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Related Experiment Videos

Relating single-molecule measurements to thermodynamics.

David Keller1, David Swigon, Carlos Bustamante

  • 1Department of Chemistry, University of New Mexico, Albuquerque, New Mexico, USA. dkeller@unm.edu

Biophysical Journal
|January 28, 2003
PubMed
Summary

Thermodynamics rules change for single molecules. Experiments show results depend on fixed versus fluctuating variables, unlike large molecule ensembles. This impacts interpreting single-molecule behavior.

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Area of Science:

  • Statistical Mechanics
  • Polymer Physics
  • Physical Chemistry

Background:

  • Thermodynamics traditionally applies to large ensembles of molecules.
  • Single-molecule measurements present unique challenges to established thermodynamic principles.

Purpose of the Study:

  • To investigate how thermodynamic interpretations differ for single molecules compared to ensembles.
  • To explore the impact of experimental variable control (fixed vs. fluctuating) on single-molecule measurements.

Main Methods:

  • Utilized a polymer stretching experiment as a model system.
  • Analyzed the influence of fixing end-to-end distance versus fixing force on experimental outcomes.
  • Examined the role of end-to-end distance distribution asymmetry.

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Main Results:

  • Single-molecule polymer stretching experiments yield different results depending on whether tension or end-to-end distance is fixed.
  • This discrepancy arises from the asymmetry in single-molecule end-to-end distance distributions.
  • Differences diminish as the polymer molecule lengthens relative to its persistence length.

Conclusions:

  • Standard thermodynamic formalisms require adaptation for single-molecule studies, potentially using potentials of mean force.
  • Potentials of mean force are not homogeneous functions, altering thermodynamic concepts like intensive/extensive quantities.
  • Interpreting single-molecule experiments necessitates a nuanced application of thermodynamic concepts due to these differences.