We show that iron(II)-EDTA-catalyzed cleavage of duplex DNA is much more rapid than cleavage of single-stranded DNA that does not form intramolecular base pairs. Comparisons of the extent of cleavage of the fully single-stranded oligonucleotides d(pT)70, d(pA)70, and d(pC)35 and the duplex DNA d(pT)70. d(pA)70 indicate that the extent of cleavage increases significantly upon formation of the duplex structure. These observations indicate that accessibility of the DNA sugars to the presumed cleaving agent, hydroxyl radical, is not the major determinant for cleavage and that most likely a direct interaction between Fe(II) and the DNA is required. As a result, the interpretation of DNA cleavage experiments performed with this reagent to obtain detailed structural information should be pursued with caution until the mechanism of the cleavage reaction is better understood.
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