A NUCLEIC-ACID TRIPLE-HELIX FORMED BY A PEPTIDE NUCLEIC-ACID DNA COMPLEX

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.