Impact of pyrophosphate and O-ethyl-substituted pyrophosphate groups on DNA structure

Design of novel DNA probes to inhibit specific repair pathways is important for basic science applications and for use as therapeutic agents. As shown previously, single pyrophosphate (PP) and O-ethyl-substituted pyrophosphate (SPP) modifications can inhibit the DNA glycosylase activities on damaged DNA. To understand the structural basis of this inhibition, the influence of the PP and SPP internucleotide groups on the helical parameters and geometry of a double-stranded DNA was studied by using molecular modeling tools including molecular dynamics and quantum mechanical-molecular mechanical (QM/MM) approaches. Native and locally modified PP- and SPP-containing DNA duplexes of dodecanucleotide d(C1G2C3G4A5A6T7T8C9G10C11G12) were simulated in aqueous solution. The energies and forces were computed by using the PBE0/6-31+G** approach in the QM part and the AMBER force-field parameters in the MM part. Analysis of the local base-pair helical parameters, internucleotide distances, and overall global structure at the located stationary points revealed a close similarity of the initial and modified duplexes, with only torsion angles of the main chain being altered in the vicinity of introduced chemical modification. Results show that the PP and SPP groups are built into a helix structure without elongation of the internucleotide distance due to flipping-out of phosphate group from the sugar-phosphate backbone. The mechanism of such embedding has only a minor impact on the base pairs stacking and Watson-Crick interactions. Biochemical studies revealed that the PP and SPP groups immediately 5', but not 3', to the 8-oxoguanosine (8oxodG) inhibit translesion synthesis by a DNA polymerase in vitro. These results suggest that subtle perturbations of the DNA backbone conformation influence processing of base lesions.