Electron magnetic resonance and SQUID measurement study of natural $A$-DNA in dry state

We present electron magnetic resonance study on the magnetization of fibrous salmon sperm $A$-DNA in dry state. Appearance of a unusually broad signal around $500\mathrm{G}$ overlapped with a narrower signal at about $3000\mathrm{G}$ persisted over the whole temperature range of $4.5–300\mathrm{K}$. The results are explained by assuming coexistence of an applied magnetic field-induced mesoscopic orbital magnetism associated with a long-range coherent transport in well ordered regions and a Pauli paramagnetism arisen from an incoherent hopping transport in disordered regions. In the molecularly ordered regions, the partial interbase hybridization of ${\pi }_z$ orbitals along the double helix axis is envisaged, whereas it is inhibited in the disordered regions due to structural defects and conformational irregularities along the backbone of the duplex. The spins located in the former regions exhibit the $g$ value greater than 10, while those in the latter regions exhibid the $g$ value of about 2.0. dc magnetic behavior of the natural $A$-DNA observed by the SQUID measurements also supports the explanation by a temperature-independent S-shaped magnetization accompanied with a linear Pauli susceptibility. We learn from the experimental results and the proposed model that the charge carriers are coherently transported giving rise to a strong orbital magnetic moment in the ordered regions.

Figure 1

(a) The representative EMR signals of dry $A$-DNA obtained at various room temperatures. One realizes that the EMR spectra consist of two distinct signals. The one is very broad with the linewidth of more than $1000\mathrm{G}$ centered at about $500\mathrm{G}$, and the other is relatively narrow at about $3000\mathrm{G}$. (b) Deconvolution of an EMR peak at $300\mathrm{K}$ by Lorentzian fitting.

Figure 2

The temperature dependence of susceptibility ${\chi }^{″}\left(\omega \right)$, $\mathrm{\Delta }{H}_{\mathrm{fwhm}}$, and $g_{\mathrm{avg}}$ that were produced by fitting the EMR line shapes with two Lorentzian functions. (a) One notices that the susceptibilities are constant over the whole temperature range, (b) the $\mathrm{\Delta }{H}_{\mathrm{fwhm}}$ show the contrast temperature-dependent behaviors with increasing temperature: the $\mathrm{\Delta }{H}_{\mathrm{fwhm}}$ value for the peak of $g>10$ increases with increasing temperature, whereas the value for the peak of $g\sim 2$ decreases with increasing temperature. The dashed line corresponds to the fitted result to the equation of $\mathrm{\Delta }{H}_{\mathrm{fwhm}}=a+bT$ $\left[\right(a=3822$ $(\pm 3)\mathrm{G}$, $b=3.274$ $(\pm 0.02326)$ G/K]. (c) For each $g$ factor, a common ferromagnetic shift is manifest.

Figure 3

(Color online) The $M\text{−}H$ curves at room and liquid helium temperatures. Both the $M\text{−}H$ curves show positive slopes up to $2500\mathrm{G}$, but above this field, they exhibit negative slopes. The solid and dashed lines represent the fitted curves by linear combination of the calculated molecular diamagnetism [$-2.71\times {10}^{-4}\mathrm{emu}∕\mathrm{G}\left(\mathrm{mol}\mathrm{nucleotide}\right)$; (blue) dot], the Pauli paramagnetism [(green) dashed dotted line], and the mesoscopic magnetism [(orange) solid line] due to the helical current loops induced by applied magnetic field.

Figure 4

The temperature dependence of molar susceptibilities, $\chi$, obtained at 1000 and $10000\mathrm{G}$. The solid and dashed lines indicate the fitting results using the equation of $\chi \left(T\right)=C∕T+{\chi }_{\mathrm{const}}$.