Solid-state NMR. Solid-state NMR experiments were recorded on an AVANCE 300 Bruker spectrometer operating at v( 1 H) = 300.13 MHz and v( 31 P) = 121.50 MHz, using a 4 mm double resonance wide variable-temperature probe head. Samples were packed in 4 mm zirconia rotors and sealed with zirconia caps. Magic angle spinning (MAS) frequency was manually adjusted and maintained at 2 or 5 kHz (± Sample spinning and cooling were achieved by passing nitrogen gas through an exchange dewar filled with liquid nitrogen. The 1 H-31 P Cross-Polarization (CP) MAS NMR experiments were recorded using an excitation pulse t90 0 ( 1 H) = 3.2 s followed by a contact time tCP = 1.5 ms for all experiments presented in the main text (except variable tCPexperiments). Each spectrum was recorded with 2560 scans and a recycle delay of 6 s. The 31 P direct polarization experiment was performed using a 30 0 excitation pulse with a recycle delay of 120 s to ensure full relaxation with a total of 16 or 32 scans acquired. During the acquisition of all NMR experiments, SPINAL-64 decoupling was used (RF( 1 H) = 70 kHz). All spectra were fitted using DMFit software, employing 'CSA mas' fitting model. The chemical shifts were referenced to commercial hydroxyapatite at room temperature with an iso centered at 2.7 ppm.