title: Ellipsometric study of molecular orientations of Thermomyces lanuginosus lipase at the air-water interface by simultaneous determination of refractive index and thickness.
abstract: Ellipsometric studies of very thin organic films suffer from the low refractive index contrast between layer and bulk substrate. We demonstrate that null ellipsometry can not only provide detailed information about the adsorption kinetics and surface excess values, but in addition on layer thicknesses with submonolayer resolution of a lipase from Thermomyces lanuginosus at the air-water interface. While measuring very close to the Brewster angle, refractive indices and layer-thicknesses can both be determined with a precision that is sufficiently high to make conclusions on the density and orientation of the molecules at the interface. The orientation was found to be concentration- and pH value-dependent. At the isoelectric point, the lipase was almost vertically oriented with respect to the surface, while for pure distilled water and low lipase concentration a rather horizontal alignment was found. Further experiments, varying the size of the interfacial area in a Langmuir trough, confirm the different layer structures.
authors.affiliation: Chair of Chemical Reaction Engineering, Brandenburg University of Technology Cottbus-Senftenberg, Platz der Deutschen Einheit 1, D-03046 Cottbus, Germany; Institute of Physics and Chemistry, Brandenburg University of Technology Cottbus-Senftenberg, Platz der Deutschen Einheit 1, D-03046 Cottbus, Germany.
authors.forename: Marco
authors.initials: M
authors.lastname: Muth
authors.affiliation: Institute of Physics and Chemistry, Brandenburg University of Technology Cottbus-Senftenberg, Platz der Deutschen Einheit 1, D-03046 Cottbus, Germany. Electronic address: schmid@b-tu.de.
authors.forename: Reiner P
authors.initials: RP
authors.lastname: Schmid
authors.affiliation: Chair of Chemical Reaction Engineering, Brandenburg University of Technology Cottbus-Senftenberg, Platz der Deutschen Einheit 1, D-03046 Cottbus, Germany.
authors.forename: Klaus
authors.initials: K
authors.lastname: Schnitzlein
mesh_terms: D000327:Adsorption; D000388:Air / Q000032:analysis*; D001203:Ascomycota / Q000201:enzymology*; D005656:Fungal Proteins / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007526:Isoelectric Point; D008049:Lipase / Q000378:metabolism*; D012031:Refractometry / Q000379:methods*; D013499:Surface Properties; D013997:Time Factors; D014867:Water / Q000737:chemistry*
chemical_list: D005656:Fungal Proteins; D014867:Water; D008049:Lipase
keywords: Adsorption; Air–water interface; Ellipsometry; Lipase; Thermomyces lanuginosus