title: Interaction of biological molecules with clay minerals: a combined spectroscopic and sorption study of lysozyme on saponite.
abstract: 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/mm(2) with HEWL occupying 526 m(2)/g based on a cross-sectional area of 13.5 nm(2)/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 D(2)O 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 H(2)O molecules per molecule of HEWL.
authors: Birck Nanotechnology Center, Purdue University, 915 West State Street, West Lafayette, Indiana 47907-2054, United States. cliffjohnston@purdue.edu
authors: Cliff T
authors: CT
authors: Johnston
authors: Gnanasiri S
authors: GS
authors: Premachandra
authors: Tamas
authors: T
authors: Szabo
authors: Joyce
authors: J
authors: Lok
authors: Robert A
authors: RA
authors: Schoonheydt
mesh_terms: D000538:Aluminum Silicates / Q000737:chemistry*; D000077215:Clay; D008903:Minerals / Q000737:chemistry*; D009113:Muramidase / Q000737:chemistry*; D013818:Thermogravimetry
chemical_list: D000538:Aluminum Silicates; D008903:Minerals; C099453:saponite; D009113:Muramidase; D000077215:Clay