M. Heim et al., HYDRATION SCANNING-TUNNELING-MICROSCOPY OF DNA AND A BACTERIAL SURFACE PROTEIN, Journal of structural biology, 119(2), 1997, pp. 212-221
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.