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Measuring molecular forces

At Leeds we make use of an instrument called the Atomic Force Microscope (AFM) which was originally developed by two Nobel Prize winning physicists. The AFM is used to measure the important molecular forces which hold small, nanometre sized molecules together.

In the ‘force measuring’ mode of the AFM, a single molecule is stretched between the microscopic cantilever tip and a flat substrate that is mounted on a piezoelectric device. As the distance between the tip and substrate increases, extension of the molecule generates a restoring force that causes the cantilever to bend. The displacement of the cantilever and the restoring force are related by Hooke’s Law, F = -kx where k is the spring constant of the cantilever. Since the spring constant of the cantilever is known, and the displacement of the cantilever can be measured with the photodetector, we can measure the force which is applied to the molecule.

This simple system allows measurement of forces which are only piconewtons in magnitude. We use the AFM to measure molecular forces of molecules from extreme organisms-extremophiles!

Find out more about this area of research: www.mnp.leeds.ac.uk/ldougan

Further reading:
A-level physics magazine - Physics Review, volume 22, number 1 (2012), ‘Measuring molecular forces’ by Lorna Dougan

T. Hoffmann and L.Dougan, Single molecule force spectroscopy using polyproteins, ChemSocRev, 41, 4781 (2012)