DNA under tight bending exhibits twist-bend coupling that increases the natural helical repeat from 10.45 to >11 bp/turn at biologically relevant curvatures, and remains mechanically stable without kinking only when the radius of curvature r ≥ ~3 nm and helical repeat h ≤ ~10.9 bp/turn, with histone proteins mechanically stabilizing DNA through overwinding—findings that require reassessing the energetics of fundamental biological processes including nucleosome dynamics, transcription factor binding, and DNA packaging.

DNA is under high bending and torsional strain in many biological contexts, but elastic limits of tightly bent DNA under simultaneous torsional strain are not fully understood. We synthesized DNA circles with all possible radii of curvature between r ≈ 2.7 – 5.7 nm in increments of Δr = 0.05 nm and defined helical repeats ranging from h ≈ 10 – 13 base pairs per helical turn. Nuclease digest reveals that DNA can be bent to r ≈ 3.0 nm without kinking, but only when h < ~10.9 bp/turn, while underwound DNA kinks irrespective of curvature. Histone proteins overwind DNA and thereby mechanically stabilize it. The natural helical repeat h0 increases from 10.45 to >11 bp/turn due to twist-bend coupling, which is not caused by kinking before ligation. These findings require reassessing our models and the energetics of molecular mechanisms involving DNA under mechanical stress.