Chemistry Reference
In-Depth Information
FIGURE 19.13
Molecular correspondence of the inorganic
e
organic interface in the nacreous shell layer of Nautilus repertus. (a) structural
relationships between protein sheets, aragonite crystals, and chitin fibres. There is close geometric match between the periodicity of the
b
-sheet
and lattice spacings in the ab plane of aragonite. (b) Possible modes of molecular complementarity between Ca atoms in the aragonite ab face
and Asp residues in the sequence Asp-X-Asp along
b
-sheet matrix interface. (From
Mann, Webb, & Williams, 1989
:
pp. 541.)
calcite crystals, whereas the corresponding extract from the ACC layer inhibits crystal formation. In contrast,
macromolecules from antler spicules (composed only of pure ACC) favoured formation of the stable hydrated
form of ACC. This clearly shows that the proteins and other macromolecules occluded with the mineral phase play
an important role both in stabilisation of unstable forms like ACC and in selection of the polymorphic form which
is selected (in this case ACC or calcite).
The shells of molluscs, like clam, oyster, abalone, scallop, and fresh water snail, use CaCO
3
as the principal
constituent for an extraordinary array of diverse structures. Several approaches have led, and will lead further in
the future, to a greater understanding of how these complex forms are generated.
