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Soft Matter and Biological Physics

Eur. Phys. J. E 3, 315-321

Force probing of protein crystals: An atomic force microscopy study

V. Mollica1 - A. Relini1 - R. Rolandi1 - M. Bolognesi12 - A. Gliozzi1

1Department of Physics, University of Genoa, and INFM, Via Dodecaneso 33, I-16146 Genoa, Italy
2 Advanced Biotechnology Center, Largo R. Benzi 10, I-16132, Genoa, Italy
relini@fisica.unige.it

Received 10 February 2000 and Received in final form 4 May 2000

Abstract
The atomic force microscope (AFM) was used for measuring force-distance curves on horse spleen ferritin crystals in liquid environment. In the region of the approach curve which corresponds to tip-surface contact, discrete jumps were recorded, as predicted by molecular dynamics simulations in the case of low tip-sample interaction. The observed jumps can be related to the removal of individual molecules from the surface by the AFM tip. A simple steric model, which takes into account tip and ferritin molecule size, can explain the displacements observed with excellent agreement. The elemental force jump resulting from the approach curves is a direct measure of the force required to remove a single molecule from the crystal face. We discuss the conditions under which the cantilever potential energy difference along the elemental force step provides the energy of extraction of a single molecule. The estimate of the intermolecular binding energy turns out to be in good agreement with the value calculated independently from the surface free energy of ferritin crystals.

PACS
87.14.Ee Proteins - 87.64.Dz Scanning tunneling and atomic force microscopy

Copyright EDP Sciences, Società Italiana di Fisica, Springer-Verlag