Following Transient Conformational Adaptations during Peptide-Guided Calcium Phosphate Biomineralization with Hyperpolarized NMR

Abstract Peptide-guided biomineralization is a key process involved in the formation of calcium phosphate (CaP) in bone, yet the peptide–ion encounter complexes at the onset of this process have remained structurally invisible: methods with sufficient resolution to decipher these intermediates would already have lost them to precipitation. Here, we resolve how a mineralization peptide adapts its conformation to recruit inorganic ions during the earliest stages of CaP formation. Using a peptide derived from the calcium-binding domain of secreted phosphoprotein 1 (SPP1), a key regulator of bone formation, we find that Ca2+ association reorganizes the conformational ensemble toward states with increased solvent exposure of Ca2+-coated Asp-rich stretches. These polyionic microenvironments subsequently exhibit spectral perturbations upon phosphate recruitment, consistent with the formation of encounter complexes during the onset of CaP nucleation at the peptide–solvent interface. Access to these transient states is enabled by NMR sensitized with hyperpolarized water (HyperW), which provides a two-order-of-magnitude sensitivity gain and reduces multidimensional acquisition times from hours to seconds─fast enough to capture peptide–ion assemblies immediately after encounter and before precipitation. Together, these results reveal a stepwise mechanism by which an intrinsically disordered mineralization peptide senses and organizes calcium and phosphate ions at the onset of biomineralization.