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Radical trapping properties of imidazolyl nitrones

The ability of ten imidazolyl nitrones to directly scavenge free radicals (R√) generated in polar (√OH, cysteinyl, √CH3) or in apolar (CH3–√CH–CH3) media has been studied. When oxygen or sulfur-centered radicals are generated in polar media, EPR spectra are not or weakly observed with simple spectral features. Strong line intensities and more complicated spectra are observed with the isopropyl radical generated in an apolar medium. Intermediate results are obtained with √CH3 generated in a polar medium. EPR demonstrates the ability of these nitrones to trap radicals to the nitrone C(α) atom (alpha radical adduct) and to the imidazol C(5) atom (5-radical adduct). Beside the nucleophilic addition of the radical to the C(α) atom, the EPR studies suggest a two-step mechanism for the overall reaction of R√ attacking the imidazol core. The two steps seem to occur very fast with the √OH radical obtained in a polar medium and slower with the isopropyl radical prepared in benzene. In conclusion, imidazolyl nitrones present a high capacity to trap and stabilize carbon-centered radicals

Molecular envelopes derived from protein powder diffraction Molecular envelopes derived from protein powder diffraction data

The preparation of single crystals suitable for X-ray analysis is frequently the most difficult step in structural studies of proteins.With the aid of two examples, it is shown that de novo solution of the crystallographic phase problem can be achieved at low resolution using microcrystalline powder samples via the single isomorphous replacement method. With synchrotron radiation and optimized instrumentation, high-quality powder patterns have been recorded, from which it was possible to generate phase information for structure factors up to 6 A resolution. pH- and radiation-induced anisotropic lattice changes were exploited to reduce the problem of overlapping reflections, which is a major challenge in protein powder diffraction. The resulting data were of sufficient quality to compute molecular envelopes of the protein molecule and to map out the solvent channels in the crystals. The results show that protein powder diffraction can yield low-resolution data that are potentially useful for the characterization of microcrystalline proteins as novel micro- and mesoporous materials as well as for structural studies of biologically important macromolecules