The electronic structure of DNA is theoretically investigated by use of the coherent potential approximation. Even when the sequence of the four kinds of bases is nonperiodic, guanine block forms the persistent highest valence band edge, and adenine block forms the persistent lowest conduction band edge state. According to the calculated joint density-of-states energy profiles, the site first attacked by the lowest excitation is adenine block. After this excitation, electrons are generated at adenine sites, and holes are generated at guanine sites. The resulting electronic structures of the valence band and conduction band suggest that the base complementarity in DNA produces the complementarity in the density-of-states divergences of the excited electrons and holes. This complementarity lowers the excitation instability in the DNA chains.
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