The interaction of hen egg white lysozyme (HEWL) with Na-and Cs-exchanged saponite was investigated using sorption, structural, and spectroscopic methods as a model system to study clay_protein interactions. HEWL sorption to Na- and Cs-saponite was determined using the bicinchoninic acid (BCA) assay, thermogravimetric analysis, and C and N analysis. For Na-saponite, the TGA and elemental analysis-derived sorption maximum was 600 mg/g corresponding to a surface coverage of 0.85 ng/mm2 with HEWL occupying 526 m2/g based on a cross-sectional area of 13.5 nm2/molecule. HEWL sorption on Na-saponite was accompanied by the release of 9.5 Na+ ions for every molecule of HEWL sorbed consistent with an ion exchange mechanism between the positively charged HEWL (IEP 11) and the negatively charged saponite surface. The d-spacing of the
HEWL_Na-saponite complex increased to a value of 4.4 nm consistent with the crystallographic dimensions of HEWL of 3 _ 3 _ 4.5 nm. In the case of Cs-saponite, there was no evidence of interlayer sorption; however, sorption of HEWL to the “external” surface of Cs-saponite showed a high affinity isotherm. FTIR and Raman
analysis of the amide I region of the HEWL_saponite films prepared from water and D2O showed little perturbation to the secondary structure of the protein. The overall hydrophilic nature of the HEWL_Na-saponite complex was determined by water vapor sorption measurements. The clay retained its hydrophilic character with a water content of 18% at high humidity corresponding to 240 H2O molecules per molecule of HEWL.