  We demonstrate a technique based on noise measurements which can be utilized
to study dynamical processes in protein assembly. Direct visualization of
dynamics in membrane protein system such as bacteriorhodopsin (bR) upon
photostimulation are quite challenging. bR represents a model system where the
stimulus-triggered structural dynamics and biological functions are directly
correlated. Our method utilizes a pump-probe near field microscopy method in
the transmission mode and involves analyzing the transmittance fluctuations
from a finite size of molecular assembly. Probability density distributions
indicating the effects of finite size and statistical correlations appear as a
characteristic frequency distribution in the noise spectra of bR whose origin
can be traced to photocycle kinetics. Valuable insight into the molecular
processes were obtained from the noise studies of bR and its mutant D96N as a
function of external parameters such as temperature, humidity or presence of an
additional pump source.
