  The collective behavior of the shortest DNA oligomers in high concentration
aqueous solutions is an unexplored frontier of DNA science and technology. Here
we broaden the realm of DNA nanoscience by demonstrating that single-component
aqueous solutions of the DNA 4-base oligomer GCCG can spontaneously organize
into three-dimensional (3D) periodic mesoscale frameworks. This oligomer can
form B-type double helices by Watson-Crick (WC) pairing, into tiled
brickwork-like duplex strands, which arrange into mutually parallel arrays and
form the nematic and columnar liquid crystal phases, as is typical for long WC
chains. However, at DNA concentrations above 400mg/mL, these solutions nucleate
and grow an additional mesoscale framework phase, comprising a periodic network
on a three dimensional body-centered cubic (BCC) lattice. This lattice is an
array of nodes (valence-8, each formed by a pair of quadruplexes of GCCG
terminal Gs), connected with a separation of 6.6 nm by struts (6-GCCG-long WC
duplexes). This 3D-ordered DNA framework is of low density (DNA volume fraction
~0.2), but, due to its 3D crystal structure, is osmotically incompressible over
its phase range. Atomistic simulations confirm the stability of such
structures, which promise to form the basis of novel families of simply and
inexpensively made nanoscale frameworks for templating and selection
applications.
