STRUCTURAL-CHANGES IN PROTEIN MOLECULES ADSORBED ON ULTRAFINE SILICA PARTICLES

The structural changes in horse cytochrome c, bovine ribonuclease A (RNase A), sperm whale myoglobin, ovalbumin, human hemoglobin, and bovine serum albumin (BSA) during adsorption on ultrafine silica particles have been studied. Since the intensity of light scattered by the ultrafine silica particles (average diameter 15 nm) is negligible, the structure of adsorbed protein molecules on the surface is directly estimated by measuring the circular dichroism (CD) spectra of the suspension of particles on which protein molecules were adsorbed. While the CD spectra of soft proteins such as hemoglobin and BSA were changed extensively by adsorption on ultrafine silica particles, those of rigid proteins such as cytochrome c and RNAse A were changed little. Changes in the CD spectrum of BSA during adsorption increased with decreasing pH. Thus, the magnitude of the structural changes is affected by both the flexibility of the protein molecules and the affinities of the particles for proteins. The BSA molecules desorbed from the ultrafine silica particles by addition of morpholine showed CD spectra similar to those of native BSA and hence were refolded. Thus, the conformational changes in BSA molecules produced by adsorption were highly reversible. The adsorption amounts of all these proteins on ultrafine silica particles were maximum at around their isoelectric points regardless of structural adaptability. © 1991 Academic Press, Inc. All rights reserved.