Measure the force and extension of an individual molecular tether under controlled displacement or load. A calibrated cantilever or optical trap converts deflection to force. Synthetic polymer stretching provides a nonbiological application; the observed curve also contains instrument and attachment compliance, and its loading-rate dependence can distinguish equilibrium elasticity from activated events.
Force peaks in membrane-protein extraction can reflect steric barriers, membrane anchoring and electrostatic interactions. Changing ionic strength changes screening and possibly specific ion binding; it need not weaken every barrier in the same direction. The rising branch of an exposed chain and rupture of a remaining structural barrier are different physical processes.
A measurement protocol that holds applied force approximately constant while allowing displacement to vary. Feedback can adjust an actuator in response to a calibrated load signal; alternatively, a passive field gradient can be sufficiently uniform over the displacement range. Finite feedback bandwidth, drift and spatial force variation limit the approximation. This differs from fixing actuator position or prescribing a force ramp.
For a simple single-tether geometry, imposed displacement divides into cantilever deflection and molecular extension . Applying the displayed correction prevents attributing the instrument's stretch to the molecule. Geometric offsets, handles and substrate compliance require additional calibrated terms.
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