Spectral Studies on the Binding Behavior of Cationic Dyes and Surfactants with Bacterial Polysaccharide of Klebsiella K43

The binding behavior of acidic capsular polysaccharide (SPS), isolated from Klebsiella serotype K43, with oppositely charged dyes and surfactants have been studied by way of absorbance and emission spectroscopic measurements. Each repeating unit of the SPS consists of three D-mannose, one D-galactose, and one D-glucuronic acid residue. The anionic polysaccharide exhibited chromotropic character and induced strong metachromasy in the cationic dye, pinacyanol chloride (PCYN) through the formation of a 1:1 stoichiometric complex. Evaluation of thermodynamic parameters, viz., changes in free energy (ΔG), enthalpy (ΔH), and entropy (ΔS), for the formation of dye-polymer complex and studies on the effect of different cosolvents were also evaluated to shed light on the binding nature as well as the extent of stability of the dye-polymer complex. Fluorescence of the cationic dye acridine orange (AO) was quenched with the progressive addition of SPS, which was found to be of Stern-Volmer type. Cationic surfactants in their pure form as well as in the mixed state with nonionic surfactant (Tween-20), replaced the dye bound to the polymer matrices; thus the original band intensities of the dyes could be reverted. Such studies revealed the involvement of both electrostatic as well as hydrophobic interactions between the dye-polymer as well as surfactant-polymer aggregates.