ORIGINAL PAPER The Mechanism of Benzothiazole Styrylcyanine Dyes Binding with dsDNA: Studies by Spectral-Luminescent Methods N. Akbay & M. Yu. Losytskyy & V. B. Kovalska & A. O. Balanda & S. M. Yarmoluk Received: 7 May 2007 / Accepted: 4 September 2007 / Published online: 28 September 2007 # Springer Science + Business Media, LLC 2007 Abstract In the presented work studies of the interaction mode of monomer and two homodimer benzothiazole styryl dyes containing spermine-like linkage/tail group with the double stranded (ds) DNA are reported. For these dyes, equilibrium constant of dye binding to DNA (K b ), as well as the number of dsDNA base pairs occupied by one bound dye molecule (n) were determined. The data obtained show that the presence of spermine-like group containing quaternary nitrogen (Bos-5) results in increase of K b value as compared to this of unsubstituted analogue (Sbt). Besides, for the dimer dyes containing benzothiazole styryl chromophores, the K b value is either five times higher (DBos-13) or almost the same (DBsu-10) as compared to this of corresponding monomer Sbt, depending on the position in the benzothiazole ring where the linker is attached. Moreover, the n values for both dimers are significantly different as well, pointing to the bis-intercala- tive binding mechanism for DBos-13 and for the groove- binding one for DBsu-10. The conclusion about the dimer dyes-dsDNA binding mechanisms is also supported by the study of the fluorescent response of these dyes on the presence of AT- and GC-containing polynucleotides. Keywords Styrylcyanine dyes . DNA . Fluorescent detection . Binding constant . Interaction mechanism Introduction For today styrylcyanines are among the fluorescent dyes widely used for biomolecules detection. These dyes are successfully applied as sensitive probes for unspecific fluorescent staining of proteins in gels [1]. Due to the ability of styrylcyanines to interact with DNA with up to hundred times fluorescent enhancement and ability to penetrate into the cell these dyes were proposed for intra cellular DNA imaging [2, 3]. Recently benzothiazolium styrylcyanines were proposed as probes for DNA detection upon two-photon excitation (TPE). Styryl dyes are known as having high two-photon absorption cross-section values [4, 5]; one of the benzo- thiazolium styryls was shown to give strong fluorescence upon TPE by 1064 nm irradiation [6]. On the other hand significant fluorescent response on the double-stranded (ds) DNA presence is observed for these dyes. Thus series of monomer and homodimer styrylcyanines was synthesized and studied for their efficiency as TPE excited nucleic acids sensitive dyes. It was shown that dyes modified with spermine-like linkage/tail group demonstrate increased sensitivity to DNA. They have low intrinsic emission intensity and enhance their fluorescence intensity up to three orders of magnitude in presence of DNA. Complexes of studied dyes with DNA also demonstrate intensive emission upon the TPE. The values of two-photon J Fluoresc (2008) 18:139–147 DOI 10.1007/s10895-007-0252-7 N. Akbay Department of Chemistry, Hacettepe University, 06800 Ankara, Turkiye M. Y. Losytskyy : V. B. Kovalska : A. O. Balanda : S. M. Yarmoluk (*) Institute of Molecular Biology and Genetics, National Academy of Sciences of Ukraine, 150 Zabolotnogo St., 03143 Kyiv, Ukraine e-mail: sergiy@yarmoluk.org.ua M. Y. Losytskyy Physics Department, Kyiv National Taras Shevchenko University, Acad. Glushkova ave. 2, build. 1, 03680 Kyiv, Ukraine