PAsp stabilized proto-vaterite ACC (Abbreviation: PAsp_ACC)

100 mM CaCl₂ solution was added at a rate of 0.01 mL/min into 150 mL of 10 mM carbonate buffer containing 0.1 g/L PAsp at pH 9.8. During the experiment, the pH was kept constant by the automatic addition of 100 mM NaOH. The experiment was performed in a sealed beaker to prevent in-diffusion of CO₂. After 10,000 s, the reaction was stopped by pouring the reaction solution into 2 L of pure ethanol (99.9%, VWR No. 20820.293) that was stirred in a plastic beaker. The beaker was sealed with parafilm, and after stirring for 30 min, the stirrer was removed, and the beaker was resealed. The beaker was left standing for 60 min, then the supernatant was slowly decanted, and the remaining opaque sediment of ACC was isolated by centrifugation at 6000 g for 15 min. The sediment was washed with ethanol (VWR No. 20820.293), followed by another centrifugation for 15 min. The procedure was repeated once more with ethanol and once with pure acetone (VWR No. 20066.296). The ACC was dried and stored at 40 °C in a vacuum until the filling of the rotor. The synthesis yielded 20 mg of ACC per batch. Five experiments were performed to gather enough material to fill a 4 mm MAS NMR rotor. The quality of the ACC was confirmed by ATR-FTIR spectroscopy for each batch. After each titration experiment, the beaker and electrodes were washed two times using acetic acid (10%) to remove traces of mineral precipitate from the vessel, electrodes, and dosing tips. After washing with acetic acid, the equipment was rinsed using Milli-Q water and dried with dust-free tissue paper. ATR-FTIR and TGA analysis of the product is shown in Supplementary Figs. ⁶ and ⁸, respectively.