Abstract
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.