Scanning tunneling microscopy imaging of uncoated biological material

Abstract

The scanning tunneling microscopy (STM) imaging of biological structures such as protein or cell membranes without metal coating has had limited success. A major difficulty is the mechanical interaction between needle and sample, which often results in a sideways displacement of, or damage to, the biological material during imaging. We recently reported a new scanning mode for the STM (referred to as hopping) in which the needle is periodically withdrawn from the surface. The buildup of lateral stress is thereby prevented and material that is weakly bonded to a substrate can be imaged. Using the new STM scanning technique we have obtained images of uncoated, large fragments of the cell wall sheath of the archaebacterium Methanospirillum hungatei (Mh). The sheath thickness was found to be 80 ű10% which agrees with previous STM results on metal coated complete Mh sheaths. High resolution images taken on top of the sheaths show parallel rows, or corrugations, having a dominant lateral period of 60 ű10% and heights of ≲4 Å. The minimum expected period of 30 Å was also present but the two-dimensional crystalline structure expected from diffraction studies was not seen in the STM images. We also report the first STM images of bare pepsin molecules and find the overall dimensions (∼70×40×25 Å) to be in good agreement with those expected from x-ray diffraction studies. The STM images also show reproducible structure of 10–20 Å lateral size on the surface of the pepsin molecules which has a resemblance to the x-ray crystallographically determined shape. For comparison, we also present normally scanned STM images of carbon coated pepsin molecules which show a complicated surface structure strongly dependent on the tunneling tip bias.