Continuous and time-shared multiple optical tweezers for the study of single motor proteins

We present a comparison between continuous (CW) and time-shared (TS) multiple optical tweezers applied to the study of the interaction between a single motor protein (myosin) with its track (an actin filament). In the experimental assay, named “three-bead assay”, a single actin filament is stretched between two beads trapped in a CW or in a TS double trap. The actin filament is presented to a single myosin molecule lying on a third bead attached to the coverslide. The CW double trap is obtained by splitting a single laser source into two orthogonally polarized beams, while the TS one is obtained by rapidly scanning a single laser beam with acusto-optic modulators. When using the CW traps, position detection of the left or right bead is obtained by means of a single quadrant detector photodiode (QDP) placed in the back focal plane of the condenser and selected with a polarizer; when using the TS traps, the position of multiple traps with the same QDP can be collected independently using triggered and synchronized generation and acquisition. The two techniques are thoroughly analysed and compared, evidencing advantages and disadvantages of each one.