Force Feedback Effects on Single Molecule Hopping and Pulling Experiments
Pith reviewed 2026-05-24 19:03 UTC · model grok-4.3
The pith
Nonequilibrium pulling experiments yield accurate folding parameters with force feedback at all temperatures, unlike hopping experiments.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
In nonequilibrium pulling experiments on a short DNA hairpin, the constant force mode with feedback produces reliable determinations of folding reaction parameters at all tested temperatures, whereas the same feedback in equilibrium hopping experiments overestimates parameters when transition rates exceed the feedback response time at higher temperatures.
What carries the argument
The force feedback algorithm in constant force mode (CFM) that adjusts trap position to hold force constant, compared against passive mode (PM) with fixed trap position.
If this is right
- Nonequilibrium pulling protocols can safely employ constant force mode even when molecular kinetics are fast.
- Equilibrium hopping measurements require passive mode when transition rates approach or exceed feedback response speed to avoid parameter bias.
- Temperature variation provides a direct test of feedback limitations because it changes kinetic rates while keeping the feedback algorithm fixed.
- The Bell-Evans model applied to lifetime or rupture-force data remains valid in pulling experiments under both control modes.
Where Pith is reading between the lines
- Feedback response time may limit accuracy in other dynamic single-molecule protocols that rely on rapid state detection.
- For molecules with temperature-sensitive kinetics, pulling protocols could be systematically preferred over hopping when feedback hardware is fixed.
- The observed mode difference suggests that irreversible trajectories average out brief feedback lags that distort equilibrium lifetime statistics.
Load-bearing premise
The discrepancy in constant force mode hopping experiments at high temperatures arises solely because feedback response time is slower than the molecular transition rates.
What would settle it
Measuring folding parameters in pulling experiments at a temperature above 45°C or with a faster hairpin and finding systematic overestimation in constant force mode comparable to the hopping case would falsify the reliability claim for pulling.
read the original abstract
Single-molecule experiments with optical tweezers have become an important tool to study the properties and mechanisms of biological systems, such as cells and nucleic acids. In particular, force unzipping experiments have been used to extract the thermodynamics and kinetics of folding and unfolding reactions. In hopping experiments, a molecule executes transitions between the unfolded and folded states at a preset value of the force (constant force mode -CFM- under force feedback) or trap position (passive mode -PM- without feedback) and the force-dependent kinetic rates extracted from the lifetime of each state (CFM) and the rupture force distributions (PM) using the Bell-Evans model. However, hopping experiments in the CFM are known to overestimate molecular distances and folding free energies for fast transitions compared to the response time of the feedback. In contrast, kinetic rate measurements from pulling experiments have been mostly done in the PM while the CFM is seldom implemented in pulling protocols. Here, we carry out hopping and pulling experiments in a short DNA hairpin in the PM and CFM at three different temperatures (6$^\circ$C, 25$^\circ$C and 45$^\circ$C) exhibiting largely varying kinetic rates. As expected, we find that equilibrium hopping experiments in the CFM and PM perform well at 6$^\circ$C (where kinetics is slow) whereas the CFM overestimates molecular parameters at 45$^\circ$C (where kinetics is fast). In contrast, nonequilibrium pulling experiments perform well in both modes at all temperatures. This demonstrates that the same kind of feedback algorithm in the CFM leads to more reliable determination of the folding reaction parameters in irreversible pulling experiments.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports single-molecule optical tweezers experiments on a short DNA hairpin comparing passive mode (PM) and constant-force mode (CFM) in both equilibrium hopping and nonequilibrium pulling protocols at 6°C, 25°C and 45°C. It finds that CFM hopping overestimates molecular distances and folding free energies at the highest temperature (fast kinetics) while CFM pulling yields consistent parameters with PM across all temperatures, concluding that the same feedback algorithm is more reliable for irreversible pulling than for hopping.
Significance. If the central empirical observation holds, the work provides direct evidence that force-feedback artifacts are protocol-dependent rather than universal, supporting wider use of CFM in pulling experiments to extract folding parameters without the overestimation seen in fast hopping.
major comments (2)
- [Abstract] Abstract and main text: the central claim that CFM pulling 'performs well' at 45°C (where molecular transitions are fast) rests on the untested assumption that feedback-response-time mismatch affects only hopping; no measurement of feedback bandwidth, no model of how ramp rate interacts with transition times, and no quantitative comparison of effective sampling rates between the two protocols are provided to substantiate why the identical mismatch is irrelevant during force-ramped pulling.
- [Abstract] Abstract: the statement that CFM hopping overestimation is 'as expected' for fast transitions is used to interpret the pulling results, yet the manuscript supplies neither direct timing data on feedback response nor error bars on the extracted parameters that would allow assessment of whether the pulling agreement is statistically robust or merely consistent within large uncertainties.
minor comments (2)
- The temperature values are given in °C while kinetic rates are discussed qualitatively; explicit conversion to absolute temperature and tabulation of observed transition rates would improve clarity.
- No mention of the number of molecules or trajectories analyzed per condition; adding these statistics would strengthen the experimental description.
Simulated Author's Rebuttal
We thank the referee for their careful reading and constructive comments on our manuscript. We address the major comments point by point below, with clarifications based on the experimental design and results, and indicate revisions where they strengthen the presentation without altering the central findings.
read point-by-point responses
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Referee: [Abstract] Abstract and main text: the central claim that CFM pulling 'performs well' at 45°C (where molecular transitions are fast) rests on the untested assumption that feedback-response-time mismatch affects only hopping; no measurement of feedback bandwidth, no model of how ramp rate interacts with transition times, and no quantitative comparison of effective sampling rates between the two protocols are provided to substantiate why the identical mismatch is irrelevant during force-ramped pulling.
Authors: We agree that a direct measurement of feedback bandwidth and a quantitative model would provide additional support. However, the empirical data show that CFM pulling yields folding parameters consistent with PM across all temperatures (including 45°C), while the same CFM algorithm produces clear overestimation only in hopping at fast kinetics. This protocol dependence arises because hopping extracts parameters from state lifetimes at fixed force (where feedback lag directly distorts dwell times), whereas pulling extracts parameters from rupture-force distributions during continuous ramps; the short duration of each transition relative to the ramp rate limits the impact of feedback lag. We will add a qualitative discussion of this distinction and a comparison of effective sampling rates based on the reported experimental parameters (trap stiffness, feedback loop rate, and pulling speed). revision: partial
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Referee: [Abstract] Abstract: the statement that CFM hopping overestimation is 'as expected' for fast transitions is used to interpret the pulling results, yet the manuscript supplies neither direct timing data on feedback response nor error bars on the extracted parameters that would allow assessment of whether the pulling agreement is statistically robust or merely consistent within large uncertainties.
Authors: The phrase 'as expected' is grounded in prior literature on feedback artifacts in constant-force hopping (cited in the manuscript). We will add error bars to all extracted molecular distances and free energies in the revised figures and tables to permit statistical evaluation of the pulling agreement. Direct timing data on feedback response were not recorded during these experiments; the temperature-dependent pattern (no discrepancy at 6°C and 25°C, clear overestimation at 45°C in hopping only) nevertheless provides internal consistency for the interpretation. revision: yes
- Direct measurement of feedback bandwidth and timing data on feedback response time, which were not collected in the original experiments.
Circularity Check
No circularity: purely empirical comparison of experimental protocols
full rationale
The paper reports direct measurements of folding parameters from hopping (lifetime analysis) and pulling (rupture-force distributions) experiments in PM vs. CFM modes across three temperatures. No derivation, ansatz, or parameter-fitting step is claimed; the central observation (CFM overestimation only in fast hopping, not in pulling) follows from the raw data without reduction to inputs by construction. Bell-Evans model is invoked only as a standard analysis tool, not as a self-referential prediction. Self-citations, if present, are not load-bearing for any claimed result.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Bell-Evans model accurately describes force-dependent kinetic rates from lifetimes and rupture forces
discussion (0)
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