Magnetic tweezers force calibration for molecules that exhibit conformational switching

High spatial and temporal resolution magnetic tweezers experiments allow for the directcalibration of pulling forces applied to short biomolecules. In one class of experiments, a force is applied to a structured RNA or protein to induce an unfolding transition; when the force is maintained at particular values, the molecule can exhibitconformational switching between the folded and unfolded states or between intermediate states. Here, we analyze the degree to which common forcecalibration approaches, involving the fitting ofmodel functions to the Allan variance or power spectral density of the bead trajectory, are biased by thisconformational switching. We find significant effects in two limits: that of large molecular extension changes between the two states, in which alternative fitting functions must be used, and that of very fast switching kinetics, in which the forcecalibration cannot be recovered due to the slowdiffusion time of the magnetic bead. We use simulations and high-resolution RNA hairpin data to show that most biophysical experiments do not occur in either of these limits.

Source: Review of Scientific Instruments - September 28, 2016 Category: Physics Authors: David R. Jacobson and Omar A. Saleh Source Type: research