An unfolded polypeptide chain is held at a fixed end-to-end extension in an optical trap
PHYSICS · Multi-part · Fermi University Physics · Thermal, Statistical & Complex Systems · Biophysics & Medical Physics · Macromolecules, membranes and cell architecture
Forces, Machines and Signals, Ch Free energy at work: states, weights and entropic forces
Problem
An unfolded polypeptide chain is held at a fixed end-to-end extension x in an optical trap. At T=293K the restoring force needed to hold it there is measured to be f1=4.20pN, and this force is known to be purely entropic in origin (the chain's conformational entropy falls as it is stretched, at essentially constant internal energy).
((a)) State the general test that distinguishes an entropic force from an energetic one at fixed configuration, and say what it predicts for f at a higher temperature if the force is purely entropic.
((b)) Using that test, predict the force needed to hold the same extension x at T2=313K.