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 nm (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, O 3 PO • . These (oxidizing) radicals function as traps with respect to hydrogens attached to α-, β-, or, possibly, γ-carbons, whereby in the case of the β-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 × 10 7 s - 1 ) for H-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 β-elimination of the phosphate-ester group, this step representing the DNA chain break. The apparently easy H-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.