In-phase ligated DNA containing T(n)A(n) segments fail to exhibit the retarded polyacrylamide gel electrophoresis (PAGE) migration observed for in-phase ligated A(n)T(n) segments, a behavior thought to be correlated with macroscopic DNA curvature. The lack of macroscopic curvature in ligated T(n)A(n) segments is thought to be due to cancellation of bending in regions flanking the TpA steps. To address this issue, solution-state NMR, including residual dipolar coupling (RDC) restraints, was used to determine a high-resolution structure of [d(CGAGGTTTAAACCTCG)2], a DNA oligomer containing a T3A3 tract. The overall magnitude and direction of bending, including the regions flanking the central TpA step, was measured using a radius of curvature, Rc, analysis. The Rc for the overall molecule indicated a small magnitude of global bending (Rc = 138 +/- 23 nm) towards the major groove, whereas the Rc for the two halves (72 +/- 33 nm and 69 +/- 14 nm) indicated greater localized bending into the minor groove. The direction of bending in the regions flanking the TpA step is in partial opposition (109 degrees), contributing to cancellation of bending. The cancellation of bending did not correlate with a pattern of roll values at the TpA step, or at the 5' and 3' junctions, of the T3A3 segment, suggesting a simple junction/roll model is insufficient to predict cancellation of DNA bending in all T(n)A(n) junction sequence contexts. Importantly, Rc analysis of structures refined without RDC restraints lacked the precision and accuracy needed to reliably measure bending.
Quantum calculations on duplex DNA trimers were used to model the changes in structure, hydrogen bonding, stacking properties, and electrostatic potential induced by oxidized purine bases and abasic (AP) sites. Results for oxidized purine bases were consistent with experimental data that show small structural and energetic perturbations induced by isolated 8-oxoguanine (8oG). Watson-Crick base pairing was preserved, and no major distortions of the backbone were induced. The thermal destabilization of DNA induced by 8oG was comparable to the energy of a single hydrogen bond. In contrast, AP sites caused substantial distortions of the DNA backbone that were accompanied by relocation of counterions. The loss of Watson-Crick hydrogen bonds in AP sites had the potential to destabilize DNA by 10-20 kcal/mol (0.4-0.8 eV); however, new inter- and intrastrand hydrogen bonds formed after removal of a nucleic acid base that significantly affected the energy of AP sites and introduced a strong dependence on sequence context. Quantum calculations on small DNA fragments provided starting conformations and force-field parameters for classical molecular dynamics simulations of radiation-induced single-strand breaks that most often combine hydrolysis of a phosphate-oxygen (P-O) bond with an AP site and fully or partially degraded sugar ring. P-O bond hydrolysis increased the freedom in backbone torsion angles, which allowed the broken strand to compress and partially fill the hole in the DNA created by the AP site. Results for strand breaks with a 3'phosphoglycolate were similar to those with phosphate end groups.
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