  We report theoretical studies of charge transport in single-stranded DNA in
the direction perpendicular to the backbone axis. We find that, if the
electrodes which sandwich the DNA have the appropriate spatial width, each
nucleotide carries a unique signature due to the different electronic and
chemical structure of the four bases. This signature is independent of the
nearest-neighbor nucleotides. Furthermore, except for the nucleotides with
Guanine and Cytosine bases, we find that the difference in conductance of the
nucleotides is large for most orientations of the bases with respect to the
electrodes. By exploiting these differences it may be possible to sequence
single-stranded DNA by scanning its length with conducting probes.
