Spectroscopic probes of the interactions of the dye Stains-all with deoxycholate and cholate

Abstract

The biological surfactants sodium cholate (NaC) and sodium deoxycholate (NaDC) differ from the normal surfactants such as sodium dodecyl sulphate (SDS) by having hydrophilic OH groups in their hydrophobic moieties, and they failed to induce sharp blue shifted metachromasia in the common cationic dyes such as acridine orange, methylene blue, pinacyanol etc. However, both the cholates induce extremely sharp and stable blue-shifted metachromasia in the cyanine dye Stains-all (Stal), at concentrate much below the critical micellar concentrations and not disrupted by the excess of surfactants unless at above the respective critical micellar concentrations. Both chiral cholates induce very strong biphasic negative circular dichroism (CD) in Stal. At high surfactant dye both NaDC-Stal and NaC-Stal exhibit a second positive biphasic CD spectrum, indicating the formation of second species of the complexes, not immediately discernible from the respective absorption spectrum. Though the reported structures of micelles and crystals of NaDC are distinctly different from that of NaC, the induced metachromasia and circular dichroism in Stal by the two surfactants are remarkably similar. It is reasonably thought that Stal cations bound at NaC and NaDC are arranged with systematic twists in one sense, responsible for metachromasia and dichroism of the dye aggregates, the formation of NaDC-Stal and NaC-Stal are probably followed by some self organization of the complexes formed in the premicellar range of concentrations, excess surfactants added to these complexes form just part of the solvent. Only above cmc the micelles start disruption of the dye aggregates. Our results fit well with the helical model of NaDC and the Small's model of NaDC micellar aggregate with the hydrophobic surfaces oriented inside.