A Two-Dimensional DNA Array: The Three-Layer Logpile

We describe the three-layer logpile (3LL), a two-dimensional DNA array which self-assembles from four synthetic oligonucleotides via a four-armed Holliday junction motif. It consists of three layers of helices, each running at 60° to the others. DNA arrays can be used as periodic templates to create, for example, synthetic protein crystals: this array is designed to maximize structural order by ensuring that helices run continuously, without bending, through the structure. UV absorbance measurements show a rate-dependent hysteresis associated with the assembly of the 3LL. Negative-stain transmission electron microscopy (TEM) of 3LL samples shows that the arrays form extensive sheets (∼μm2) and a process of iterative correlation mapping and averaging of small subsets of digitized TEM micrographs yields an averaged projection image that is consistent with a computer-generated model of the crystal.