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Abstract: Kinetics and equilibria of cyanine dyes thiazole orange (TO) and benzothiazole orange (BO) self-aggregation and binding to CT-DNA are investigated in aqueous solution at 25°C and pH 7. Absorbance spectra and T-jump experiments reveal that BO forms J-aggregates while TO forms more stable H-aggregates. Fluorescence and absorbance titrations show that TO binds to DNA more tightly than BO. TO stacks externally to DNA for low polymer-to-dye concentration ratios (C P/C D) while dye intercalation occurs for high values of C P/C D. T-jump and stopped-flow experiments performed at high C P/C D agree with reaction scheme D+S⇆D,S⇆DSI ⇆DSII where the precursor complex D,S evolves to a partially intercalated complex DSI which converts to the more stable intercalate DSII. Non-electrostatic forces play a major role in D,S stabilization. Last step is similar for both dyes suggesting accommodation of the common benzothiazole residue between base pairs. Experiments using poly(dA-dT)·poly(dA-dT) and poly(dG-dC)·poly(dG-dC) confirm base pair preference for TO. [Copyright &y& Elsevier]

Abstract: A T-jump investigation of the binding of Cyan40 [3-methyl-2-(1,2,6-trimethyl-4(1H)pyridinylidenmethyl)-benzothiazolium ion] and CCyan2 [3-methyl-2-[2-methyl-3-(3-methyl-2(3H)-benzothiazolylidene)-1-propenyl]-benzothiazolium ion] with poly(dA–dT)·poly(dA–dT) and poly(dG–dC)·poly(dG–dC) is performed at I =0.1M (NaCl), 25°C and pH 7. Two kinetic effects are observed for both systems. The binding process is discussed in terms of the sequence D+P⇆P,D⇆PDI ⇆PDII, which leads first to fast formation of a precursor complex P,D and then to a partially intercalated complex PDI which converts to the fully intercalate complex PDII. Concerning CCyan2 the rate parameters depend on the polymer nature and their analysis shows that in the case of poly(dG–dC)·poly(dG–dC) the most stable bound form is the fully intercalated complex PDII, whereas in the case of poly(dA–dT)·poly(dA–dT) the partially intercalated complex PDI is the most stable species. Concerning Cyan40, the rate parameters remain unchanged on going from A–T to G–C indicating that this dye is unselective. [Copyright &y& Elsevier]