What sets the handedness of liquid crystals made of DNA origami?
The twist direction of liquid-crystal phases formed by DNA origami rods is set by their thermal wiggling into gentle helices, not by the twist of their rigid ground-state shape.
Source
Chiral shape fluctuations and the origin of chirality in cholesteric phases of DNA origamis
Study at a glance
- Design
- Computational / modelling — oxDNA molecular dynamics of single six-helix-bundle origami filaments combined with an extended Onsager density functional theory of cholesteric order, compared with published experiments.
- N
- No sample; four origami twist variants were simulated, each with on the order of a thousand uncorrelated conformations.
- Population
- Simulated DNA origami six-helix bundle filaments with different designed axial twists
- Outcome
- Cholesteric pitch and handedness, backbone helicity/writhe, duplex over- or underwinding
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The authors modelled four DNA origami filaments designed with different amounts of left- or right-handed twist. Using a nucleotide-level coarse-grained DNA model, they simulated each filament's thermal shape fluctuations, then fed either the ground-state shape or the full set of fluctuating shapes into a molecular theory that predicts the cholesteric pitch without adjustable parameters. Results were compared with earlier experimental measurements.
What they found
Using only rigid ground-state shapes predicted the wrong handedness: phases twisted opposite to the filaments, while experiments showed the same handedness. Adding electrostatic repulsion mainly loosened the pitch and did not fix the sign. Including thermal fluctuations reversed the handedness and tightened the pitch, giving good overall agreement with experiment. The fluctuating filaments writhe into long, weak helices of opposite sense to their twist, because writhing relieves over- or underwinding of the constituent DNA duplexes.
The limits
What it doesn't show
The predicted handedness inversion happens at a slightly different filament twist than in experiment, and predicted pitches are somewhat too large. The oxDNA model uses sequence-averaged mechanics and simple electrostatics, simulations used a higher salt concentration than experiments, and the theory neglects biaxial correlations. The work is purely computational and relies on external experiments for validation.
Key terms
- Cholesteric liquid crystal
- A liquid-crystal phase in which the preferred molecular direction rotates steadily along one axis, forming a helix.
- Cholesteric pitch
- The distance over which the molecular alignment direction rotates by a full turn.
- Writhe
- How much a filament's backbone coils in space, as opposed to twisting about its own axis.
- Onsager theory
- A theory predicting ordering of elongated particles from how their excluded volume depends on relative orientation.
- DNA origami
- Nanostructures made by folding a long DNA strand with many short staple strands into a designed shape.
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Quiz yourself
What did calculations using only rigid ground-state origami shapes predict?
Common questions
Why is it surprising that the phases share the filaments' handedness?
Simple packing arguments predict weakly twisted rods pack best in the opposite sense, which is what the ground-state calculation gave.
Was electrostatics the missing ingredient?
No. Adding Debye-Hückel repulsion mainly unwound the pitch; thermal shape fluctuations were needed to flip the handedness.
Why do filaments writhe opposite to their twist?
Writhing in that sense partly relieves over- or underwinding strain in the DNA duplexes caused by the design's base-pair deletions or insertions.
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