Photochemistry of 4'-benzophenone-substituted nucleoside derivatives as models for ribonucleotide reductases: competing generation of 3'-radicals and photoenols

Ribonucleotide reductases (RNRs) catalyze the 2'-reduction of ribonucleotides, thus providing 2'-deoxyribonucleotides, the monomers for DNA-biosynthesis. The current mechanistic hypothesis for the catalysis effected by this class of enzymes involves a sequence of radical reactions. A 3'-hydrogen abstraction, effected by a radical at the enzyme's active site, is believed to initiate the catalytic cycle. As models for this substrate-enzyme interaction, the photochemically induced intramolecular hydrogen abstraction in a series of 4'-benzophenone-substituted nucleoside analogues was studied. Model compounds with hydroxy-, methoxy-, mesyloxy-groups or a cyclic carbonate in 2'- and 3'-positions were investigated. Depending on the substitution pattern, two different types of photoproducts were observed: Those which result from photoenol formation (gamma-H-abstraction) and those which result from abstraction of the 3'-H-atom (delta-H-abstraction). Photoenol formation was further supported by H/D-exchange experiments. Thus, the 3'-H-abstraction postulated as the initial step in RNR action was successfully modeled by photolysis of 4'-benzophenone-substituted nucleoside analogues. The regioselectivity of the photochemical H-abstraction and thus of the product distribution as a function of the 2'- and 3'-substituents was rationalized on the basis of a conformational analysis of the four model systems, utilizing molecular mechanics simulations.     

New electron centers in neutron-irradiated natural quartz

EPR measurements on neutron-irradiated natural quartz crystals from Brazil revealed the presence of two new electron centers, each with hyperfine structure due to one proton. In one of these centers this hyperfine splitting is typical for H^? which most likely substitutes for an oxygen whereas in the other a much smaller splitting due to an OH group was observed. The properties of these centers are compared with those of the E^′_n centers with closely related structures.     

Initial State Interaction in the (π+,3p) Reaction on N, Ar and Xe

The pion absorption reaction (π^{+,3 p} ) on Ar was studied at pion energies of 70, 118, 162 and 239 MeV, and on N and Xe at 239 MeV. The 3p cross section with a 50 MeV cut on the missing energy is presented. The existence of an initial state interaction (ISI) component to the reaction was investigated. The portion of the 3p cross section assigned to the ISI process is less than one half and at lower energies significantly so. Received: 11 December 1997     

Magnetic excitations in the two dimensional planar antiferromagnets K2FeF4 and Rb2FeF4

The magnetic excitation spectrum of K₂FeF₄ and Rb₂FeF₄, two K₂NiF₄-structure planar antiferromagnets with rather large anisotropy and spins perpendicular to the c-axis, has been measured by Raman and FIR-spectroscopy. One of the two predicted one-magnon transitions and the two-magnon mode have been observed in K₂FeF₄ (Rb₂FeF₄) at 48.5 cm^-1 (37.6 cm^-1) and 182.0 cm^-1 (160.5 cm^-1) respectively. The magnetic field and temperature dependence of the spectra are reported too. The data are discussed on the basis of an easy plane spin model Hamiltonian. In K₂FeF₄: Mn²⁺ a low lying magnetic impurity mode is observed at 40.5 cm^-1.     

Luminescence spectra of Mn(II) in different symmetries

Luminescence emission and excitation spectra of high-spin Mn(II) were studied in crystals with different site symmetries of the divalent ion. For cubic or uniaxial site symmtries only one emission band is observed with a maximum asymmetry of 20% and larger width on the low energy side. Lower site symmetries may result in spectra of considerable complexity. A second emission band of lower intensity near 14 000 cm^-1 was found in systems with hexa- as well as tetra-coordinated Mn(II). It evidently originates from the same Mn(II) that gives rise to the stronger emissions at higher energies. A qualitative explanation for its occurence on the basis of close-lying split components of the ⁴T₁(G) or ⁴T₂(G) states is presented. In two systems emission bands from higher excited states and shifts of emission maxima with excitation energy were observed. The shifts are explained on the basis of overlapping bands and variable intensity ratios for these unresolved components. The results indicate that the degree of distortion is more important for the occurrence of these complications than the actual site symmetry of Mn(II).     

Observation of the decay τ → πv

The decay of the heavy lepton τ into πv has been established using the magnetic detector PLUTO. The branching ratio is determined to be BR(τ→πv)=(9.0 ± 2.9)% with an additional systematic uncertainty of 2.5%. This value is in good agreement with the theoretical prediction.     

The structure of quartz at 25 and 590°C determined by neutron diffraction

Analysis of single-crystal data on the α to β transformation in quartz, which takes place at 573°C, showed that the model which, till now, has best agreed with available X-ray data, does not hold for the neutron data. In an earlier model both oxygen and silicon atoms move to special positions (6j and 3c in the space group P6₂22). The new data for β-quartz were best explained by assuming a disorder for the oxygen atom around the 6j position, and in order to keep regular SiO₄ tetrahedra, a corresponding small disorder in the silicon atom is assumed.