  We report the observation of a Twist Grain Boundary (TGB) phase of DNA, a
striking motif of three dimensional (3D) equilibrium self-assembly of the DNA
tetramer 5'-GTAC-3', a base sequence that is self-complimentary, pairing to
form 4-base long, blunt-end Watson/Crick (WC) duplexes. Hydrophobic blunt ends
and liquid crystal ordering enable these short duplexes to aggregate into
long-DNA-like columns, even though the double helix has a break every 4 bases.
A further step assembles these columns into monolayer sheets in which the
columns are mutually parallel, and, finally, these sheets stack into lamellar
arrays in which the column axis of each layer rotates in helical fashion,
through a 60 degree angle with each passage to the next layer. This
reorientation in a left-handed TGB helix enables each 2nm diameter WC column to
be parallel to, and to partially enter, the major grooves of its neighbors.
