HYDRATION EFFECTS ACCOMPANYING THE SUBSTITUTION OF COUNTERIONS IN THEIONIC ATMOSPHERE OF POLY(RA)CENTER-DOT-POLY(RU) AND POLY(RA)CENTER-DOT-2POLY(RU) HELICES

The physical properties of nucleic acid helices strongly depend on the ir interaction with counterions and water. Of special interest is the structure of their ionic atmosphere which is closely related to its ov erall hydration because the distances between counterions and the atom ic groups of a helix are comparable to the size of a water molecule. I n an effort to answer whether the putative higher linear charge densit y of triple helices, relative to duplexes, exerts special effects on t he structural and hydration parameters of their ionic atmosphere, we u sed a combination of high-precision ultrasonic velocity and density te chniques to follow the hydration effects upon substituting Na+ for Cs or Mg+ ions in the ionic atmosphere of poly(rA). 2poly(rU) and poly(r A) poly(rU). The titration of the Na+ salt of each helix with Cs+ resu lts in marginal hydration effects. This indicates that both Na+ and Cs + form outer-sphere counterion-RNA complexes, where the counterion kee ps its coordinated water molecules. The substitution of Na+ for Mg2+ r esults in positive compressibility values (3 x 10(-4) cm(3)/bar per mo l of nucleotide) and volume effects (2 cm(3) per mol of nucleotide), a nd reflects a small dehydration of the whole Mg2+-RNA complexes. The o verall dehydration level corresponds to the formation of outer-sphere complexes, indicating that Mg2+ keeps most of its coordinated water in the ionic atmosphere of each helical structure. The resulting small e ffects in the hydration parameters of the tripler ionic atmosphere sug gest that the structure of the counterion-triplex complex may be deter mined by short-range interactions, including the immobilization of wat er molecules, while the long-range electrostatic interactions that det ermine the condensation of counterions do not show a significant influ ence on the local structure of the ionic atmosphere of these helices.