Optical and Magnetic Tweezers

Instruments that grip a microscopic bead with light or magnetic fields to apply and measure piconewton forces on a single attached biomolecule.

Definition

Optical and magnetic tweezers are single-molecule instruments that hold a microsphere with optical or magnetic forces to exert and measure controlled piconewton forces, and in the magnetic case torques, on a tethered molecule.

Scope

This topic covers the two principal force-manipulation methods of single-molecule biophysics: optical tweezers, which trap a dielectric bead at a laser focus, and magnetic tweezers, which pull and twist a magnetic bead. It explains how each generates and calibrates force, what observables they provide, and their complementary strengths, with mechanical unfolding and trajectory analysis treated in neighbouring topics.

Core questions

• How does a focused laser beam trap a microscopic bead?
• How are the forces exerted by tweezers calibrated and measured?
• How do magnetic tweezers apply torque as well as force?
• What molecular processes are best studied with each method?

Key theories

Gradient-force optical trap

A tightly focused laser beam exerts a restoring gradient force that holds a dielectric bead near the focus, so small displacements report force and the bead acts as a calibrated handle on an attached molecule.

Force–extension interrogation of molecules

Pulling on a tethered molecule and recording its extension versus force probes elasticity and structural transitions directly, as in the overstretching transition of double-stranded DNA.

Mechanisms

In optical tweezers, the intensity gradient of a focused laser polarises a dielectric bead and pulls it toward the focus; the trap behaves like a soft spring, so the bead's displacement, tracked with nanometre precision, gives the applied force after calibration against thermal fluctuations or viscous drag. In magnetic tweezers, an external field gradient pulls a magnetic bead and a rotating field twists it, allowing both stretching force and controlled torque. A single molecule tethered between the bead and a surface is thereby loaded mechanically while its extension is recorded.

Clinical relevance

Tweezer measurements reveal the mechanics of motors, polymerases, and nucleic acids that are biomedically important, providing educational insight into molecular mechanism rather than clinical recommendation.

History

Ashkin's demonstration of the single-beam gradient optical trap in 1986, recognised by a later Nobel Prize, enabled single-molecule force experiments such as the stretching of individual DNA molecules, while magnetic tweezers added controlled torque for studying twist and supercoiling.

Key figures

• Arthur Ashkin
• Steven Chu
• Carlos Bustamante
• Steven Block

Seminal works

• ashkin1986
• smith1996

Frequently asked questions

How can light hold a bead in place?

A tightly focused laser creates an intensity gradient that polarises a transparent bead and pulls it toward the brightest point, forming a stable trap that acts like a tiny spring.

What is the difference between optical and magnetic tweezers?

Optical tweezers offer high spatial and temporal resolution and stiff trapping, while magnetic tweezers naturally apply constant force and can twist the molecule, making them suited to studying torque and supercoiling.