title: Structure and Property Changes in Self-Assembled Lubricin Layers Induced by Calcium Ion Interactions.
abstract: Lubricin (LUB) is a "mucin-like" glycoprotein found in synovial fluids and coating the cartilage surfaces of articular joints, which is now generally accepted as one of the body's primary boundary lubricants and antiadhesive agents. LUB's superior lubrication and antiadhesion are believed to derive from its unique interfacial properties by which LUB molecules adhere to surfaces (and biomolecules, such as hyaluronic acid and collagen) through discrete interactions localized to its two terminal end domains. These regionally specific interactions lead to self-assembly behavior and the formation of a well-ordered "telechelic" polymer brush structure on most substrates. Despite its importance to biological lubrication, detailed knowledge on the LUB's self-assembled brush structure is insufficient and derived mostly from indirect and circumstantial evidence. Neutron reflectometry (NR) was used to directly probe the self-assembled LUB layers, confirming the polymer brush architecture and resolving the degree of hydration and level of surface coverage. While attempting to improve the LUB contrast in the NR measurements, the LUB layers were exposed to a 20 mM solution of CaCl2, which resulted in a significant change in the polymer brush structural parameters consisting of a partial denaturation of the surface-binding end-domain regions, partial dehydration of the internal mucin-domain "loop", and collapse of the outer mucin-domain surface region. A series of atomic force microscopy measurements investigating the LUB layer surface morphology, mechanical properties, and adhesion forces in phosphate-buffered saline and CaCl2solutions reveal that the structural changes induced by calcium ion interactions also significantly alter key properties, which may have implications to LUB's efficacy as a boundary lubricant and wear protector in the presence of elevated calcium ion concentrations.
authors.affiliation: Institute for Frontier Materials and ARC Centre of Excellence for Electromaterials Science, Deakin University , Waurn Ponds Campus, Geelong, Victoria 3216, Australia.
authors.forename: George W
authors.identifier: 0000-0003-2250-8334
authors.initials: GW
authors.lastname: Greene
authors.affiliation: School of Chemistry, Monash University , Clayton 3800 Australia.
authors.forename: Rajiv
authors.initials: R
authors.lastname: Thapa
authors.affiliation: Australian Centre for Neutron Scattering, Australia Nuclear Science and Technology Organization , Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia.
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Holt
authors.affiliation: Institute for Frontier Materials and ARC Centre of Excellence for Electromaterials Science, Deakin University , Waurn Ponds Campus, Geelong, Victoria 3216, Australia.
authors.forename: Xiaoen
authors.initials: X
authors.lastname: Wang
authors.affiliation: Australian Centre for Neutron Scattering, Australia Nuclear Science and Technology Organization , Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Garvey
authors.affiliation: School of Chemistry, Monash University , Clayton 3800 Australia.
authors.forename: Rico F
authors.initials: RF
authors.lastname: Tabor