Solid-State NMR Rationalizes the Bone-Adhesive Properties of Serine and Phosphoserine Bearing Calcium Phosphate Cements by Unveiling their Organic/Inorganic Interface

From a multitude of homonuclear and heteronuclear correlation magic-angle-spinning (MAS) NMR experiments, we present thorough structural and phase-quantification analyses of calcium phosphate cements (CPCs) that incorporate either L-serine (Ser) or O-phospho-L-serine (Pser), thereby rendering the cements strongly bone-adhesive and suitable for biomedical implants with capacity to glue both soft and hard tissues together. In the absence of organic additives, the CPCs comprise disordered hydroxyapatite (HA), which forms from the reaction of alpha-Ca-3(PO4)(2) with water. However, the presence of even a few mol % of Pser/Ser drastically changes the cement reactions: the HA formation is quenched, while MAS NMR experiments reveal intimate contacts between the Pser/Ser molecules and amorphous calcium phosphate (ACP) that incorporate HPO42- groups: these organic/inorganic species form a homogeneous amorphous ACP/Pser or ACP/Ser cement component. The amount of ACP/Pser in the cement is shown to correlate qualitatively with its shear strength, also rationalizing why Pser-bearing CPCs exhibit stronger adhesive properties than their Ser-based counterparts, for which the ACP/Ser content does not increase concomitantly with that of Ser (as for the Pser-based CPCs). The Pser-bearing CPCs feature the strongest shear strength for 23-72 mol % Pser, whereas the decline of the adhesive properties for the Pser-richest CPCs (>72 mol %) stems from unreacted Pser and formation of its Ca salt, as well as several minor Ca phosphate phases involving HPO42- and H2PO4- groups. By combining information from various one- and two-dimensional MAS NMR experiments with H-1, C-13, and P-31 as structural probes, we examined the inorganic/organic contacts of the ACP/Pser and ACP/Ser phases, and monitored the alterations of the cement reactions for variable amounts of the organic additives.