Inverse 31P → 1H NMR cross-polarization in hydrated nanocrystalline calcium hydroxyapatite

Hydrated nanocrystalline calcium hydroxyapatite was studied using unconventional solid-state NMR technique with cross-polarization from phosphorus-31 to protons (31P → 1H CP) under magic-angle spinning (MAS). Two peaks were detected: at 0.0 ppm from structural hydroxyl groups and at 5.4 ppm from water. Their intensities were monitored as a function of the contact time. It was found that there were two types of water adsorbed on the crystal surface: typical strongly bound water and distinct, more mobile water associated with a particular 31P environment capable of fast spin–lattice relaxation in the rotating frame. Relative contents of protons in various material compartments were estimated.