2. Materials and methods
Lysozyme from chicken egg white (≥ 98%) and n-dodecyl β-d-maltoside (≥ 98%) were purchased from Sigma, USA. Tris(hydroxymethyl)aminomethane hydrochloride (Tris–HCl) was of analytical grade and also obtained from Sigma. A stock solution of lysozyme was made in 20 mM of pH 7.4 tris HCl buffer and protein concentration of 7 μM was used throughout except in FTIR and DLS measurements. Tris–HCl buffer was filtered through a 0.45 μm Millipore Millex-HV PVDF filter and Cy5.5 NHS ester was measured by using Mettler-Toledo pH meter (model S20).
2.2. Surface tension measurements
Surface tension measurements were carried out by using Kruss Type 10 tensiometer by Du Nouy ring detachment method. In the case of pure surfactant, the equilibration time was 15 min, whereas the surfactant–lysozyme solutions were equilibrated at least for 30 min. Thus, the values of surface tension (γ) were noted when pilus did not vary with time. The average values of equilibrium γ were obtained by repeating the measurement three times. The critical micelle concentration (cmc) values were estimated by the intersection between the two linear portions of γ − log [surfactant] isotherms. The surface tension values were accurate within ± 0.1 mNm− 1.