Secondary radical attack on DNA nucleotides: reaction by addition to DNA bases and abstraction from sugars.

In this work radicals generated by dissociative electron attachment to iodoacetamide (H2NCOCH2.) and 6-chloromethyluracil (U5CH2.) are suggested to react with DNA nucleotides in frozen aqueous solutions via either hydrogen abstraction or addition to the double bonds of the bases. Methyl hydrogens of TMP are the preferential sites of the attack by H2NCOCH2. radical. For dCMP the C1' site on the sugar group is found to be the preferred site of hydrogen abstraction by H2NCOCH2. while for dGMP and to a lesser extent dAMP attack at the C8 position of the purine ring is found to be competitive with sugar attack. In general allylic U6CH2. and U5CH2. radicals are found to be poor hydrogen abstractors and the only reaction pathway found is the addition to double bonds at C6 in thymine and C8 in adenine and guanine. Whereas, the cytosine 5,6 double bond appears to be unreactive towards addition at low temperatures. Some evidence is found for sugar radical addition to the adenine C8 position.