solution at room temperature with rate constants ≥ 5 × 109/mol s. These reactions lead to the formation of a broad transient absorption band with a maximum at ca. 370 nm, which is attributed to OH-adducts with cyclohexadienyl structures. 2-Hydroxy-2-propyl radicals react with diphenyliodonium ions in aqueous solution at room temperature with k2 = 6 × 107l/mol s. The transient absorption band formed in this reaction has a maximum at about 390 nm and is attributed to diphenyliodo radicals, Ph2I. In contrast, no detectable reaction occurred with 2-hydroxy-2-propyl radical and triphenylsulphonium ion.

Polyriboadenylic acid, polyribocytidylic acid and polyribouridylic acid were irradiated with 16 MeV electrons in aqueous solution. Part of the OH radicals generated during the radiolysis of water reacted with the sugar moieties of the polynucleotides. Subsequently formed peroxyl radicals reacted with each other thus forming oxyl radicals whose's decay led to main-chain scissions with the consequence of a decrease of the light scattering intensity (LSI). In the case of single-stranded polynucleotides, the analysis of the LSI decay curves revealed the occurrence of two processes. The rapid mode was assigned to the intramolecular and the slow mode to the intermolecular reaction of peroxyl radicals. For the rate constant of the rapid process (activation energy: 5 kcal/mol) a pronounced kinetic salt effect was observed. In the case of double-stranded polynucleotides (polyA and polyC at pH 4, poly(A+U) at pH 8) the same effects were observed as with native calf thymus DNA in a former study [9]: The LSI decreased only after a critical dose was surpassed. A rapid process was followed by a slow one. The slow process is assigned to the melting of H-bridges located at bases between main-chain scissons in the double helix. The rapid process corresponds to the separation of fragments formed by two single strand breaks laying very close to each other on opposite sites in the double helix.

Upon irradiation with UV light (λ = 347 nm), 2,4,6-trimethylbenzoyldiphenylphosphine sulfide was found to be fragmented into free radicals by α-scission (F(k) = 0.3 ± 0.1): Flash photolysis studies revealed that the optical absorption spectrum of diphenylthiophosphonyl radicals, S = P(Ph) 2 . possesses a strong band with λ max = 340 nm and a somewhat weaker band with λ max ≈ 500 nm (e 340nm = 1.2 ± 0.2) · 10 ⁴ 1/mol cm). The reactivity towards olefinic compounds, M, is 10 to 30 times lower than in the case of O = P(Ph) 2 radicals. Typical bimolecular rate constants (in 1/mol s) of the reaction of S = P(Ph) 2 with M are: 4 x 10 ⁶ (styrene), 6.2 x 10 ⁵ (methylacrylate), 4.2 x 10 ⁴ (vinyl acetate).

Poly(ribonucleic acids), poly A, poly C and poly U, were irradiated in O2-free dilute aqueous solution at pH 7 to 8 with single pulses (50 to 200 ns) of 16 MeV electrons. With the aid of Rayleigh light scattering measurements main-chain scission, induced by OH radicals, was observed with the three polynucleotides. From the time dependence of the decrease of the light scattering intensity (LSI), the existence of two modes of decrease was inferred, indicating at least two different chemical mechanisms were operative in main-chain degradation. Evidence for the assignment of the slow mode of LSI decrease to the lifetime of a free radical was obtained from quenching experiments with cysteamine. It is noteworthy, that the extent and the lifetime of LSI decrease are not the same for the three polynucleotides. The differences indicate the influence of the chemical nature of the bases on main-chain scission. Consequently, it is concluded that OH attack at carbons in 1' and/or 2' position of the ribose moiety contributes essentially to the degradation mechanism.

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The photoinitiated cationic, polymerization of typical vinyl ether monomers, such as isobutyl vinyl ether, di(ethylene glycol) divinyl ether, tri(ethylene glycol) divinyl ether, 1,4-butanediol divinyl ether, and 1,6-hexanediol divinyl ether, by use of aryl-substituted vinyl halides was studied. The cationic polymerization of these monomers was initiated at 0 degrees C or room temperature upon irradiation lambda = 350 nm in CH(2)Cl(2) solutions with one of the following compounds: 1-bromo-1,2,2-tris(p)-methoxyphenyl) ethene, 1-bromo-1-(p-methoxyl)henyl)-2,2-diphenylethene. 1-bromo-1,2,2-triphenylethene, and 1-chloro-2,2-bis(p-methoxyphenyl)ethene in the presence of zinc iodide (ZnI(2),). A mechanism involving formation of ail adduct between the monomer and the products yielded from the photoinduced homolysis of the vinyl halide followed by electron transfer is proposed. In the subsequent step, the terminal carbon-halide bond in this adduct is activated by the coordinating effect of ZnI(2). This polymerization exhibited some characteristics of pseudoliving cationic polymerization.

The combustion performance of poly(butylene terephthalate) (PBT) can be improved by the addition of red phosphorus provided it is intermolecularly cross‐linked upon irradiation with ⁶⁰ Co γ‐rays in the presence of triallyl cyanurate (TAC). At a content of 3 or 4 wt.‐% the latter significantly promotes cross‐linking in the presence of air. From combustion tests with samples containing red phosphorus (P red ) and having been γ‐irradiated in the presence of TAC it turned out that an improved fire resistance of PBT is achieved if the red phosphorus content is at least 12.5 wt.‐%. In this case test samples were self‐extinguishing and the UL 94 rating corresponded to V‐1. Product analysis and thermal gravimetric analysis revealed that P red stimulates aromatization and charring. These processes involve the reaction of P red with the polymer. ³¹ P NMR spectroscopy revealed that the residue contained chemically bonded phosphorus. Decomposition of anhydride groups resulting in phenyl radicals. magnified image Decomposition of anhydride groups resulting in phenyl radicals.

Poly(A + U), the 1:1 complex of poly(riboadenylic acid), poly A, and poly(ribouridylic acid), poly U, at pH 8 and self-complexed poly A at pH 4 exist as double helices in dilute aqueous solution. These complexes exhibit a similar behavior as native calf thymus DNA upon irradiation with 16 MeV electron pulses. Thus time resolved Rayleight light scattering measurements showed that crosslinking and double strand breakage can be clearly separated, the former proceeding faster than the latter. The extent to which the two processes occur depends on the ionic strength of the solution. At ionic strenghts exceeding 10 −1 mol/l crosslinking is the dominant process indicating that hcrit, the critical length between two single strand breaks for the accomplishment of double strand breaks, is strongly reduced. The investigation of complexes of poly A and Mg2+ ions revealed that the destruction of salt bridges is the rate determining process for the decrease of the light scattering intensity due to mainchain scission. This implies that life-times of salt bridges can be determined.