X-ray induced luminescence spectroscopy for DNA damaging intermediates aided by a monochromatic synchrotron radiation

To identify the bonding sites of initial radiation interaction with DNA and to trace the following chemical reaction sequences on the pathway of damage induction, we carry out a spectroscopy XIL (X-ray induced luminescence) using soft X-ray synchrotron radiation. This is a nondestructive analysis of the excited intermediate species produced in a molecular mechanism on the damage induction pathway.

We introduce aqueous samples of UMP (uridine-5'-monophosphate) in the vacuum by the use of a liquid micro-jet technique. The luminescence in the region of UV-VIS (from visible to ultraviolet) radiation induced after the absorption of monochromatic soft X-ray by aqueous UMP is measured with sweeping the soft X-ray energy in the region of 370-560 eV.

The enhanced XIL intensities for aqueous UMP in the region of soft X-ray of 410-530 eV (in "water window" region) are obtained. The enhancement of XIL intensities in the UV-VIS region, relative to the water control, is explained by the excitation and ionization of a K-shell electron of nitrogen atoms in the uracil moiety. The enhanced XIL intensities do not match the structure of XANES (X-ray absorption near-edge structure) of the aqueous UMP. This suggests that the XIL intensities reflect the quantum yields of luminescence, or the quantum yields for conversion by UMP of an absorbed X-ray into UV-VIS radiation. In this paper, spectra of luminescence are shown to be resolved by combining low pass filters. The filtered luminescence spectra are obtained at the center of gravity (λc) of the band pass wavelength regions at λc = 270nm, 295 nm, 340 nm, 385 nm, 450 nm, and 525 nm., which show a trend similar to the fluorescence of nucleobases induced by ultraviolet radiation.

It is concluded that the origin of the observed XIL is the hydrated uracil moiety in aqueous UMP, decomposition of which is suppressed by the migration of excess charge and internal energy after the double ionization due to Auger decay.