Chemistry Reference
In-Depth Information
substrates that facilitate the formation of massive skeletons. Examples of organisms believed
to mineralize primarily by extracellular biologically controlled processes include the
external tests of certain foraminifera (Towe and Cifelli 1967; Zeebe and Sanyal 2002; Erez
2003), cephalopod statoliths (Bettencourt and Guerra 2000), shells of mollusks (Gregoire et
al 1955; Crenshaw 1980; Weiner and Traub 1980, 1984; Falini et al. 1996; Pereira-Mouries
et al. 2002; Weiss et al. 2002; Gotliv et al. 2003); exoskeletons of bryozoans (Rucker and
Carver 1969), scleractinian corals (Constantz 1986; Constantz and Meike 1989), bones and
teeth (e.g., discussion in Lowenstam and Weiner 1989; Veis 2003).
To understand mineralization mechanisms, one must possess knowledge regarding the
structures of the organic matrix components and of the entire framework. One of the most
thoroughly investigated matrices in this respect is the mollusk shell nacreous layer. X-ray
and electron diffraction reveal that the most ordered component is
-chitin, and the most
abundant component is silk fibroin, but it shows little evidence of order based on
diffraction (Weiner and Traub 1984). It has also been shown that there is a well defined
spatial relation between the chitin of the framework and the associated aragonite mineral.
This implies that the nucleation mechanism is most likely epitaxial (Weiner and Traub
1984). The acidic components impart a chemical activity and template structure that direct
the resulting mineralization process. Not much is known about their secondary structures.
Mollusk acidic matrix proteins are thought to mainly adopt the
β
-sheet structure (Worms
and Weiner 1986). More recent structural information on the nacreous layer raises the
interesting possibilities that the silk fibroin component of the matrix is a gel (Levi-
Kalisman et al. 2001) (Figure 6) and that it does not form layers on either side of the chitin,
as was proposed by Weiner and Traub (1984). Several mollusk shell matrix proteins have
been sequenced. Most have been purified by gel electrophoresis and are not highly acidic.
β
Figure 6.
Model of the nacreous layer organic matrix, proposing that chitin and acidic macromolecules
constitute the major framework constituents, and that the silk fibroin component forms a gel between
the layers (Levi-Kalisman et al. 2001).
