3-STRANDED CLIP OF THE OLIGONUCLEOTIDE )PO(CH2CH2O)(3)P(DT)(10)PO(CH2CH2O)(3)P(DA)(10)-3'

Temperature dependence of UV and CD spectra of the oligonucleotide 5'- (dT)10-L-(dT)10-L-(dA)10-3' [tripl(ATT)] [L = -pO(CH2CH2O)3p-] in phos phate buffer, pH 7, at various NaCl concentrations and in the presence or absence of 0.01 M MgCl2 has been studied. At low oligonucleotide c oncentrations (2.2 x 10(-5) M nucleotide concentration) all structural transitions proceed intramolecularly. Tripl(ATT) exists in three form s: as a three-stranded clip (at low temperatures), a double-stranded h airpin (at intermediate temperatures), and as an open strand (at high temperatures). Thermodynamic parameters of the triplex formation depen ding on the NaCl concentration were calculated. The CD spectra were as signed to the single-, double-, and three-stranded forms. Ethidium bro mide (EtBr) binding to the three-stranded clip was studied. Ethidium b romide molecules were shown to intercalate into the triple helix with the stable complex formation (association constant is 10(6)). One mole cule of three-stranded clip binds not more than three EtBr molecules. The proposed synthetic model (oligonucleotide blocks coupled by hydrox yalkyl chains) has been shown to be convenient for studies of the phys ical and chemical properties of the triplex and other multistranded co mplexes of DNA.