Chemistry Reference
In-Depth Information
CONCLUSION
The physical processes of nucleation and crystal growth are inherent to the
biological process of mineralization. Deciphering the control mechanisms exerted by
organisms over nucleation and growth is one of the central challenges facing us as we try
to understand how biominerals are formed. The concept of the energy landscape, though
quite detached from the biology, provides a framework for relating the interactions
between the controlling agents and the physical changes in shape, structure, and phase,
regardless of whether those agents act as members of solid molecular scaffolds or ions in
solutions. In this chapter, we have attempted to show how the principles of crystallization
relate to these physical properties.
The single most important concept for understanding nucleation is that of the critical
size and its relationship to the main external control parameter, the supersaturation, and
the primary materials control parameter, the interfacial energy. Most importantly, the
magnitude of the interfacial energy depends on the atomic-scale at the interface and,
consequently, surfaces can be used by organisms to control both the location and
orientation of biominerals. Secondarily, the details of the free energy landscape that
separates the solvated phase from the final bulk crystal has a large impact on the pathway
of nucleation and its manipulation may allow organisms to precipitate inorganic
compounds into an easily shaped but metastable amorphous phase, which is then driven
to convert to the final crystalline phase. Unfortunately, while these concepts are based on
a firm physical foundation, clear and quantifiable links to real systems have yet to be
established.
Critical size and free energy barriers continue to be central concepts during the
growth phase of crystals, but the potential mechanisms for controlling the process appear
to be greater in number and diversity. The critical parameters continue to include
supersaturation and free energy, but the free energy of the interface is supplanted by that
of the step. Moreover, the kinetics of attachment and detachment become of equal
importance in determining growth rates and crystal shapes. These processes are typically
quantified by a single parameter, the kinetic coefficient, which unfortunately masks the
physics behind solute incorporation. Surprisingly, while many processes including
desolvation, adsorption, surface diffusion, chemical reaction, and eventual incorporation
can, in principle, both impact this coefficient and depend on the details of system
chemistry, AFM studies of recent years are pointing towards rearrangement of waters to
allow the solute to access the surface as the dominant factor. Consequently, the kinetic
coefficient shows a clear scaling with molecular size. Once size is taken into account, the
apparent energy barrier to step motion is surprisingly similar for all systems analyzed.
Due to the combined importance of both the thermodynamics of the step edge and
the kinetics of solute attachment and detachment, there are a number of opportunities for
organisms to modulate the growth phase. Unlike the situation with nucleation of
biominerals, a widely accepted model for shape modification has been developed based
on the concept of stereochemical recognition. Within this model, the modulators of
growth exhibit stereochemical recognition for otherwise unexpressed faces of the
mineral. Attachment to those faces then stabilizes them, producing a new crystal habit.
Proposed examples include calcium oxalate monohydrate with citrate, ice with anti-
freeze glycoproteins, and calcite with inorganic impurities such as Li and Mg, as well as
organic modifiers containing carboxyl groups such as aspartic acid, glutamic acid, and
acidic peptides. In principle, this mechanism can and probably does occur for some
systems. However, as shown in this chapter, many of the classic systems for which this
model was proposed clearly display behavior better explained through specific impurity-
step interactions on existing faces. The differences between these two models of shape
