Osteopontin Stabilizes Metastable States Prior to Nucleation during Apatite Formation

Transcription

1 Supplemental Osteopontin Stabilizes Metastable States Prior to Nucleation during Apatite Formation Casper Jon Steenberg Ibsen1, Denis Gebauer2, Henrik Birkedal1 * 1 inano and Department of Chemistry, Aarhus University, 14 Gustav Wieds Vej, 8000 Aarhus, Denmark 2 University of Konstanz, Department of Chemistry, Universitätsstr. 10, Box 714, D Konstanz, Germany * Corresponding author, hbirkedal@chem.au.dk Experimental Materials and methods All solutions were prepared with DI water. The phosphate buffer was prepared by mixing aqueous solutions of Na2HPO4 and NaH2PO4 (both 10.0 mm) such that the ph was The protein, extracted from bovine milk, was supplied by ARLA Food Ingredients and used without further purification. Titrations A starting volume of phosphate buffer (20 ml, 10.0 mm, ph 8.00) was placed in a beaker and a gentle flow of N2 gas saturated with water vapour was passed over it, to prevent uptake of atmospheric CO2. Calcium chloride (2.0 mm) was then slowly titrated (0.05 ml/min) into the phosphate buffer while stirring, with simultaneous countertitration of sodium hydroxide (10.0 mm) to ensure constant ph. The titrations were automated by a Tiamo autotitration unit equipped with a ph electrode (Aquatrode, Metrohm) and a calcium ion selective electrode (Ca-ISE, Metrohm). The Ca-ISE was used to monitor the concentration free calcium in the solution in real time. The Ca-ISE was calibrated by titration of calcium chloride (2 mm) into pure water and measuring the response. When investigating the effect of the osteopontin on the crystallization behaviour, small amounts

2 of the phosphoprotein was dissolved in ph 8.00 phosphate buffer to form stock solutions with which to work. The protein used in these studies originated from bovine milk and therefore has a different degree of phosphorylation etc. than bone OPN. Because of its origin, the protein also contained some residual calcium ions meaning that the measured free calcium at time zero was slightly offset from zero at the highest protein concentrations. All titration experiments were

3 performed in duplicate or triplicate, but only single, representative datasets are presented herein for visibility. Ultracentrifugation To learn more about speciation in the supersaturated solution prior to precipitation, small amounts of sample were subjected to analytical ultracentrifugation (AUC). 300 µl aliquots were drawn from the reaction volume at time points throughout the titration experiments and spun at 60k rpm. 300 µl of the OPN-free phosphate buffer at ph 8.00 was used as a reference. Sedimentation inside the volume was followed by UV absorption, detecting only solutes and solids with bound protein as well as the native protein itself. We found that a protein concentration of 0.30 mg/ml was necessary to measure a reasonable signal. DLS The speciation during titration was also followed by dynamic light scattering (DLS). The apparatus used was a NanoFlex (Microtrac) working in reflection. At roughly 20 minute intervals throughout a titration, 2.0 ml reactant solution was pipetted into a small vial and allowed to rest for 5 min. Then 30 scans of each 30 seconds were measured before returning the volume to the batch mixture and a new one drawn. This was repeated for the duration of a titration experiment FTIR and XRD Samples for ex situ characterization were obtained by running titrations for a certain amount of time and then isolating formed material using preparative ultracentrifugation (55k rpm, 1h). To amass enough material for characterization, the entire reaction volume had to be processed for each sample. Therefore, no two samples characterized by FTIR originated from the same titration. Precipitate isolated this way was rinsed superficially with a few ml of water before being washed twice with ethanol and dispersed in acetone for storage.

4 FTIR spectra were recorded on a Spectrum 100 FT-IR (PerkinElmer) using 128 scans with a spectral width of 4 cm -1 by adding a few drops of the dispersion on the ATR diamond objective and letting the acetone evaporate. A single sample was investigated by synchrotron x-ray diffraction at beamline i711, Maxlab, Lund, Sweden. Sample preparation was done on site immediately prior to the measurement by evaporating the acetone and packing the powder in a glass capillary. Figure S1: (A) Titration curves. (B) Point of transition from prenucleation stage to MP-stage (black), first drop in free calcium (dark grey) and second drop in free calcium (light grey). Lines show linear correlation between stabilization and amount of protein.

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6 Figure S2: TEM images of intermediates. (A-C) MP stage composed of strings/patches of 4-6 nm spheres and larger agglomerates of 20 nm spheres. (D) SAED of area in (C) showing no crystalline scattering. (E-F) PN stage: Same overall picture as in MP but lower abundancy of particles.