Photoionization of organic phosphates by 193 nm laser light in aqueous solution: Rapid intramolecular H-transfer to the primarily formed phosphate radical. A model for ionization-induced chain-breakage in DNA?

In aqueous solution, 193 MI (6.4 eV) photolysis of inorganic and organic phosphates such as ribose-5-phosphate leads to ionization with formation of the corresponding oxygen-centered phosphate radicals, O3PO.. These (oxidizing) radicals function as traps with respect to hydrogens attached to alpha-, beta-, or, possibly, gamma-carbons, whereby in the case of the beta-hydrogens a six-membered transition state for transfer of the hydrogen to the phosphate oxygen is possible, leading to high rate constants (up to >5 x 10(7) s(-1)) for II-transfer in these unimolecular reactions. In the case of (deoxy)ribosephosphates the six-membered transition state is possible for transfer of the hydrogen at C4 to the phosphate group at C5 as well as at C3. In DNA, the resulting C4'-radical will undergo a rapid beta-elimination of the phosphate-ester group, this step representing the DNA chain break. The apparently easy ii-transfer from a carbon to a phosphate radical, by which these radicals are "repaired", is why phosphate radicals are not observed in irradiated DNA. Insofar as hereby the C4'-radical is formed, the mechanism of DNA chain breakage is the same for the "direct" and the "indirect" effect.