We report differences in the interaction of two structurally similar phenanthroline intercalators, ethidium and propidium, with poly(dA)•poly(dT) and poly[d(A-T)] as a function of ionic strength based on titration microcalorimetry, fluorescence titration, and hydrostatic pressure measurements. Both ethidium and propidium bind more strongly to poly[d(A-T)]•poly[d(A-T)] than to poly(dA)•poly(dT). Ethidium intercalation into the latter polymer displays titrations with positive cooperativity; this is not found with propidium. The enthalpy of intercalation (ΔH°) is exothermic for both dyes with poly[d(A-T)]•poly[d(A-T)]; however, the value of this parameter is nearly zero in the case of poly(dA)•poly(dT). The molar volume change (ΔV°) accompanying dye intercalation is negative under all conditions for poly[d(A-T)]•poly[d(A-T)] whereas it is positive for poly(dA)•poly(dT). The changes observed in ΔV° correlate well with the entropy changes derived from the titration and calorimetric data for this reaction. The results, interpreted in terms of the relative hydration of these two polymers, are consistent with a higher extent of hydration of poly-(dA).poly(dT) relative to poly[d(A-T)]•poly[d(A-T)].
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