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Binding of lysozyme with cetyltrimethylammonium bromide (CTAB) and dodecyl-trimethylammonium bromide (DTAB) at various detergent concentrations and pH was studied at 25 degrees C by equilibrium dialysis technique. In the case of CTAB, binding isotherms at pH 5.0, 7.0, and 9.0 show cooperative binding at all the concentrations of the detergent and the binding ratios increase with pH. Cooperative binding is also shown by DTAB at all the concentrations and pH, but the binding ratios are lower compared to CTAB. The Gibb's free energy change calculated on the basis of Wyman's binding potential concept increases with pH, indicating increased binding strength of CTAB at higher pH. The UV difference spectra of CTAB and DTAB with lysozyme and its model compounds such as L-Trp, L-Tyr X HCI and L-Phe show two peaks at 297 nm and 250 nm at pH 9.0 indicating the possible involvement of tryptophans as the binding sites along with the carboxylate anion or the phenolic group of a tyrosine on lysozyme. The effect of higher ionic strength on the binding of CTAB with lysozyme at pH 9.0 is evidenced by lower binding ratios and decreased intensities of the UV difference bands, thus indicating the involvement of electrostatic interactions. However, the hydrophobic interactions between the detergents and the aromatic amino acid residues in lysozyme contribute more to the binding strength. The binding of these cationic detergents by lysozyme induces conformational changes in the enzyme. They are followed by the circular dichroism (CD) technique which shows a decrease in the aromatic bands in the 320-250 nm region.(ABSTRACT TRUNCATED AT 250 WORDS)

The binding of lysozyme with bromophenol blue (BPB) at various dye concentrations and pH was carried out at 25 degrees 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 degrees C, also indicate the biphasic nature similar to those observed at 25 degrees 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.(ABSTRACT TRUNCATED AT 250 WORDS)

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