Vectorially-Oriented Monolayers of Cytochrome Oxidase:  Fabrication and Profile Structures

Vectorially-oriented monolayers of detergent-solubilized bovine heart cytochrome c oxidase have been formed by self-assembly from solution and Langmuir−Blodgett (LB) deposition. Both quartz and Ge/Si multilayer substrates, the latter fabricated by molecular beam epitaxy, were alkylated with an amine-terminated alkylsiloxane monolayer prior to introduction to the protein. For the self-assembled protein monolayers, the amine end group surface provided for primarily electrostatic interactions with the protein, thereby encouraging a nearly unidirectional vectorial orientation of the so-adsorbed integral membrane protein. This was demonstrated by the analysis of meridional X-ray diffraction data from the monolayers so-adsorbed onto the Ge/Si multilayer substrates, which directly provided electron density profiles of the protein along the axis normal to the substrate plane to a spatial resolution of 10 Å. These profiles are consistent with the three-dimensional structure of the protein, obtained from electron microscopy. Patterson function analysis of meridional X-ray diffraction from the LB-deposited monolayers has shown the profile structure of the so-deposited protein monolayers to be qualitatively similar to that obtained via self-assembly from solution, thereby suggesting that the LB-deposited monolayers are similarly vectorially-oriented. Optical spectroscopy using quartz substrates has also indicated that the LB monolayers tend to be more densely packed than their self-assembled counterparts. Optical linear dichroism has confirmed that the planes of the oxidase's two heme groups and, hence, the molecule's long axis are more perpendicular to the monolayer plane in the LB case than for the self-assembled monolayers, consistent with the profile length of the molecule along the axis normal to the monolayer plane. Such densely packed, vectorially-oriented monolayers in a fully hydrated state now provide a unique opportunity to perform directly correlated structural−functional studies on this membrane protein.