We assembled an ultra-fast infrared optical trapping system to detect mechanical events that occur less than a millisecond
after a ligand binds to its filamentous substrate, such as myosin undergoing its 5 – 10 nm working stroke after actin binding.
The instrument is based on the concept of Capitanio et al.1, in which a polymer bead-actin-bead dumbbell is held in two
force-clamped optical traps. A force applied by the traps causes the filament to move at a constant velocity as
hydrodynamic drag balances the applied load. When the ligand binds, the filament motion stops within 100 μs as the total
force from the optical traps is transferred to the attachment. Subsequent translations signal active motions, such as the
magnitude and timing of the motor’s working stroke. In our instrument, the beads defining the dumbbell are held in
independent force clamps utilizing a field-programmable gate array (FPGA) to update the trap beam positions at 250 kHz.
We found that in our setup, acousto-optical deflectors (AODs) steering the beams were unsuitable for this purpose due to
a slightly non-linear response in the beam intensity and deflection angle vs. the AOD ultra-sound wavelength, likely caused
by low-amplitude standing acoustic waves in the deflectors. These aberrations caused instability in the force feedback
loops leading to artefactual ~20 nm jumps in position. This type of AOD non-linearity has been reported to be absent in
electro-optical deflectors (EODs)2. We demonstrate that replacement of the AODs with EODs improves the performance
of our instrument. Combining the superior beam-steering capability of the EODs, force acquisition via back-plane
interferometry, and the dual high-speed FPGA-based feedback loops, we smoothly and precisely apply constant loads to
study the dynamics of interactions between biological molecules such as actin and myosin.