Effect of Ca2+ ions on the adhesion and mechanical properties of adsorbed layers of human Osteopontin
Bruno Zappone 1*, Philipp Thurner 2, Jonathan Adams 3, Georg E. Fantner 4 and Paul K. Hansma 3
1 LICRYL, CNR-INFM Regional Laboratory and Centro di Eccellenza Cemif.Cal
2 Bioengineering Science Research Group, University of Southampton, Southampton
3 Physics Department, University of California Santa Barbara
4 Massachusetts Instituter of Technology
* To whom correspondence should be addressed. E-mail: zappone{at}fis.unical.it.
Submitted on April 23, 2008
Revised on May 27, 2008
Accepted on 3 June 2008
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Abstract |
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Using an Atomic Force Microscope (AFM) and a Surface Force Apparatus (SFA) we have measured the surface coverage, adhesion and mechanical properties of layers of Osteopontin (OPN), a phosphoprotein of the human bones, adsorbed on mica. OPN is believed to connect collagen fibrils of the bone in a matrix that dissipates energy, reducing the risk of fractures. AFM normal force measurements showed large adhesion and energy dissipation upon retraction of the tip, which were due to the breaking of the many OPN-OPN and OPN-mica bonds formed during tip-sample contact. The dissipated energy increased in the presence of Ca2+ ions due to the formation of additional OPN-OPN and OPN-mica salt bridges between negative charges. The forces measured by SFA between two macroscopic mica surfaces were mainly repulsive and became hysteretic only in the presence of Ca2+: adsorbed layers underwent an irreversible compaction during compression due to the formation of long-lived calcium salt bridges. This provides an energy storage mechanism, which is complementary to energy dissipation and may be equally relevant to bone recovery after yield. The prevalence of one mechanism or the other appears to depend on the confinement geometry, adsorption protocol and loading-unloading rates.
Key Words:
bone mechanics, mechanical properties of proteins, phosphoproteins, protein adhesion, surface force measurements
