Polarization selectivity of interaction of DNA molecules with X-ray radiation

Abstract

The optimum form of a long helical molecule, which DNA is, has been calculated in terms of the classical electromagnetic theory. Three different methods of classical electrodynamics are used: the theory of dipole radiation of electromagnetic waves, the energetic power approach, and a helical model of molecules of chiral medium. In all three cases, an identical result for the optimum geometrical form of a long spiral molecule has been obtained. The lead angle between the tangent to the helix and the plane normal to the axis of the helix should be equal to 24.5 degrees. This condition imposes restrictions on the radius and the pitch of the helical molecule. The experimentally measured geometrical characteristics of the DNA molecule satisfy the theoretically calculated condition precisely enough. Having the optimum geometrical form, the DNA molecule is not influenced by a circularly right-polarized electromagnetic wave in the soft X-ray range λ = 7–8 nm. This wave, for which the right-handed DNA molecule is “transparent,” should propagate orthogonally to the helix axis and form a right-handed screw in space. The wave radiated by the right-handed DNA molecule orthogonally to helix axis in the range of λ ≈ 7–8 nm has, accordingly, the left-handed circular polarization. The polarization selectivity of the DNA molecule by the action of X-ray radiation is exhibited strongly enough in the wavelength range of λ ≈ 1–35 nm. The results obtained are valid for any distribution of electric currents in DNA, i.e., for any sequence of nitrogenous bases in DNA.