  The dynamics of peptide-protein binding and unbinding of a variant of the
RNase S system has been investigated. To initiate the process, a
photoswitchable azobenzene moiety has been covalently linked to the S-peptide,
thereby switching its binding affinity to the S-protein. Transient fluorescence
quenching was measured with the help of a time-resolved fluorometer, which has
been specifically designed for these experiments and is based on inexpensive
LED's and laser diodes only. One mutant shows on-off behaviour with no specific
binding detectable in one of the states of the photoswitch. Unbinding is
barrier-less in that case, revealing the intrinsic dynamics of the unbinding
event, which occurs on a few 100 microseconds timescale in a strongly
stretched-exponential manner.
