The adsorption-desorption cycle. Reversibility of the BSA-silica system

The reversibility of the adsorption-desorption cycle was established by comparing the thermostability (determined by differential scanning calorimetry) and secondary structure (obtained by circular dichroism spectroscopy) of BSA before adsorption, adsorbed on, and exchanged from silica particles. Circular dichroism was also measured as a function of temperature at a given wavelength. Adsorbed BSA presents a higher thermostability and a lower alpha -helix content than the native protein while it regains its conformation when released from the surface back into the solution; the homomolecular exchange is reversible. The changes in ellipticity (at a given wavelength) as a function of the temperature show that the thermal denaturation of native, adsorbed, and exchanged BSA proceeds in two steps. For the dissolved protein, the first step up to 50 degreesC involves a slight change in the structure while in the 50-90 degreesC temperature range the actual unfolding takes place. For the adsorbed BSA, the first step proceeds up to 60 degreesC and includes some intermolecular association between the adsorbed protein molecules, which may be responsible for the increased thermostability. The unfolding occurs in the 60-90 degreesC range; it is less cooperative and involves a lower enthalpy change than the native protein. Adsorbed BSA presents the same secondary structure as that observed for dissolved BSA that has passed a heating-cooling cycle. (C) 2001 Academic Press.