title: Poly(dA).poly(dT) exists in an unusual conformation under physiological conditions: propidium binding to poly(dA).poly(dT) and poly[d(A-T)].poly[d(A-T)].
abstract: The binding of propidium to poly(dA).poly(dT) [poly(dA.dT)] and to poly[d(A-T)].poly[d(A-T)] [poly[d(A-T)2]] has been compared under a variety of solution conditions by viscometric titrations, binding studies, and kinetic experiments. The binding of propidium to poly[d(A-T)2] is quite similar to its binding to calf thymus deoxyribonucleic acid (DNA). The interaction with poly(dA.dT), however, is quite unusual. The viscosity of a poly(dA.dT) solution first decreases and then increases in a titration with propidium at 18 degrees C. The viscosity of poly[d(A-T)2] shows no decrease in a similar titration. Scatchard plots for the interaction of propidium with poly(dA.dT) show the classical upward curvature for positive cooperativity. The curvature decreases as the temperature is increased in binding experiments. A van't Hoff plot of the observed binding constants yields an apparent positive enthalpy of approximately +6 kcal/mol for the propidium-poly(dA.dT) interaction. Propidium binding to poly[d(A-T)2] shows no evidence for positive cooperativity, and the enthalpy change for the reaction is approximately -9 kcal/mol. Both the magnitude of the dissociation constants and the effects of ionic strength are quite similar for the dissociation of propidium from poly(dA-T)2] and from poly[d(A-T)2], suggesting that the intercalated states are similar for the two complexes. The observed association reactions, under pseudo-first-order conditions, are quite different. Plots of the observed pseudo-first-order association rate constant vs. polymer concentration have much larger slopes for propidium binding to poly[d(A-T)2] than to poly(dA.dT).(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D007700:Kinetics; D009690:Nucleic Acid Conformation; D010617:Phenanthridines; D011067:Poly dA-dT / Q000378:metabolism; D011089:Polydeoxyribonucleotides / Q000378:metabolism; D011419:Propidium; D012967:Sodium Dodecyl Sulfate; D013329:Structure-Activity Relationship; D013816:Thermodynamics; D014783:Viscosity
chemical_list: D010617:Phenanthridines; D011089:Polydeoxyribonucleotides; D011067:Poly dA-dT; D011419:Propidium; D012967:Sodium Dodecyl Sulfate