Exposed and buried biomineral interfaces in the aragonitic shell of perna canaliculus revealed by solid-state NMR

A comprehensive molecular description of the inorganic-bioorganic interfaces and internal structure of the aragonitic shells of Perna canaliculus is derived by employing solid-state NMR spectroscopy. The primary component of the shell, the highly ordered aragonite polymorph of CaCO₃, is shown to possess a small fraction of disordered carbonates whose average chemical-structural identity is similar to that of aragonite. These disordered carbonates were found to interact with bioorganics, bicarbonates, and water molecules and are denoted as interfacial. Characterization of the bleached and of the annealed shells enables the distinguishing of two classes of interfacial carbonates: exposed, solvent accessible, which interact primarily with bioorganics, and buried, solvent inaccessible, which interact exclusively with spatially separated water and bicarbonates. Shell annealing shows that the decomposition of the buried bicarbonate defects correlates with removal of lattice distortions, as detected by XRD, a phenomenon often found in biogenic calcium carbonates. The solid-state NMR investigation exposes the molecular bioorganic-inorganic interfaces in a mollusk shell and demonstrates the unique capability of NMR to determine comprehensively the structure of biogenic composite materials.