Thermodynamic analysis of proteins adsorbed on silica particles: Electrostatic effects

Electrostatic effects on protein adsorption were investigated using differe ntial scanning calorimetry (DSC) and adsorption isotherms, The thermal dena turation of lysozyme, ribonuclease A (RNase), and alpha -lactalbumin in sol ution and adsorbed onto silica nanoparticles was examined at three concentr ations of cations: 10 and 100 mM of sodium and 100 mM of sodium to which 10 mM of calcium was added. The parameters investigated were the denaturation enthalpy (DeltaH), the temperature at which the denaturation transition wa s half-completed (T-m), and the temperature range of the denaturation trans ition. For lysozyme and RNase, adsorption isotherms depend strongly on the ionic s trength. At low ionic strength both proteins have a high affinity for the s ilica particles and adsorption is accompanied by a 15-25% reduction in Delt aH and a 3-6 degreesC decrease in T-m, indicating that the adsorbed state o f the proteins is destabilized, Also, an increase in the width of the denat uration transition is observed, signifying a larger conformational heteroge neity of the surface bound proteins. At higher ionic strengths, both with a nd without the addition of calcium, no significant adsorption-induced alter ation in DeltaH was observed for all three proteins. The addition of calciu m, however, decreases the width of the denaturation transition for lysozyme and RNase in the adsorbed state. (C) 2001 Academic Press.