The crystal structure of a nucleic acid triplex reveals a helix, desig
nated P-form, that differs from previously reported nucleic acid struc
tures. The triplex consists of one polypurine DNA strand complexed to
a polypyrimidine hairpin peptide nucleic acid (PNA) and was successful
ly designed to promote Watson-Crick and Hoogsteen base pairing. The P-
form helix is underwound, with a base tilt similar to B-form DNA. The
bases are displaced from the helix axis even more than in A-form DNA.
Hydrogen bonds between the DNA backbone and the Hoogsteen PNA backbone
explain the observation that polypyrimidine PNA sequences form highly
stable 2:1 PNA-DNA complexes. This structure expands the number of kn
own stable helical forms that nucleic acids can adopt.