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Biological organisms are capable of controlling inorganic crystal growth to a remarkable degree (1, 2). This exquisite control is usually achieved with the use of an organic polymeric "matrix" of highly acidic macromolecules. In certain cases, the minerals that are formed by biological organisms are uniquely oriented or co-aligned relative to the organic matrix (3). The in vitro synthesis of novel organic-inorganic composites with properties analogous to those produced by Nature continues to challenge the materials scientist. The use of simplified surfacrant molecular assemblies (4, 5) resembling biological membranes (for example, monolayers, multilayers, or vesicles) is one approach toward achieving this goal. These structures provide modifiable interfacial functionalization and well-defined spatial organization. A number of elegant examples demonstrate the nucleation and growth of organic (6) and inorganic crystals at monolayer assemblies (7-9). To our knowledge, the crystals produced by these methods are oriented only in the direction normal to the membrane plane (8, 9). The crystal axes in the plane of nucleation do not appear to be aligned with a structural parameter of the nucleation surface.
Previous studies have indicated that stereochemical match between the organic and inorganic interfaces is a predominant factor in determining the specific nucleation face type, in addition to lattice match and electrostaric interactions (10-12). One difficulty is that structural information regarding the organic template has so far been obtained in the absence of mineralization at the monolayer [for example, from grazing incidence x-ray diffraction, electron diffraction, and x-ray reflectivity (13, 14)], yielding the average spacing between organic functional groups and the two-dimensional unit cell dimensions. Furthermore, it has been suggested that synergistic changes in the organic template structure occur upon interaction with solid interfaces, as is the case when crystals are forming at the hydrophilic head-group region (12). In such cases, in situ structural information regarding the organic template is even more elusive as dynamic changes in monolayer organization may occur.
In this report, we demonstrate that cooperativity at the organic-inorganic interface can result in complete alignment of calcite crystals along an identifiable structural feature oi the acidic p-PDA matrix. Lattice match between calcite and p-PDA dominates along the a axis of the calcite crystal. Symmetry reduction in the p-PDA template coupled with proper scereochemical match ultimately controls the co-alignment...