Interaction of Lysozyme with Dyes

II. Binding of Bromophenol Blue

Abstract

The binding of lysozyme with bromophenol blue (BPB) at various dye concentrations and pH was carried out at 25°C by equilibrium dialysis, ultraviolet (UV) difference and circular dichroism (CD) spectral techniques.
Binding isotherms at pH 5.0 show non-cooperative binding at low dye concentrations, which change over to cooperative binding at higher concentrations indicating biphasic nature. However, binding isotherms at pH 7.0 and 9.0 show cooperative binding only, at all concentrations of the dye. The number of available binding sites decreases with the increase of pH. Gibbs free energy change, calculated on the basis of Wyman's binding potential concept, decreases with the increase of pH. Binding isotherms at pH 5.0 obtained at a lower temperature of 8°C, also indicate the biphasic nature similar to those observed at 25°C, but with a slight decreased strength of binding.
The UV difference spectra of the complex do not show any distinct peaks in the 285 to 297 nm region eliminating any possible interaction of BPB with tryptophan and tyrosine residues of the lysozyme molecule. The CD spectra of lysozyme-BPB complex show a decrease in ellipticities with reference to native lysozyme in the near UV and far UV regions. This indicates that the lysozyme-BPB complex has a lower helical content probably due to the conformational changes induced into the native enzyme. The appearance of new positive peaks at 315 nm in the near UV region and at 592 nm in the visible region of the CD spectra may be due to the induced asymmetry into the BPB molecule as a result of its binding to a cationic residue (probably a lysine residue) of lysozyme.
Thus, the types of interactions involved in the binding of BPB to lysozyme are mainly electrostatic in nature in addition to hydrogen bonding and van der Waals forces.