  The electronic structure of an infinite poly-guanine - poly-cytosine DNA
molecule in its dry A-helix structure is studied by means of density-functional
calculations. An extensive study of 30 nucleic base pairs is performed to
validate the method. The electronic energy bands of DNA close to the Fermi
level are then analyzed in order to clarify the electron transport properties
in this particularly simple DNA realization, probably the best suited candidate
for conduction. The energy scale found for the relevant band widths, as
compared with the energy fluctuations of vibrational or genetic-sequence
origin, makes highly implausible the coherent transport of electrons in this
system. The possibility of diffusive transport with sub-nanometer mean free
paths is, however, still open. Information for model Hamiltonians for
conduction is provided.
