Preferential binding and structural distortion by Fe2+ at RGGG-containing DNA sequences correlates with enhanced oxidative cleavage at such sequences

Certain DNA sequences are known to be unusually sensitive to nicking via the Fe2+-mediated Fenton reaction. Most notable are a purine nucleotide followed by three or more G residues, RGGG, and purine nucleotides flanking a TG combination, RTGR. Our laboratory previously demonstrated that nicking in the RGGG sequences occurs preferentially 5' to a G residue with the nicking probability decreasing from the 5' to 3'end of these sequences. Using 1H NMR to characterize Fe2+ binding within the duplex CGAGTTAGGGTAGC/GCTACCCTAACTCG and 7-deazaguanine-containing (Z) variants of it, we show that Fe2+ binds preferentially at the GGG sequence, most strongly towards its 5' end. Substitutions of individual guanines with Z indicate that the high affinity Fe2+ binding at AGGG involves two adjacent guanine N7 moieties. Binding is accompanied by large changes in specific imino, aromatic and methyl proton chemical shifts, indicating that a locally distorted structure forms at the binding site that affects the conformation of the two base pairs 3' to the GGG sequence. The binding of Fe2+ to RGGG contrasts with that previously observed for the RTGR sequence, which binds Fe2+ with negligible structural rearrangements.