Effect of physicochemical conditions on the ultrafiltration of β-lactoglobulin: Fluorescence probing of induced structural changes

This work aims for determining the impact of different environmental conditions, such as pH, ionic strength (salt concentration) and the chemistry of the membrane surface (hydrophilic/hydrophobic character) on the structure of permeating proteins after ultrafiltration. In the permeation experiments reported in this paper, different solutions of a model protein – β-lactoglobulin – at pH 3, 5 and 8 and salt concentrations of 1, 10 and 100 mM were processed with membranes of different molecular weight cut-off (10 and 30 kDa) and materials (regenerated cellulose—RC, and polyethersulfone—PES). The analysis of the structural alterations induced in the permeating molecules of β-lactoglobulin after ultrafiltration was performed using a methodology that combines the use of complementary fluorescence techniques: steady-state fluorescence and steady-state fluorescence anisotropy. From the results obtained it was concluded that permeation performed under pH values below the isoelectric point of β-lactoglobulin, at high salt concentrations and using membranes with hydrophobic character allows for minimization of the impact of the membrane processing in the structure of β-lactoglobulin.