Spontaneous cleavage of single and double stranded 4 '-DNA radicals under aerobic conditions

The O-2-induced strand scission of 4'-DNA radicals is initiated by a revers ible O-2 addition reaction. The rate coefficient of the O-2 release from th e 4'-DNA peroxyl radical is 1.00 s(-1) in single strands and 0.05 s(-1) in double strands at 20 degrees C. Because of this reversibility, an O-2-depen dent strand cleavage occurs only in the presence of H-donors which trap the 4'-DNA peroxyl radicals yielding DNA hydroperoxides. At very low H-donor c oncentrations the strand scission is the result of an O-2-independent, spon taneous reaction even under aerobic conditions.