  To monitor conformational changes of individual membrane transporters in
liposomes in real time, we attach two fluorophores to selected domains of a
protein. Sequential distance changes between the dyes are recorded and analyzed
by Forster resonance energy transfer (FRET). Using freely diffusing membrane
proteins reconstituted in liposomes, observation times are limited by Brownian
motion through the confocal detection volume. A. E. Cohen and W. E. Moerner
have invented and built microfluidic devices to actively counteract Brownian
motion of single nanoparticles in electrokinetic traps (ABELtrap). Here we
present a version of an ABELtrap with a laser focus pattern generated by
electro-optical beam deflectors and controlled by a programmable FPGA. This
ABELtrap could hold single fluorescent nanobeads for more than 100 seconds,
increasing the observation times of a single particle by more than a factor of
1000. Conformational changes of single FRET-labeled membrane enzymes FoF1-ATP
synthase can be detected in the ABELtrap.
