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 organ ic phosphates such as ribose-5-phosphate leads to ionization with form ation 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 th e case of the beta-hydrogens a six-membered transition state for trans fer of the hydrogen to the phosphate oxygen is possible, leading to hi gh rate constants (up to >5 x 10(7) s(-1)) for II-transfer in these un imolecular reactions. In the case of (deoxy)ribosephosphates the six-m embered transition state is possible for transfer of the hydrogen at C 4 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-es ter 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 me chanism of DNA chain breakage is the same for the ''direct'' and the ' 'indirect'' effect.