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 sample preparation : the dna origami structure was designed by using cadnano software ( version 0.2.3 ; http://cadnano.org/legacy;7 design schematics in figure  s1 ) . curvature along the length of the lever was introduced by a pattern of base deletions and insertions as described previously ( deletion pattern in figure  s2).13 the 8634nucleotide singlestranded scaffold dna , derived from m13mp18 , was prepared as previously described.12 , 25 scaffold dna ( 10  nm ) was mixed with each staple strand ( 100  nm ; high purity salt free , mwg eurofins operon ) in te buffer ( tris ( 10  mm , ph  7.6 ) , edta ( 1  mm ) ) containing mgcl2 ( 18  mm ) and nacl ( 5  mm ) .
the mixture was subjected to a nonlinear thermal annealing ramp from 65 to 25  c over 24  h ( see the supporting information ) .
folded structures were electrophoresed on 0.7  % agarose gels containing 0.5tbe buffer ( tris ( 45  mm ) , boric acid ( 45  mm0 , edta ( 1  mm ) ) withmgcl2 ( 11  mm ) and ethidium bromide ( 0.5  g  ml ) at 5.5  v  cm for 2  h in an icewater bath .
dna was recovered by manually squeezing the excised gel slice and collecting the liquid . 

tem imaging : gelpurified origami solution ( 3  l ) was adsorbed onto glowdischarged tem grids ( formvar / carbon , 300  mesh cu ; ted pella , redding , ca ) at 20  c , and then stained with aqueous uranyl formate ( 2  % ) containing sodium hydroxide ( 25  mm ) .
imaging was performed at 30  000 magnification ( zoomout images in figure  s5 at 12  000 magnification ) with a jem1011 transmission electron microscope ( jeol ) operated at 80  kv , equipped with a fastscanf114 camera ( tvips , gauting , germany ) .
particles in tem images were picked by the interactive boxing routine ( e2boxer.py ) of eman2 software ( http://blake.bcm.edu/emanwiki/eman ; example set of particles in figure  s6).26 

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abstractthe application of commonly used force spectroscopy in biological systems is often limited by the need for an invasive tether connecting the molecules of interest to a bead or cantilever tip . here 
 we present a dna origamibased prototype in a comparative binding assay . 
 it has the advantage of in situ readout without any physical connection to the macroscopic world . 
 the seesawlike structure has a lever that is able to move freely relative to its base . 
 binding partners on each side force the structure into discrete and distinguishable conformations . 
 model experiments with competing dna hybridisation reactions yielded a drastic shift towards the conformation with the stronger binding interaction . with reference dna duplexes of tuneable length on one side 
 , this device can be used to measure ligand interactions in comparative assays .