Solid state NMR of salivary calculi: Proline-rich salivary proteins, citrate, polysaccharides, lipids, and organic–mineral interactions

Abstract

Solid state NMR (ssNMR) can characterize mineral ( 31 P) and organic ( 13 C) components of human salivary stones ( n = 8). All show apatitic 31 P spectra. 13 C ssNMR indicates more protein, of more consistent composition, than apatitic uroliths, with prominent signals from Tyr, Phe, and His. Citrate and lipids, identified by dipolar dephasing (DD), and polysaccharides are also observable in varying amounts. 13 C{ 31 P} rotational echo double resonance ( 13 C{ 31 P} REDOR) identifies carbon atoms in close (<ca. 0.5 nm) proximity to phosphorus and therefore probably binding with mineral. Citrate, sugar, and carboxylate signals exhibit strong 13 C{ 31 P} REDOR effects, also observed for signals between 50 and 60 ppm, from protein α-carbons and, possibly, phosphoserines and phospholipids, and sometimes for a 35–40 ppm envelope containing Asp–Cβ and Glu–Cγ signals. Amino acid analysis indicates high proline and aromatic content. 13 C ssNMR and amino acid analyses are consistent with the preponderance of proline-rich salivary proteins such as statherin.