title: Ethidium binding to left-handed (Z) DNAs results in regions of right-handed DNA at the intercalation site.
abstract: The equilibrium binding of ethidium to the right-handed (B) and left-handed (Z) forms of poly(dG-dC).poly(dG-dC) and poly(dG-m5dC).poly(dG-m5dC) was investigated by optical and phase partition techniques. Ethidium binds to the polynucleotides in a noncooperative manner under B-form conditions, in sharp contrast to highly cooperative binding under Z-form conditions. Correlation of binding isotherms with circular dichroism (CD) data indicates that the cooperative binding of ethidium under Z-form conditions is associated with a sequential conversion of the polymer from a left-handed to a right-handed conformation. Determination of bound drug concentrations by various titration techniques and the measurement of circular dichroism spectra have enabled us to calculate the number of base pairs of left-handed DNA that adopt a right-handed conformation for each bound drug; 3-4 base pairs of left-handed poly(dG-dC).poly(dG-dC) in 4.4 M NaCl switch to the right-handed form for each bound ethidium, while approximately 25 and 7 base pairs switch conformations for each bound ethidium in complexes with poly(dG-dC).poly(dG-dC) in 40 microM [Co(NH3)6]Cl3 and poly(dG-m5dC).poly(dG-m5dC) in 2 mM MgCl2, respectively. The induced ellipticity at 320 nm for the ethidium-poly(dG-dC).poly(dG-dC) complex in 4.4 M NaCl indicates that the right-handed regions are nearly saturated with ethidium even though the overall level of saturation is very low. The circular dichroism data indicate that ethidium intercalates to form a right-handed-bound drug region, even at low r values where the CD spectra show that the majority of the polymer is in a left-handed conformation.
mesh_terms: D055598:Chemical Phenomena; D002621:Chemistry; D002942:Circular Dichroism; D004247:DNA; D004996:Ethidium; D009690:Nucleic Acid Conformation; D011089:Polydeoxyribonucleotides; D013056:Spectrophotometry, Ultraviolet; D013329:Structure-Activity Relationship
chemical_list: D011089:Polydeoxyribonucleotides; C019530:poly(dC-dG); C029988:poly(dG-m(5)dC).poly(dG-m(5)dC); D004247:DNA; D004996:Ethidium