DISSOCIATION OF A MODEL DNA COMPOUND DAPDA BY MONOCHROMATIC SOFT X-RAYS IN SOLIDS AND COMMENTS ON THE HIGH SELECTIVITY FOR 3' BREAKAGE IN THE PHOSPHOESTER BOND

The dissociation products of dApdA (2'-deoxyadenylyl-(3'-5')-2'-deoxyadenosine) irradiated in the solid state by highly monochromatic soft X-rays (energy resolution, < 10(-3)) near the K-edge of phosphorus were analysed using thin-layer chromatography. The major chemical species identified were adenine base and 5'-dAMP in approximately equal amounts, indicating that the dissociation occurred between the 3' carbon (C3') of deoxypentose and its adjacent phosphorus over the energy range of 2.147-2.167 keV, including the K-edge resonance absorption of phosphorus at 2.153 keV. In conjunction with the accumulated data on the degradation of dApdA and related oligonucleotides by vacuum-UV radiation (above 7 eV), which generally indicate a simple, selective dissociation at the 3' side of the deoxypentose as observed with the soft X-rays, an hypothesis is presented on the molecular mechanism of radiation-induced breakage of phosphoester bonds that led to the selective 3' breakage, based on the differential flexibility of torsion angles of C3'-O3'-P and C4'-C5'-O5'-P groups in relation to those of the sugar ring.