A P-31 NMR-STUDY OF THE ADSORPTION OF BOVINE SERUM-ALBUMIN ON MONTMORILLONITE USING PHOSPHATE AND THE PARAMAGNETIC CATION MN2+ - MODIFICATION OF CONFORMATION WITH PH

The specific interfacial area of bovine serum albumin (BSA) adsorbed on montmorillonite was deduced from the ratio between the quantity of cations exchanged on adsorption of BSA and the quantity of protein bound. A paramagnetic cation, Mn2+, was used, and its release from the clay surface was followed by measuring the line broadening effect of displaced Mn2+ on the 31P NMR signal from orthophosphate in the solution. From experiments conducted at different protein/clay ratios and different pH, it was deduced: (i) that no more than one monolayer was adsorbed; (ii) that the specific interfacial area was the same at low and at high surface coverage; (iii) that below the isoelectric point (i.e.p.) the specific interfacial area of BSA increased with decreasing pH, with a constant surface coverage of the clay surface; and (iv) that above the i.e.p. the surface coverage of the clay decreased with further increases in pH. It is assumed that electrostatic interactions between the protein and the clay surface play a major role in these phenomena. The correlation with the behavior of extracellular enzymes in soils is emphasized. © 1992.