A KNOWLEDGE of possible conformational states of double-helical nucleic acids is fundamental to an understanding of how they work1,2. Models for a series of conformations have been made on the basis of X-ray data3–5. Investigations of nucleic acids in solution point to the existence of a continuum of forms but do not make possible the quantitative determination of the conformational parameters. Here, theoretical investigation of the intramolecular interaction energy as a function of conformational parameters and a search for minimal energy regions in the space of these parameters become essential.