  We use plasmon rulers to follow the conformational dynamics of a single
protein for up to 24 h at a video rate. The plasmon ruler consists of two gold
nanospheres connected by a single protein linker. In our experiment, we follow
the dynamics of the molecular chaperone heat shock protein 90, which is known
to show open and closed conformations. Our measurements confirm the previously
known conformational dynamics with transition times in the second to minute
time scale and reveals new dynamics on the time scale of minutes to hours.
Plasmon rulers thus extend the observation bandwidth 3/4 orders of magnitude
with respect to single-molecule fluorescence resonance energy transfer and
enable the study of molecular dynamics with unprecedented precision.
