Time and temperature aspects of beta-lactoglobulin removal from methylated silica surfaces by sodium dodecyl sulphate

The adsorption of beta-lactoglobulin onto methylated silica surfaces and the subsequent protein removal by the anionic surfactant sodium dodecyl sulphate (SDS) were followed using in-situ ellipsometry. Experiments were performed at pH 6.0 in phosphate-buffered saline solution. Parameters varied include temperature, length of time for protein adsorption from solution and surface residence time of beta-lactoglobulin. The temperature was kept constant throughout a trial, and the majority of experiments were carried out at a few degrees below the protein denaturation temperature as reported from differential scanning calorimetry studies. beta-Lactoglobulin adsorption at high temperatures resulted in aggregation at the surface after a lag phase of several minutes. Varying the protein adsorption time and thus the amount adsorbed while keeping the protein surface residence time fixed did not seem to affect the amount desorbed upon rinsing or the amount eluted by surfactant. For short beta-lactoglobulin adsorption times, the adsorbed amounts were comparable at all temperatures studied. The temperature hardly affected the amount desorbed during rinsing, but did however have a pronounced influence on the protein removed by surfactant. Up to around 60 degrees C practically all beta-lactoglobulin was eluted by the SDS. The fraction removed then decreased with temperature, with a sharp drop between 70 and 73 degrees C, and a further decline at higher levels. SDS was seen to be highly inefficient at removing beta-lactoglobulin adsorbed at temperatures above 70 degrees C. The trend observed is attributed to temperature-dependent changes in the protein resident on the surface. The beta-lactoglobulin surface residence time was seen to significantly affect the elutability. At short residence times the removal efficiency was comparably high, but decreased with time. However, no significant difference could be detected between two sufficiently long residence times. The behaviour is consistent with the assumption of multiple states of adsorbed proteins, together with slow conformational changes in the adsorbed protein layer.