Characteristics of the Watson‐Crick type hydrogen‐bonded DNA base pairs: An ab initio molecular orbital study

Characteristics of the Watson‐Crick‐type hydrogen‐bonded base pairs, thymine‐adenine and cytosineguanine, are presented. These were established using an ab initio molecular orbital method. Base–base interactions are revealed to have dominant roles in the structures of nucleic acids and also in their functions. The most stable conformations in Watson‐Crick‐type base pairs are almost in accord with the structure observed in fiber X‐ray diffraction study. Explanations are given of the origin of the sequencedependent local structures which differ from one base pair to the next, as observed in single‐crystal X‐ray analyses. In the case of the thymine‐adenine base pair, it is shown that various geometrical parameters having almost the same stability are available. According to the propeller twist, the instability is not large enough for a base pair to be twisted readily.