Electronics Research Laboratory
Department of Physics
P. O. Box 64 (Gustaf Hällströmin katu 2)
FIN-00014
UNIVERSITY OF HELSINKI
FINLAND

Tel. +358-9-191 50695
Fax +358-9-191 50694

Optical Tweezers

We are developing an Optical Tweezers instrument for use in single molecule biophysics experiments. Our aim is to characterize viral molecular motors (RNA polymerase, Hexameric packaging motor) on single molecule level. Our collaborators include Roman Tuma (University of Leeds, UK) and Dennis Bamford (Institute of Biotechnology, University of Helsinki).

For more information, please contact:

Anders Wallin

 

An optical position-clamp with predictive control

We have developed a predictive control-algorithm which overcomes some of the limitations of our previous proportional real-time control strategy. With this new algorithm the resonance peak due to the loop-delay is smaller, which allows us to use higher gains and achieve higher effective trap stiffness.

See our latest (November 2009) APL paper:
H. Ojala, A. Korsbäck, A.E. Wallin, and E. Haeggström, "Optical position-clamping with predictive control", Applied Physics Letters 95 181104 (2009). (doi:10.1063/1.3257693)

Real-Time optical force-clamp

We are developing an FPGA-based real-time optical force-clamp (see video below).

This video shows two optically trapped polystyrene beads as seen through a 100x bright-field microscope. The optical traps are indicated by cross-hairs. A DNA-molecule (invisible) is tethered between the beads. The scale-bar on the right is 20 micrometers long.

The yellow-trace shows the force set-point. When the feedback-control is activated the FPGA steers the upper trap using AODs in order to achieve the set-point force on the lower bead. The actual force on the lower bead is shown in blue. The distance between the the two beads is indicated in green.

See our SPIE conference paper:
Anders E. Wallin, Heikki Ojala, Gabija Žiedaitė, Linda Degerth, Dennis Bamford, and Edward Hæggström, High-resolution optical tweezers for investigating DNAbinding/translocating molecular motors, Proc SPIE 7400 (2009) (doi:10.1117/12.826470)

Real-Time feedback control of optical tweezers

We are employing an FPGA-based data acquisition card which allows real-time control of trap position and stiffness. A description of this appplication won the Life Science & Biotechnology categor of the Paper Contest at NIWeek 2007.

Beam Steering with AODs

We are using acousto optic deflectors (AODs) to rapidly steer the trap laser. This allows manipulating a single bead in the steerable trap, or creating multiple time-shared traps. An FPGA-based controller is used to drive the instrument in feedback mode, i.e. measured bead positions are used as input to either a position-clamp or force-clamp control algorithm.

Monte Carlo simulation of step length measurement in a constant-force tethered bead optical tweezers measurement.

We have developed a Monte Carlo simulation for modeling a typical molecular motor(MM) experiment. in which a microsphere (radius r), tethered through a WLC, is held in a force-clamp (F_opt) while the WLC contour length (L) is successively shortened with a step length (dL). The end-to-end distance (d) is measured and from the position vs. time trace the step length of the molecular motor is determined. Our simulation can predict the SNR for different experimental parameters.

Anders E. Wallin, Ari Salmi and Roman Tuma “Step Length Measurement - Theory and Simulation for tethered Bead Constant-Force Single Molecule Assay”, Biophysical Journa, August 2007 Vol 93 No 3 (doi:10.1529/biophysj.106.097915)
(also presented as a poster at the International Biophysics Congress 2005, August 2005, Montpellier, France)