Position of caesium ions in crystalline B-form DNA determined by synchrotron radiation diffraction: Preliminary results

Abstract

The diffraction patterns of CsDNA taken with copper X-irradiation are considerably impaired because of the strong X-ray absorption by caesium ions. The use of high power synchrotron radiation of wavelength λ = 1.2 Å has yielded photographs suitable for intensity measurements. The structure of phage T2 CsDNA at 76% relative humidity is isomorphous to the crystalline B-form of LiDNA, and the disposition of cations appears to conform to the 10-fold screw symmetry of B-DNA. The structure factor amplitudes of 20 reflections in the diffraction pattern of phage T2 CsDNA are noticeably different from those of the same reflections in the LiDNA diffraction pattern, and the positions of the Cs + ions could thus be found. The best model has the cations located significantly close to dyad axes lying between the planes of successive nucleotide pairs. One of the two cations “belonging” to a nucleotide pair touches the surface of the narrow groove of the B-DNA double helix, while the other is on the wide groove side, rather far from its “own” DNA molecule.