  Conformational changes of individual fluorescently labeled proteins can be
followed in solution using a confocal microscope. Two fluorophores attached to
selected domains of the protein report fluctuating conformations. Based on
F\"orster resonance energy transfer (FRET) between these fluorophores on a
single protein, sequential distance changes between the dyes provide the real
time trajectories of protein conformations. However, observation times are
limited for freely diffusing biomolecules by Brownian motion through the
confocal detection volume. A. E. Cohen and W. E. Moerner have invented and
built microfluidic devices with 4 electrodes for an Anti-Brownian
Electrokinetic Trap (ABELtrap). Here we present an ABELtrap based on a laser
focus pattern generated by a pair of acousto-optical beam deflectors and
controlled by a programmable FPGA chip. Fluorescent 20-nm beads in solution
were used to mimic freely diffusing large proteins like solubilized FoF1-ATP
synthase. The ABELtrap could hold these nanobeads for about 10 seconds at the
given position. Thereby, observation times of a single particle were increased
by a factor of 1000.
