SURFACE-TOPOGRAPHY OF ACETYLCHOLINESTERASE IN LANGMUIR AND LANGMUIR-BLODGETT-FILMS

The surface topography of the enzyme acetylcholinesterase was studied at the air/aqueous and the air/solid interfaces using the Brewster ang le and the atomic force microscopies, respectively. Surface potentials of the enzyme monolayer have been measured in conjunction with the su rface pressure. The surface potential and the surface dipole moment da ta show that the orientation of the molecular dipoles occurs before th e orientation of the hydrophobic groups of the acetylcholinesterase mo nolayer. The variations of the surface potential observed at large mol ecular area suggest the presence of domains in the film. The Brewster angle images confirm the formation of domains at the air/aqueous inter face. The size of these domains increases with decreasing the molecula r area. Furthermore, the Brewster angle microscopy allowed us to detec t a reversible formation of the domains upon the compression and the d ecompression of the monolayer. On the other hand, the atomic force mic roscope images of the Langmuir-Blodgett films show that the enzyme mol ecules are more close-packed at a surface pressure of 25 mN/m than at 20 mN/m. Size measurements of the enzyme particles indicate that acety lcholinesterase has an ellipsoidal shape and that the tetramer form of this enzyme is the most abundant.