1999 Fisher Trends Biochem. Sci


measurement of the force-induced un-folding of single protein domains andthat offer a new perspective on the func-tion of proteins exposed to stretchingforces.

Single-molecule force spectroscopy

n the force-measuring mode of theatomic force microscope (AFM)10–12, asingle molecule is stretched betweenthe microscopic silicon nitride tip of aflexible cantilever and a flat substratethat is mounted on a highly accuratepiezoelectric positioner (Fig. 1b). Alayer of protein, or other biological poly-mer, is either adsorbed to the substrateor linked to it through the formation ofcovalent bonds. When the tip and sub-strate are brought together and thenwithdrawn, one or more molecules canattach to the tip by adsorption. As thedistance between the tip and substrateincreases, extension of the moleculegenerates a restoring force that causesthe cantilever to bend. This causes de-flection of a laser beam directed towardthe upper surface of the cantilever,which is measured using a photodetec-tor. The output of the photodetector canbe related to the angle of the cantileverand therefore to the applied force, if theelastic properties of the cantilever areknown. This system allows spatial ma-nipulations of less than a nanometerand can measure forces of a few piconewtons (pN).

Entropic elasticity

When a polymer is relaxed, it forms acoiled structure because this maximizesthe entropy of its segments. Extensionof the polymer generates an opposingforce due to the reduction in entropy.This phenomenon, referred to as en-tropic elasticity, suggests that small ex-tensions require little force but that theresistance to extension rises rapidly asthe polymer approaches its full length.The behaviour of polymers under mechanical stress is described by theworm-like chain (WLC) model of elas-ticity13–15. This model describes a poly-mer as a continuous string of a giventotal (or contour) length (Fig. 2a).Bending of the polymer at any point in-fluences the angle of the polymer for adistance, referred to as the persistencelength, that reflects the polymer flexibil-ity. The smaller the persistence length,the greater the entropy of the polymerand the greater the resistance to exten-sion. The persistence length and thecontour length comprise the adjustableparameters of the WLC model.

0968–0004/99/$ – See front matter © 1999, Elsevier Science Ltd. All rights reserved. PII: S0968-0004(99)01453-X

1999 Fisher Trends Biochem. Sci

1999 Fisher Trends Biochem. Sci


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