ABSTRACT
A conventional Na+ counterion in dry natural DNA can be replaced by other metallic ions from alkali metals, alkaline earth metals,
and transition metals [264]. A natural DNA sample with a metallic
ion (M) is written as M-DNA. Using divalent metallic ions (Co2+, Ni2+, and Zn2+) incorporated inside the helix [16, 57], metallic conductions were suggested by the I-V characteristics [17, 29]. Furthermore, the reduction of π-π∗ in Zn-DNA was pointed out in ultraviolet spectroscopy [46], and strong electronic correlation was
reported [24]. The ESR signal due toπ electrons, however, was never
detected in M-DNA (M = Zn2+, Mn2+, Ca2+, and Mg2+) [27, 94, 157]. From the viewpoint of ESR studies, the incorporation of divalent
metallic ions is not so easy to inject a carrier in M-DNA. On the other hand, the valence of Fe ions incorporated in Fe-DNA is found to be
3+ [94], and the charge transfer from Fe2+ to DNA is indicated by the intragap absorption [27].
