title: Micelle-sequestered dissociation of cationic DNA-intercalated drugs: unexpected surfactant-induced rate enhancement.
abstract: Detergent sequestration using micelles as a hydrophobic sink for dissociated drug molecules is an established technique for determination of dissociation rates. The anionic surfactant molecules are generally assumed not to interact with the anionic DNA and thereby not to affect the rate of dissociation. By contrast, we here demonstrate that the surfactant molecules sodium dodecyl sulfate (SDS), sodium decyl sulfate, and sodium octyl sulfate all induce substantial rate enhancements of the dissociation of intercalators from DNA. Four different cationic DNA intercalators are studied with respect to surfactant-induced dissociation. Except for the smallest intercalator, ethidium, the dissociation rate constants increase monotonically with surfactant concentration both below cmc and (more strongly) above cmc, much more than expected from electrostatic effects of increased counterion concentration. The rate enhancement, most pronounced for the bulky, multicationic, hydrophobic DNA ligands in this study, indicates a reduction of the activation energy for the ligand to pass out from a deeply penetrating intercalation site of DNA. The discovery that surfactants enhance the rate of dissociation of cationic DNA-intercalators implies that rate constants previously determined by micelle-sequestered dissociation may have been overestimated. As an alternative, more reliable method, we suggest instead the addition of excess of dummy DNA as an absorbent for dissociated ligand.
authors.affiliation: Physical Chemistry Section at the Department of Chemistry and Bioscience, Chalmers University of Technology, SE-41296 Gothenburg, Sweden.
authors.forename: Fredrik
authors.initials: F
authors.lastname: Westerlund
authors.forename: L Marcus
authors.initials: LM
authors.lastname: Wilhelmsson
authors.forename: Bengt
authors.initials: B
authors.lastname: Nordén
authors.forename: Per
authors.initials: P
authors.lastname: Lincoln
mesh_terms: D001583:Benzoxazoles / Q000737:chemistry*; D002412:Cations; D004247:DNA / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D007364:Intercalating Agents / Q000737:chemistry*; D007700:Kinetics; D008823:Micelles; D011067:Poly dA-dT / Q000737:chemistry; D011806:Quinolinium Compounds / Q000737:chemistry*; D012967:Sodium Dodecyl Sulfate / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D013501:Surface-Active Agents / Q000737:chemistry*
chemical_list: D001583:Benzoxazoles; D002412:Cations; D007364:Intercalating Agents; D008823:Micelles; D011806:Quinolinium Compounds; D013501:Surface-Active Agents; C075296:1,1'-((4,4,7,7-tetramethyl)-4,7-diazaundecamethylene)bis-4-(3-methyl-2,3-dihydro(benzo-1,3-oxazole)-2-methylidene)quinolinium; D011067:Poly dA-dT; D012967:Sodium Dodecyl Sulfate; D004247:DNA