Thermodynamic Analysis of Proteins Adsorbed on Silica Particles: Electrostatic Effects

Electrostatic effects on protein adsorption were investigated using differential scanning calorimetry (DSC) and adsorption isotherms. The thermal denaturation of lysozyme, ribonuclease A (RNase), and alpha-lactalbumin in solution and adsorbed onto silica nanoparticles was examined at three concentrations 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 was half-completed (T(m)), and the temperature range of the denaturation transition. For lysozyme and RNase, adsorption isotherms depend strongly on the ionic strength. At low ionic strength both proteins have a high affinity for the silica particles and adsorption is accompanied by a 15-25% reduction in DeltaH and a 3-6 degrees C decrease in T(m), indicating that the adsorbed state of the proteins is destabilized. Also, an increase in the width of the denaturation transition is observed, signifying a larger conformational heterogeneity of the surface bound proteins. At higher ionic strengths, both with and without the addition of calcium, no significant adsorption-induced alteration in DeltaH was observed for all three proteins. The addition of calcium, however, decreases the width of the denaturation transition for lysozyme and RNase in the adsorbed state. Copyright 2001 Academic Press.