The noncovalent interaction of a number of cationic and anionic meso-substituted polymethine (carbocyanine) dyes with sodium deoxycholate (NaDC) as a biologically important surfactant was studied by spectral-fluorescent methods. The following dyes were studied: 3,3'-di(β-hydroxyethyl)-9-methylthiacarbocyanine-iodide (K1), 3,3'-di(β-hydroxyethyl)-5,5'-dimethoxy-9-ethylthiacarbocyanine iodide (K2), 3,3',9-trimethylthiacarbocyanine iodide (K3, Cyan 2), 3,3'-di(γ-sulfopropyl)-9-methylthiacarbocyanine-betaine (A1), and 3,3'-di(γ-sulfopropyl)-4,5,4',5'-dibenzo-9-ethylthiacarbocyanine-betaine (A2, DEC). Upon addition of increasing concentrations of NaDC, the absorption spectra of dyes K1-K3 in aqueous solutions exhibit changes due to the formation of dye aggregates and their subsequent decomposition into monomers. The formation of aggregates of K1-K3 leads to broadening of the absorption spectra and an appearance of additional bands. Upon binding to the surfactant, the cis-monomers of the dyes undergo partial cis-trans conversion, which is accompanied by an increase in the fluorescence intensity. The binding constants of the dyes to NaDC were estimated on the basis of Scatchard and Hill models from the dependences of the absorption and fluorescence spectra on the concentration of NaDC.
Keywords: Aggregation; Fluorescent probes; Micelles; Polymethine (cyanine) dyes; Sodium deoxycholate; cis–trans conversion.