Conformational analysis of double‐stranded B‐type DNA structures

A complete conformational analysis of B‐type DNA was carried out in the independent two‐dimensional surface of the hexa‐dimensional conformational space of the helical structures. A theoretical double‐helical model was derived by locating the deepest minimum of the conformational energy. It is similar to that obtained by x‐ray refining methods; significant differences seem to be supported by relevant experimental data. Examination of the energy map allows a direct estimate of the flexibility limits of B‐DNA, which are found to be consistent with a smoothly supercoiled DNA in chromatin. The structure of the hypothetical left‐handed B‐type DNA was predicted to be as stable as the standard right‐handed model at high ionic strength. Left‐ and right‐handed helices are, however, not interconvertible without separation of the strands, and mixing of opposite screw sense fragments is unlikely to give coaxial helices, which would prevent their packing in fibers.