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Abstract

The mesocrystal system fluoroapatite-gelatine grown by double-diffusion
is characterized by hierarchical composite structure on a mesoscale. In
the present work we apply solid state NMR to characterize its structure
on the molecular level and provide a link between the structural
organisation on the mesoscale and atomistic computer simulations. Thus,
we find that the individual nanocrystals are composed of crystalline
fluorapatite domains covered by a thin boundary apatite-like layer. The
latter is in contact with an amorphous layer, which fills the
interparticle space. The amorphous layer is comprised of the organic
matrix impregnated by isolated phosphate groups, Ca3F motifs and water
molecules. Our NMR data provide clear evidence for the existence of
precursor complexes in the gelatine phase, which were not involved in
the formation of apatite crystals, proving hence theoretical predictions
on the structural pre-treatment of gelatine by ion impregnation. The
interfacial interactions, which may be described as the glue holding the
composite materials together, comprise hydrogen bond interactions with
the apatite PO43- groups. The reported results are in a good agreement
with molecular dynamics simulations, which address the mechanisms of a
growth control by collagen fibers, and with experimental observations of
an amorphous cover layer in biominerals.