Investigation of adsorption-induced structural changes of proteins at solid/liquid interfaces by differential scanning calorimetry

Adsorption of proteins on solid surfaces is widely studied because of its importance in various biotechnological, medical, and technical applications, e.g. biosensors, cardiovascular implants and chromatography. One of the main problems is to what extend adsorption-induced conformational changes occur since they often modify the biological activity of the protein. Thus, the investigation of structural rearrangement due to interaction with the solid surface is of great interest. As protein molecules are in many cases continuously exchanged between the adsorbed and dissolved states, the question arises whether protein molecules re-adopt their original native structure after release from the surface. In this study, two model proteins with well-characterized properties (human serum albumin (HSA) and α-chymotrypsin) were adsorbed from aqueous buffered solution onto finely dispersed hydrophilic silica particles. Adsorption isotherms were determined from the depletion of the supernatant, protein concentrations were analyzed by photometric methods. Reversibility of the processes was tested. The structural rearrangements in the protein molecules induced from the adsorption process were probed by highly sensitive differential scanning calorimetry (micro-DSC). Transition temperature and enthalpy measurements for thermal unfolding were compared between given amounts of protein in free (native) and adsorbed/desorbed form. From these measurements it appears that under the chosen adsorption conditions, HSA may lose essentially all of its cooperatively folded structure, while α-chymotrypsin retains full structure. The relation between the native protein structural stability on the one hand, and structural rearrangement on the other is discussed. Finally, the importance of conformational changes for spontaneous adsorption to occur is considered.