Effects of temperature, pH, and salt concentration on beta-lactoglobulin deposition kinetics studied by optical waveguide lightmode spectroscopy

Deposition kinetics of β-lactoglobulin at a solid−liquid interface was studied with optical waveguide lightmode spectroscopy (OWLS) over a range of temperatures between 61 and 83 °C. A new temperature-controlled cell for OWLS measurements allows fast, on-line monitoring of the deposit formation at elevated temperatures. Primary protein layers were deposited at 25 °C in order to precondition and stabilize the waveguide surface. Sustained deposition lasting from a few minutes (around 80 °C) to hours (below 70 °C) resulted in multilayer deposits up to several tens of nanometers thick. The measured deposition rates were strongly influenced by temperature, pH, and NaCl concentration. Deposition rates decreased with increasing pH from 5.5. to 7.4, in a trend similar to that for noncovalent aggregation of β-lactoglobulin in solution. Activation energies for deposition rates decreased with increasing pH, from 340 kJ/mol at pH 5.5 to 230 kJ/mol at pH 7.4 and were similar to the activation energies for denaturation of β-lactoglobulin in solution.