HYDRATION SCANNING-TUNNELING-MICROSCOPY OF DNA AND A BACTERIAL SURFACE PROTEIN

Hydration scanning tunneling microscopy is based on the electrical con ductivity of molecularly thin water layers which adsorb to the sample surfaces in a humid atmosphere. It allows reliable imaging of biologic al specimens and even insulators, provided they are hydrophilic. Here, we present results obtained with linearized plasmid DNA on mica and a bacterial surface protein layer (the HPI layer). A width of 3 nm was measured for the DNA molecules and a quasi-periodic structure along th e molecules with a repeat distance of about 5 nm was observed, We show that-depending on the tunneling voltage-there are two different imagi ng modes for the DNA samples: at higher voltages, real tunneling or fi eld emission is responsible for the charge transfer between tip and sa mple. In contrast, at lower voltages we found indications of a water m eniscus between tip and surface. The HPI layer, however, seems to be i maged at most voltages without a water meniscus. (C) 1997 Academic Pre ss.