PRESSURE-DEPENDENCE OF THE HELIX-COIL TRANSITION-TEMPERATURE OF POLY[D(G-C)]

The pressure dependence of the helix-coil transition temperature (T-m) of poly[d(G-C)] was studied as a function of sodium ion concentration in phosphate buffer. The molar volume change of the transition (Delta V) was calculated using the Clapeyron equation and calorimetrically d etermined enthalpies. The Delta V of the transition increased from +4. 80 (+/-0.56) to +6.03 (+/-0.76) mL mol(-1) as the sodium ion concentra tion changed from 0.052 to 1.0 M. The van't Hoff enthalpy of the trans ition calculated from the half-width of the differentiated transition displayed negligible pressure dependence; however, the value of this p arameter decreased with increasing sodium ion concentration, indicatin g a decrease in the size of the cooperative unit. The volume change of the transition exhibits the largest magnitude of any double-stranded DNA polymer measured using this technique. For poly[d(G-C)] the magnit ude of the change Delta V with sodium ion concentration (0.94 +/- 0.05 mL mol(-1)) is approximately one-half that observed for either poly[d (A-T)] or poly(dA). poly(dT). The Delta V values as interpreted as ari sing from changes in the hydration of the polymer due to the release o f counterions and changes in the stacking of the bases of the coil for m. As a consequence of solvent electrostriction, the release of counte rions makes a net negative contribution to the total Delta V, implying that disruption of the stacking interactions contributes a positive v olume change to the total Delta V. The larger magnitude of the Delta V compared with that of other double-stranded polymers may be due in pa rt to the high helix-coil transition temperature of poly[d(G-C)], whic h will attenuate the contribution of electrostriction to the total vol ume change. The data in addition show that in the absence of other cel lular components, the covalent structure of DNA is stabile under condi tions of temperature and pressure more extreme than those experienced by any known organism. (C) 1995 John Wiley & Sons, Inc.