Scanning tunneling microscopy imaging of biological structures

Abstract

We report scanning tunneling microscope (STM) images of the outer sheath of the cell wall of the archaebacterium Methanospirillum hungatei. The 120-Å-thick sheath forms a cylinder that, according to extensive electron microscope studies, is known to contain a two-dimensional crystalline lattice on its outer surface with p2 symmetry and with a=56.6 Å and b=28.1 Å. The sheath tends to fragment perpendicular to the cylinder axis to give rings or hoops but is otherwise mechanically rigid and therefore well suited for STM studies. Our STM results show extended corrugations with a strong indication that their width is a random multiple of an ∼30-Å minimum period. The p2 symmetry of the surface lattice was, however, not seen in the STM images. We also report results on the imaging of sheath material under water.