An ordered single-stranded structure for polyadenylic acid in denaturing solvents. An X-ray fiber diffraction and model building study

A new ordered structure has been observed in fibers of poly(rA) made from strongly denaturing solvents such as formamide or dimethylsulfoxide. The X-ray diffraction pattern together with other evidence suggests that this unique “denatured” structure is a single-stranded helix where the adenylyl residues are related by a 180 ° rotation and a 2.9 Å advance along the helix axis. The backbone is located in the center of the structure and the bases point outwards. The bases do not participate in either direct hydrogen-bonding or in stacking interactions.