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The application of radiosensitizing halogenated nucleotides for therapeutic cancer treatments using synchrotron X-rays is currently under investigation. It is believed that the primary radiation-induced reaction is a dehalogenation, leading to highly reactive radical species. In a different light, halogenated nucleic acids have also been used in macromolecular cystallography to solve the structure of nucleic acids by anomalous scattering techniques. However, for brominated compounds, a relative moderate X-ray dose can induce sufficient debromination to prevent structure determination. In the present study, we show that polarized absorption spectra on single crystals can yield useful information about radiationinduced reactions. Polarization effects in X-ray absorption spectra recorded on oriented single crystals enable an enhanced detection of features occurring in certain crystal and/or molecular directions. We present here time-resolved polarized X-ray absorption measurements taken on a single crystal of a brominated 23-nucleotide RNA fragment corresponding to the dimerization-initiation site (DIS) of HIV-1(Lai) genomic RNA. The radiation-induced dissociation of the bromine atom from the nucleotide is concomitant with a change in symmetry of the immediate chemical environment of the Br atoms (from highly anisotropic to spherically symmetric). This symmetry change manifests itself as a change of linear X-ray dichroism near the Br K-edge. These measurements show that the nature and the progress of the radiation-induced debromination reaction can be followed easily by absorption or fluorescence measurements, especially if the polarization direction is nearly parallel to the C-Br bonds. Since this type of spectra can be easily and quickly recorded, polarized X-ray absorption emerges as a useful tool for studying radiation-induced reactions in the solid state and for monitoring radiation damage in macromolecular crystallography. m21.p02

A comparative UV-Vis and EXAFS investigation of the coordination of the uranyl ion UO22+in solution

A comparative UV-Vis and EXAFS investigation of the coordination of the uranyl ion UO₂²⁺in solution