EFFECTS OF C2H5OH, NA+(AQ), N(CH2CH3)4+(AQ), AND MG2+(AQ) ON THE THERMODYNAMICS OF DOUBLE-HELIX-TO-RANDOM-COIL TRANSITIONS OF POLY(DA)ASTERISK-POLY(DT) AND POLY(DADT)

D.s.c. studies in PIPES buffer (pH = 7.0) have been performed on poly(dA)*poly(dT) and poly(dAdT) at different concentration of Na+, N(CH2CH3)+4, Mg2+, and C2H5OH in the 273 K to 373 K temperature range. For poly(dAdT) at all salt and ethanol concentrations, the d.s.c. curves for the double-helix-to-random-coil transition (called melting) are fitted well by a simple two-state model with ΔHm≈ 26 kJ·mol-1for dAdT pairs and low cooperativity consistent with an extended hairpin structure. For poly(dA)*poly(dT), the d.s.c. curve has an asymmetric transition region, has greater cooperativity, and ΔHm≈ 35 kJ·mol-1. We were unable to substantiate ΔHm= 57.3 kJ·mol-1reported for poly(dA)*poly(dT) for this transition by Herrera and Chaires (Biochemistry1989, 28, 1993), even on a sample supplied by them. Calorimetrically derived Tmvalues (temperature at maximum in d.s.c. curve in the transition region) vary linearly with lg{c(C+)/(mol·dm-3)}, where C+denotes a cation, with a slope slightly below that predicted from counterion condensation theory; higher values were obtained for NaCl using activity coefficients. Over the entire temperature and concentration ranges of C2H5OH, ΔHmfor the melting transition varied less than 15 per cent for both polymers; but Tmdecreased linearly with increasing mass fraction of ethanol in water. The difference in ΔHmof over 8.8 kJ·mol-1between the two polymers for breaking the hydrogen-bond structure, therefore, is probably due to interactions in the interior of the duplex. Two separate transition regions are present in the d.s.c. curves for poly(dA)*poly(dT), however, at all concentrations of C2H5OH studied when c(NA+) <1.0 mol·dm-3and in aqueous solution with c (Mg2+) <0.03 mol·dm-3. The sum of the individual ΔHmvalues for these two transition regions is equal to that for the asymmetric transition at lower c (Na+), and we propose formation of triple-helix structures (disproportionation) as intermediates in the melting process. © 1993 Academic Press Ltd.