Stabilization of the i-motif by intramolecular adenine-adenine-thymine base triple in the structure of d(ACCCT).

The crystal structure of d(ACCCT), solved by molecular replacement, shows a four-stranded i-motif conformation, where two parallel duplexes intercalate with one another in opposite orientations. Each duplex is stabilized by hemi-protonated C-C+ base pairing between parallel strands, and a string of water molecules bridge the cytosine N4 atoms to phosphate O atoms. This structure of d(ACCCT) shows examples of reversed Hoogsteen and Watson-Crick base pairing in both intermolecular and intramolecular manners to stabilize the tetraplex. Noticeably, the four-stranded complex is further stabilized at one end by a three-base hydrogen-bonding network, in which two adenines and a thymine form four hydrogen bonds via a reverse Hoogsteen and an asymmetric adenine-adenine base pairing. The structure of d(ACCCT) shows a similar local structure to that found in the d(TAA) part of the crystal structure of d(TAACCC) and provides further structural evidence that these base arrangements are essential for stabilizing these novel DNA super-secondary structures.