PRESSURE-DEPENDENCE OF THE MELTING TEMPERATURE OF DA-CENTER-DOT-DT POLYMERS

The pressure dependence of the helix-to-coil transition temperature (T (m)) of poly[d(A-T)], poly(dA).poly(dT), and poly(dA).poly(dT)2 was st udied as a function of sodium chloride concentration. The molar volume change of the transition (DELTAV) was calculated using the Clapeyron equation and calorimetrically determined enthalpies. The DELTAV of the transition varied linearly with the logarithm of the salt concentrati on for both double-stranded polymers. The DELTAV of poly[d(A-T)] chang ed from +0.36 to +3.86 cm3 mol-1 as the sodium chloride concentration changed from 0.020 to 1.0 M. For poly(dA).poly-(dT), the DELTAV varied from +2.60 to +4.59 cm3 mol-1 over the range of 0.020-0.20 M NaCl. No pressure dependence of the van't Hoff enthalpy was observed for the d ouble-helical form of either polymer. The DELTAV for denaturation of t he triple-helical species poly(dA).poly(dT)2 was found to be +7.81 and +10.4 cm3 mol-1 at 1.0 and 3.0 M NaCl, respectively. The observed DEL TAV values are interpreted in terms of changes in the hydration of the polymers arising from release of counterions and changes in the stack ing of the bases upon denaturation. Counterion release is assumed to m ake a net negative contribution to the overall DELTAV, implying that d isruption of the stacking interactions must make a positive volume cha nge to the overall DELTAV. The difference in the DELTAV values for the two polymers remains constant as the salt concentration is changed, s uggesting a difference in the partial molar volume of their single-str anded forms. The more positive DELTAV for poly(dA).poly(dT) is interpr eted as arising from the greater base stacking of this polymer. The ma gnitude and salt-induced change in the DELTAV for denaturation of trip le-stranded poly(dA).poly(dT)2 with respect to the DELTAV of the doubl e-stranded form imply that the effect of adding another strand to the helix results in a linear change in the observed DELTAV once the effec t of sodium chloride concentration on this parameter has been consider ed.