Interaction of β-lactoglobulin with ionic surfactants: Apparent molar volume and compressibility studies of ionic surfactants in aqueous solutions of β-lactoglobulin

In this work we present apparent molar volume (Φv), adiabatic compressibility (Φk) and sound velocity number [U] of sodium dodecyl sulfate (SDS), sodium n-nonyl sulfate (SNS), sodium dodecyl benzene sulfonate (SDBS), cetyl trimethyl ammonium bromide (CTAB), and di (dodecyl dimethyl ammonium bromide) (12-2-12) in aqueous solution of β-lactoglobulin (βLG) between 10 and 36 °C at an interval of 2 °C. The temperature dependence of Φv value of these surfactants is found to be completely consistent with temperature over the entire concentration range; two minima are seen to occur at 22 and 32 °C. The sharpness of the minima decreases with the increase in surfactant concentration. However, no such pattern is observed with Φk value. Corresponding experiment with βLG (i.e., in absence of surfactant) shows two maximum at the same temperatures as does the minima in Φv value of surfactants. [U] and Φk are found to contrast in their behaviour with regards to the effect of temperature and surfactant concentration. However, this is suggested to provide a consistent view as regards to the surfactant binding to protein. We fail to observe such a complex temperature dependence of Φv value in the presence of 2-mercaptoethanol (a protein denaturant). Assuming complete denaturation of βLG, the surfactant binding to βLG is believed to be diversified into electrostatic and hydrophobic interactions.