Fig. 2.
(A) 31P NMR spectra. (Bottom) 121.5-MHz CP MAS spectrum of intact gastrolith of C. quadricarinatus showing two partially resolved peaks assigned to Pi and PEP. (Top) 161.3-MHz solution NMR of the soluble matrix. Solution chemical shift values of phosphates are pH dependent and are different from the in situ values (SI Text). (B) 31P → 13C 2D-HETCOR MAS NMR spectrum of intact gastrolith of the crayfish C. quadricarinatus showing connectivity between Pi to ACC carbonate and between PEP phosphate to ACC carbonate. 1H to 31P contact time of 2 ms and 24 × 24TR TEDOR mixing were employed. (C) A schematic drawing of ACC with Pi and PEP molecularly dispersed.