CONFORMATIONAL TRANSITIONS OF DUPLEX AND TRIPLER NUCLEIC-ACID HELICES- THERMODYNAMIC ANALYSIS OF EFFECTS OF SALT CONCENTRATION ON STABILITY USING PREFERENTIAL INTERACTION COEFFICIENTS

For order-disorder transitions of double- and triple-stranded nucleic acid helices, the midpoint temperatures T-m depend strongly on a(+/-), the mean ionic activity of uniunivalent salt. Experimental determinat ions of dT(m)/d In a(+/-) and of the enthalpy change (Delta H-0) accom panying the transition in excess salt permit evaluation of Delta Gamma , the stoichiometrically weighted combination of preferential interact ion coefficients, each of which reflects thermodynamic effects of inte ractions of salt ions with a reactant or product of the conformational transition Delta Gamma = -(Delta H-0/2RT(m)(2))dT(m)/d In a(+/-) = De lta H-0/2R d(T-m(-1))/d In a(+/-).