MOLECULAR-MODELS OF NUCLEIC-ACID TRIPLE HELIXES - II - PNA AND 2'-5'-BACKBONE COMPLEXES

We describe nucleic acid triple-helical structures containing either a mide or 2'-5' linkages, the former backbone describing the chemistry o f certain peptide nucleic acids (PNA). The methodology and the startin g reference frame are the same as those described in the preceding art icle. Apart from evaluating the possible combinations of chain conform ations that connect adjacent bases on each of the three strands, we ha ve examined the feasibility of tripler formation when neighboring Wats on-Crick+Hoogsteen hydrogen-bonded base tripler are displaced by small amounts along their short and long axes. The predicted triple-helical complexes are examined in terms of relevant crystallographic, spectro scopic, and calorimetric data, The computed models clarify why PNA can not form B-like structures and also reveal principles useful for the d esign of other triplex-forming DNA mimics.