Effect of Ca2+ ions on the adhesion and mechanical properties of adsorbed layers of human osteopontin.
Using an atomic force microscope and a surface force apparatus, we 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 mineralized collagen fibrils of the bone in a matrix that dissipates energy, reducing the risk of fractures. Atomic force microscopy 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 Ca(2+) ions due to the formation of additional OPN-OPN and OPN-mica salt bridges between negative charges. The forces measured by surface force apparatus between two macroscopic mica surfaces were mainly repulsive and became hysteretic only in the presence of Ca(2+): 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.
Zappone
Bruno
B
Liquid Crystal Laboratory, Regional Laboratory, Center of Excellence for Functional Nanostructured Materials, Centro Nazionale delle Ricerche, Istituto Nazionale per la Fisica della Materia, Arcavacata di Rende (CS), Italy. zappone@fis.unical.it
Thurner
Philipp J
PJ
Adams
Jonathan
J
Fantner
Georg E
GE
Hansma
Paul K
PK
D000327:Adsorption / Q000187:drug effects; D000538:Aluminum Silicates / Q000378:metabolism; D001696:Biomechanical Phenomena; D002021:Buffers; D002118:Calcium / Q000494:pharmacology*; D006801:Humans; D018625:Microscopy, Atomic Force; D053495:Osteopontin / Q000737:chemistry* / Q000378:metabolism*; D012972:Sodium Hydroxide / Q000737:chemistry; D013499:Surface Properties; D013997:Time Factors
D000538:Aluminum Silicates; D002021:Buffers; D053495:Osteopontin; D012972:Sodium Hydroxide; D002118:Calcium; C011934:mica