Nucleic Acids Research, 1983, Vol. 11, No. 13 4467-4482
© 1983
CHEMISTRY |
1H NMR study of an ethidium dimer poly(dA-dT) complex: evidence of a transition between bis and monointercalation
1Département de Chimie Organique ERA 613 CNRS et SC 21 INSERM, UER des Sciences Phannaceutiques et Biologiques, 75006 Paris 2Unité de Physicochimie Macromoléculaire Institut Gustave Roussy, 16 bis avenue P.V. Couturier, 94800 Villejuif, France
Received May 4, 1983. Revised June 7, 1983. Accepted June 7, 1983.
Comparative 1H NMR and optical studies of the interaction between poly (dA-dT), ethidium bromide (Et) and ethidium dimer (Et2) in 0.7 M NaCl are reported as a function of the temperature. Denaturation of the complexes followed at both polynucleotide and drug levels leads to a biphasic melting process for poly(dA-dT) complexed with ethidium dimer (t1/2/ 75°C ; 93°C) but a monophasic one in poly(dA-dT) : ethidium bromide complex (t1/2=74°C). In both cases drug signals exhibit monophasic thermal dependance (Et = 81°C ; Et2=95°C). Evidence is presented showing that the ethidium dimer bisintercalates into poly(dA-dT) in high salt, based on the observation that i) dimer and monomer ring protons exhibit similar upfield shifts upon DNA binding, ii) upfield shifts of DNA sugar protons are twice as large with the dimer than with ethidium bromide. Comparison between native DNA fraction and bound drug fraction indicates that ethidium covers, n=2.5–3 base pairs. The dimer bis-intercalates and covers, n=5.7 base pairs when the helix fraction is high but as the number of available sites decreases the binding mode changes and the drug monointercalates (n=2.9).
*Present address: Department of Pharmaceutical Chemistry, University of California, San Francisco, CA 94143, USA