In order to obtain the best fit between the model and the experiment, a small positive adsorption cooperativity (alpha = -2, Table 1) was found for FITC-LDL adsorption onto the hydrophilic silica and a small but negative adsorption cooperativity ( alpha = 2.5) for adsorption onto the ODS-silica, respectively. The meaning of the adsorption cooperativity is that an additional "attractive potential" exist around already adsorbed protein molecules. Such a potential facilitates the adsorption of the next molecules and so on.... Can this be what has really happened in LDL adsorption ? One indication that such adsorption potential may indeed exist can be found in the scanning force microscopy image of adsorbed LDL on mica (Figure 3). Indeed many of the adsorbed LDL molecules are aggregated into dimer - tetramers [32].
Figure 3. LDL adsorbed on freshly-cleaved mica for 15 minutes, then rinsed and imaged by SFM in air. Note the "L-shaped" (indicated by white arrows) and "clover-shaped" (indicated by black arrow) tetramer patterns. Dimers and trimers are also apparent to a lesser extent on the surface.
In the view of the authors, the purpose of any protein adsorption experiment should be the extraction of physico-chemical parameters, which will provide us with a condensed description of the observed process and enable us to predict similar processes in other situations. This short syllabus describes one of the approaches in analyzing protein adsorption. Because protein adsorption processes may never reach true equilibrium, the choice was to measure the kinetics of protein adsorption rather than the adsorption isotherm.
