title: Spectroscopic studies of (m5dC-dG)3: thermal stability of B- and Z-forms.
abstract: The hexanucleoside pentaphosphate d(m5CpGpm5CpGpm5CpG) has been studied in solution by ultra-violet absorption, circular dichroism and 31P nuclear magnetic resonance under various experimental conditions. In 0.2 M NaClO4 at low temperature, an hexamer duplex is formed which has a B or B-like conformation. As the salt concentration is increased, a transition from a B-form to the Z-form occurs and is complete in 3 M NaClO4. In 3 M NaClO4, the behavior of the Z double helix is complex as a function of temperature. The variation of the circular dichroism at 295 nm is biphasic. A first transition occurs over a large range of temperature and corresponds to a conformational change due to a non-cooperative intramolecular process. Ultra-violet absorption and 31P nuclear magnetic resonance show that the new conformation arising from a distortion of the backbone is not similar to that observed in low salt conditions (B-form). At high hexanucleotide concentration, aggregates are formed. The second transition is cooperative and corresponds to the melting of a double stranded helix into single strands.
mesh_terms: D002942:Circular Dichroism; D004355:Drug Stability; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D009690:Nucleic Acid Conformation; D009838:Oligodeoxyribonucleotides; D009841:Oligonucleotides; D009994:Osmolar Concentration; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature
chemical_list: D009838:Oligodeoxyribonucleotides; D009841:Oligonucleotides