Nucleoside conformations: X. An X-ray fiber diffraction study of the gels of guanine nucleosides☆

The X-ray fiber diffraction study of four guanine nucleoside gels is reported. These nucleosides form helical structures of regularly stacked piles of tetramers in an arrangement proposed by Gellert, M., Lipsett, M. and Davies, D. R. ((1962) Proc. Natl. Acad. Sci U.S. 38, 2013). The calculated Fourier transforms are compatible with the tetrameric model. The helix characteristics of the tetrameric fibers are the following: M = 2 for Guo, M = 53 for 8-Br-Guo, M = 43 for 2′,3′-O-diacetyl-Guo, M = 53 for dGuo. These values correspond to a unit rotation between successive bases of 45° for Guo, 54° for 8-Br-Guo and dGuo, and about 67° for (ac)2Guo. While Guo, 8-Br-Guo and dGuo appear to form continuous head-to-tail tetramer stacks, (ac)2Guo probably has a more complicated helical symmetry, possibly involving head-to-head aligned octamers.