Fine structure of nacre revealed by solid state 13C and 1H NMR

Abstract

Solid-state NMR investigations of nacre reveal the presence of an amorphous surface layer around the aragonite platelets. This surface layer contains hydrogen carbonate groups, presumably at its outer surface and water molecules slightly hindered in their mobility in a proton ratio of 1 : 33, i.e. about 16 water molecules per HCO₃⁻ unit. The ¹H spin–lattice relaxation T₁ times of protons in the protein/polysaccharide matrix (about 300 ms), of the mobile water molecules (2 s) and of the hydrogen carbonate units (12 s) differ significantly, thus revealing no spatial proximity between the protein/polysaccharide matrix with the hydrogen carbonate units of the amorphous carbonate surface layer suggesting no significant interaction between protein matrix and the amorphous calcium carbonate (ACC) layer. These results are discussed in the context of recent publications on novel structural aspects of nacre.