Spectroscopic investigation of interaction of Nile Blue A, a potent photosensitizer, with bile salts in aqueous medium

• Bile salts, the biosurfactants, below their CMC induce dimerization of Nile Blue.
• Bile salts above their CMC solubilize monomeric Nile Blue in their micellar core.
• Both H and J dimers of Nile Blue are formed; major contribution is from H dimers.
• Extent of dimerization of Nile Blue is more in dihydroxy than trihydroxy bile salt.
• Interaction of Nile Blue with bile salt micelles is spontaneous and enthalpy-driven.

Nile Blue A (NB) is one of the most studied benzophenoxazine dyes, as a potent photosensitizer for photodynamic therapy. The dye when administered intravenously disperses throughout the body by circulating through blood and is taken up by most cells that emphasize its interaction with various biomolecules. Therefore a rational understanding of the interaction of NB with relevant biological and biomimicking systems appears important. The focus of the present work is to investigate the interaction of NB with two bile salts sodium deoxycholate (NaDC) and sodium cholate (NaC) by spectroscopic techniques. The bile salts, in their premicellar concentration range, induce NB dimerization. Both H- and J-dimers are formed, however a major contribution is from the H-dimers. The extent of NB dimerization in NaDC, the dihydroxy bile salt, is higher than that in NaC, the trihydroxy bile salt. The bile salts when present above their micellar concentrations solubilize NB in its monomeric form. NB exhibits stronger binding and partitioning efficiency toward NaDC than NaC micelles at a given temperature. Binding and partitioning of NB to these micelles are spontaneous and exothermic in nature and these are enthalpy-driven processes. The spectral profiles and thermodynamic parameters of NB point toward the dissimilar nature of its environment in the micelles formed by the above two bile salts.

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