title: Triple-helix formation by an oligonucleotide containing one (dA)12 and two (dT)12 sequences bridged by two hexaethylene glycol chains.
abstract: The triple-helix formation by the oligonucleotide (dA)12-x-(dT)12-x-(dT)12, where x is a hexaethylene glycol group, was investigated by thermal denaturation analysis and circular dichroism spectroscopy. Thermal denaturation analysis showed that this single-stranded oligonucleotide is able to fold back on itself twice to give a triple helix at low temperature. Upon an increase in the temperature, two cooperative transitions were observed: formation of a double-stranded structure with a dangling x-(dT)12 extremity, then formation of a single-stranded coil structure. Due to the intramolecular character of the transition, the triplex is much more stable than that formed by the reference mixture (dA)12 + 2(dT)12. In 0.1 M NaCl, the triplex-to-coil transition occurred at about 30 degrees C whereas the duplex-to-coil was at about 60 degrees C. Upon an increase in the salt, the increase of temperature corresponding to the triplex-to-duplex transition was larger than that of the duplex-to-coil transition. MgCl2 showed higher efficiency than NaCl to promote triplex or duplex formation. The thermodynamic parameters delta H and delta S were determined at various ionic strengths for both transitions. Both the enthalpy change and entropy change associated with triplex-to-duplex transition (Hoogsteen base pairing) were smaller than those associated to the duplex-to-coil transition (Watson-Crick base pairing). When the ionic strength increased, the parameters -delta H and -delta S showed a very small decrease for the duplex-to-coil transition whereas a strong increase was observed with the triplex-to-duplex transition.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Centre de Biophysique Moléculaire, Orléans, France.
authors.forename: M
authors.initials: M
authors.lastname: Durand
authors.forename: S
authors.initials: S
authors.lastname: Peloille
authors.forename: N T
authors.initials: NT
authors.lastname: Thuong
authors.forename: J C
authors.initials: JC
authors.lastname: Maurizot
mesh_terms: D001483:Base Sequence; D002942:Circular Dichroism; D005026:Ethylene Glycols; D008433:Mathematics; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D009691:Nucleic Acid Denaturation; D009838:Oligodeoxyribonucleotides / Q000138:chemical synthesis / Q000737:chemistry*; D009994:Osmolar Concentration; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics
chemical_list: D005026:Ethylene Glycols; D009838:Oligodeoxyribonucleotides; C027903:oligo (dT); C030570:oligodeoxyadenylic acid; C077067:hexaethylene glycol