Probing Hydration of Monovalent Cations Condensed Around Polymeric Nucleic Acids

We report the relative molar sound velocity increments, [U], partial molar volumes, Vo, and partial molar adiabatic compressibilities, KSo, of the Li+, Na+, K+, Rb+, Cs+, NH4+, and N(CH3)4+ salts of poly(dAdT)poly(dAdT), poly(dGdC)poly(dGdC), poly(dIdC)poly(dIdC), poly(rA)poly(rU), poly(rG)poly(rC), poly(rI)poly(rC), and poly(rU) at 25 °C. When analyzing these data, we take into account the Donnan membrane equilibrium effect. Comparison between the values of [U], Vo, and KSo exhibited by the nucleic acid salts and respective chlorides (LiCl, NaCl, KCl, RbCl, CsCl, NH4Cl, and N(CH3)4Cl) yields information about the state of counterion hydration in the vicinity of each nucleic acid structure studied here. Our analysis reveals that the poly(dGdC)poly(dGdC), poly(dIdC)poly(dIdC), and poly(rI)poly(rC) duplexes and single-stranded poly(rU) do not significantly influence the hydration properties of their condensed counterions. In the vicinity of these polymers, counterions retain their full hydration shells (within ±15%). By contrast, counterions condensed around the poly(dAdT)poly(dAdT), poly(rA)poly(rU), and poly(rG)poly(rC) duplexes are significantly dehydrated and retain, respectively, only 65(±18)%, 34(±21)%, and 33(±9)% of their original hydration shells. Taken together, the volumetric data reported here provide important new information that ultimately may help us understand the central role that hydration and counterions play in modulating the conformational preferences of nucleic acids and the energetics of DNA recognition events.