The conformation of single‐strand polynucleotides in solution: sedimentation studies of apurinic acid

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Abstract
To elucidate the role of the bases in single‐strand polynucleolide conformation, we have studied apurinic acid, a single‐strand polydeoxyribonucleotide from which almost half the bases have been removed. The conformation of apurinic acid in aqueous solution near θ condition has been investigated by sedimentation velocity and sedimentation equilibrium measurements. The unperturbed coil dimensions of apurinic acid are essentially identical to those of poly rU of the same degree of polymerization. The dimensions are also similar to those of poly rA at high temperature, where the adenine residues are not stacked upon one another. We conclude that the considerable rigidity of these polynuclotides is conferred not by residual, undetected base stacking, but by restrictions in rotation about the bonds of the backbone. Furthermore, the rigidity of the ribose‐phosphate backbone cannot be attributed to interactions involving the 2'—OH group.