Magnetic study of the electronic states of $B$-DNA and $M$-DNA doped with metal ions

The magnetic properties of the pristine and metal ion doped deoxyribonucleic acid (DNA) of salmon are investigated with electron paramagnetic resonance (EPR), superconducting quantum interference device and energy dispersive x-ray flourescence spectroscopy. Purified salmon DNA gives intrinsically no EPR signal, which is consistent with DNA being a semiconductor, but not with DNA having metallic or superconducting properties as reported previously. Several kinds of divalent ions (Zn, Mn, Ca, …) are used as dopants, resulting in no substantial EPR signal except in the case of Mn. This leads to the conclusion that a metal ion counterbalances two phosphate anions instead of Na counterions in $B$-DNA, which contradicts the metallic behavior reported previously [A. Rakitin et al., Phys. Rev. Lett. 86, 3670 (2001)].

Figure 1

(Color online) Schematic structure of (a) $B$-DNA and (b) $M$-DNA. Two DNA backbones are represented by the sinusoidal curves, made of a repeat of deoxyribose sugar and phosphate ion (open circle). A step of DNA “ladder” is made of a base pair, either guanine-cytosine or adenine-thymine, which is connected by three or two hydrogen bonds, respectively. (a) In $B$-DNA each phosphate anion is attended by a ${\mathrm{Na}}^{+}$ cation to form a salt. (b) Proposed structure of $M$-DNA (Ref. 5).

Figure 2

(Color online) EPR spectra of S-powder and S-fiber (salmon DNA). The relative intensities are calibrated relative to each other.

Figure 3

(Color online) EPR spectra of Mn-DNA taken at room temperature at x band. Two sharp peaks above and below the Mn spectrum are the signals from the ruby standard. The inset represents the six hyperfine split peaks in the 10% Mn-doped DNA.

Figure 4

(Color online) Magnetic susceptibility of Mn-DNA measured with SQUID susceptometer at $1\mathrm{T}$. The estimated spin concentration is about one $S=5∕2$ spin per base pair. The inset shows the expanded view at low temperatures demonstrating $\Theta \approx -0.8\mathrm{K}$.

Figure 5

(Color online) EPR spectrum of Zn-DNA. The signal intensity is much weaker than that for Mn-DNA, as implied by a low $S∕N$ ratio compared with that in Mn-DNA.