title: Titration of poly(dA-dT) . poly(dA-dT) in solution at variable NaCl concentration.
abstract: CD and uv absorption data showed that high molecular weight poly(dA-dT) . poly(dA-dT), at 298 K, undergoes an acid-induced transition from B-double helix to random coil in NaCl solutions of different concentrations, ranging from 0.005 to 0.600M. Similarly, titration of the polynucleotide with a strong base causes duplex-to-single strands transition. The base- and acid-induced transitions were both reversible by back-titration (with an acid or, respectively, with a base): the apparent pKa were the same in both directions. However, the number of protons per titratable site (adenine N1) required to reach half-denaturation was in great excess over the stoichiometric value; to a much larger extent, the same effect was observed also for the deprotonation of the N3H sites of thymine. Moreover, in the basic denaturation experiments, at low salt concentrations ([NaCl]< or =0.300M) less acid than calculated was needed to back-titrate the base excess to half-denaturation. Both effects could be qualitatively justified on the basis of the counterion condensation theory of polyelectrolytes and considering the energy barrier created by the negatively charged phosphodiester groups to the penetration of the OH- ions inside the double helix and the screening effect of the Na+ ions on such charges, in the deprotonation experiments.
authors.affiliation: Dipartimento di Chimica Inorganica e Analitica S. Cannizzaro, Viale delle Scienze, Università di Palermo, 90128 Palermo, Italy.
authors.forename: Marta
authors.initials: M
authors.lastname: Airoldi
authors.forename: C Andrea
authors.initials: CA
authors.lastname: Boicelli
authors.forename: Fabio
authors.initials: F
authors.lastname: Cadoni
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Gennaro
authors.forename: Marcello
authors.initials: M
authors.lastname: Giomini
authors.forename: Anna M
authors.initials: AM
authors.lastname: Giuliani
authors.forename: Mauro
authors.initials: M
authors.lastname: Giustini
mesh_terms: D002623:Chemistry Techniques, Analytical / Q000379:methods*; D002942:Circular Dichroism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D011089:Polydeoxyribonucleotides / Q000737:chemistry*; D011522:Protons; D012965:Sodium Chloride; D017186:Titrimetry / Q000379:methods
chemical_list: D011089:Polydeoxyribonucleotides; D011522:Protons; D012965:Sodium Chloride; C084838:polydeoxyadenylic acid-polythymidylic acid