Section 19.5.8.2. Polynucleotides

The molecular structures of a series of DNA and RNA helices have been determined and refined using data from polycrystalline fibres (Arnott et al., 1969; Chandrasekaran & Arnott, 1989). These include the canonical A, B and C forms of DNA, corresponding, respectively, to 11-, 10- and 9.3-fold right-handed antiparallel Watson–Crick base-paired helices. Structural differences between the three have been attributed to changes in furanose puckerings and helical parameters: the A form has C3-endo, but B and C have C2-endo or analogous C3-exo puckers. All RNA duplexes are members of the A family. Later important structures included the sixfold single helix of poly (C) (Arnott et al., 1976), a compact eightfold double helix for poly d(AT) and poly d(IC) (Arnott et al., 1983), and the left-handed Z-DNA for poly d(GC) (Arnott et al., 1980). Difference Fourier syntheses were instrumental in locating a spine of water molecules in the minor groove and a series of sodium ions and water molecules that bridge the phosphate groups of adjacent DNA molecules in the tenfold helices of poly (dA)·poly (dT) (Chandrasekaran et al., 1995), poly (dA)·poly (dU) and poly d(AI)·poly d(CT) (Chandrasekaran et al., 1997). Data from noncrystalline fibres have been used to determine, among others, the structures of DNA·RNA hybrid duplexes (Arnott et al., 1986), a DNA triple-stranded helix (Chandrasekaran et al., 2000a) and two RNA triple-stranded helices (Chandrasekaran et al., 2000b,c). In each case mentioned, the best model was clearly preferred statistically (Hamilton, 1965) and had an R value between 0.2 and 0.3 to about 3 Å resolution.