POLYRIBOADENYLIC AND POLYDEOXYRIBOADENYLIC ACIDS . OPTICAL ROTATORY STUDIES OF PH-DEPENDENT CONFORMATIONS AND THEIR RELATIVE STABILITY

The ultraviolet optical rotatory dispersion, circular dichroism, and absorption properties of polyriboadenylic acid, polydeoxyriboadenylic acid, and the corresponding dinucleoside phosphates were examined as a function of pH and dioxane content. The objective of the study was to examine similarities and differences among the structural forms assumed by the ribose and by the 2′-deoxyribosecontaining polymers under various conditions. Circular dichroism at neutral pH, conditions under which the polymers form single-strand helices, confirms the optical rotatory dispersion finding of low-magnitude Cotton effects for polydeoxyriboadenylic acid, especially at high wavelength. The lack of dependence upon ionic strength of this ellipticity band implies that phosphate repulsion is not responsible for this reduced rotation. However, the relative stability of neutral polydeoxyriboadenylic acid toward dioxane may indicate a unique base-stacking arrangement. Optical rotatory dispersion of polyriboadenylic acid and of polydeoxyriboadenylic acid, examined as a function of pH, shows that each polymer can exist in two different acidic forms. In both cases, one form (with higher wavelength Cotton effect) is present when the pH equals the pKa of adenyl residues in the polymer; below this pH there is a gradual conversion to the more acidic structure. Spectrophotometric titrations do not reveal the presence of the two forms. The optical rotation temperature dependence of polydeoxyriboadenylic acid in acid is complicated by depurination. Evidence that the acidic forms of polydeoxyriboadenylic acid are double strand is obtained from the pH dependence of complex formation with polyribouridylic acid. The differences in the acidic forms of polyriboadenylic acid and polydeoxyriboadenylic acid can be seen from the effect of dioxane upon these structures. © 1969, American Chemical Society. All rights reserved.