The effect of the shutter exposure time in force calibration in Magnetic Tweezers

Abstract

Magnetic tweezers is a single-molecule technique highly used in biophysics to investigate processes at molecular scale. Obtaining quantitative results relies on a precise force calibration, which depends on the measurement of bead’s transversal fluctuations. The later is influenced by the camera shutter exposure time along which the light intensity signal is averaged. Previous works have highlight the importance of reducing the camera’s exposure time in short molecules (where the bead’s fluctuations are more rapid). Here we carry out experiments with long and short DNA molecules to study the effect of using different shutter exposure times in force calibrations. We find that working at the standard conditions, where the camera exposure time is maximized, the force is highly overestimated. Whereas modulating the exposure time becomes a good strategy to correctly measure forces both in long and short molecules. Finally, we use the results to compare the elastic response of single-stranded and double-stranded DNA molecules