A method is proposed to measure global bending in DNA and RNA structures. It relies on a properly defined averaging of base-fixed coordinate frames, computes mean frames of suitably chosen groups of bases and uses these mean frames to evaluate bending. The method is applied to DNA A-tracts, known to induce considerable bend to the double helix. We performed atomistic molecular dynamics simulations of sequences containing the A4T4 and T4A4 tracts, in a single copy and in two copies phased with the helical repeat. Various temperature and salt conditions were investigated. Our simulations indicate bending by roughly 10° per A4T4 tract into the minor groove, and an essentially straight structure containing T4A4, in agreement with electrophoretic mobility data. In contrast, we show that the published NMR structures of analogous sequences containing A4T4 and T4A4 tracts are significantly bent into the minor groove for both sequences, although bending is less pronounced for the T4A4 containing sequence. The bending magnitudes obtained by frame averaging are confirmed by the analysis of superhelices composed of repeated tract monomers.
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