negative regulator, such as Nacrein and N19 (Yano et al. 2007) from P. fucata, and
Perlwapin from abalone. The formation of exquisite nacreous layer structures may
be the result of balance between positive factors and negative ones.
12.4.3 Calcium Carbonate Polymorph Specificity
Calcite and aragonite are the two most stable polymorphs of calcium carbonate.
Although they have very similar structures, gastropods and bivalves are able to
selectively deposit calcite in the outer layer and aragonite in inner layers. Which is
the key factor determining their polymorph specificity?
It has been demonstrated that it is the soluble proteins from different shell layers
that specifically control calcium carbonate crystal morphology in an appropriate
microenvironment. The macromolecules extracted from the aragonite and calcitic
shell layers of mollusks induced formation of aragonite and calcite in vitro, respectively (Falini et al. 1996). The amino acid composition of the organic matrix, their
location, and the manner in which they affect crystal nucleation, differed markedly
in relation to the shell microstructures, especially between the nacreous and prismatic matrix (Samata 1990; Nudelman et al. 2007). Moreover, the switch between
aragonite and calcite can also be controlled in vitro by soluble polyanionic proteins
extracted from abalone shell (Belcher et al. 1996). However, proteins extracted
from the abalone aragonitic nacreous layer first induce (in vitro) the formation of
amorphous calcium carbonate, and then transform into the aragonitic crystalline
form (Gotliv et al. 2003). Further studies show that some proteins control the
morphology of nacre either through selective adsorption and spatial constraint on
the growing crystal (Giles et al. 1995), or by inhibiting the growth of certain
aragonite crystal planes (Ma et al. 2007). Others induce formation of aragonite on
calcite substrate by inhibiting the growth of calcite (Treccani et al.2006; Mann et al.
2007).
Like the soluble matrix, the insoluble matrix also has an influence on the growth
of CaCO 3 . The EDTA insoluble matrix of abalone nacre induces the growth of flat
and roughly polygonal CaCO 3 crystals, and the growth of three-dimensional parallel sheets of densely packed platelets, shown to consist of oriented aragonite by
XRD (Heinemann et al. 2006).
Further investigations indicated that the polymorph specificity of calcium carbonate is dependent upon the amino acid sequence, the conformation of specific
protein(s) in the mollusk shell, and the microenvironment in which crystal nucleation and growth takes place. Synthetic polypeptides (Asp-Leu)n have been
demonstrated to be capable of specifically inducing aragonite formation in vitro
(Levi et al. 1998). Calcium carbonate polymorphs change in vivo from aragonite to
calcite accompanied by significant alterations in protein conformation in the aragonitic layer (Choi and Kim 2000).
12 Molecular Approaches to Understand Biomineralization of Shell Nacreous Layer
343
Perlwapin from abalone. The formation of exquisite nacreous layer structures may
be the result of balance between positive factors and negative ones.
12.4.3 Calcium Carbonate Polymorph Specificity
Calcite and aragonite are the two most stable polymorphs of calcium carbonate.
Although they have very similar structures, gastropods and bivalves are able to
selectively deposit calcite in the outer layer and aragonite in inner layers. Which is
the key factor determining their polymorph specificity?
It has been demonstrated that it is the soluble proteins from different shell layers
that specifically control calcium carbonate crystal morphology in an appropriate
microenvironment. The macromolecules extracted from the aragonite and calcitic
shell layers of mollusks induced formation of aragonite and calcite in vitro, respectively (Falini et al. 1996). The amino acid composition of the organic matrix, their
location, and the manner in which they affect crystal nucleation, differed markedly
in relation to the shell microstructures, especially between the nacreous and prismatic matrix (Samata 1990; Nudelman et al. 2007). Moreover, the switch between
aragonite and calcite can also be controlled in vitro by soluble polyanionic proteins
extracted from abalone shell (Belcher et al. 1996). However, proteins extracted
from the abalone aragonitic nacreous layer first induce (in vitro) the formation of
amorphous calcium carbonate, and then transform into the aragonitic crystalline
form (Gotliv et al. 2003). Further studies show that some proteins control the
morphology of nacre either through selective adsorption and spatial constraint on
the growing crystal (Giles et al. 1995), or by inhibiting the growth of certain
aragonite crystal planes (Ma et al. 2007). Others induce formation of aragonite on
calcite substrate by inhibiting the growth of calcite (Treccani et al.2006; Mann et al.
2007).
Like the soluble matrix, the insoluble matrix also has an influence on the growth
of CaCO 3 . The EDTA insoluble matrix of abalone nacre induces the growth of flat
and roughly polygonal CaCO 3 crystals, and the growth of three-dimensional parallel sheets of densely packed platelets, shown to consist of oriented aragonite by
XRD (Heinemann et al. 2006).
Further investigations indicated that the polymorph specificity of calcium carbonate is dependent upon the amino acid sequence, the conformation of specific
protein(s) in the mollusk shell, and the microenvironment in which crystal nucleation and growth takes place. Synthetic polypeptides (Asp-Leu)n have been
demonstrated to be capable of specifically inducing aragonite formation in vitro
(Levi et al. 1998). Calcium carbonate polymorphs change in vivo from aragonite to
calcite accompanied by significant alterations in protein conformation in the aragonitic layer (Choi and Kim 2000).
12 Molecular Approaches to Understand Biomineralization of Shell Nacreous Layer
343