Crystal modularity

Twin planes are most frequently internal boundaries across which the crystal matrix is reflected (Section 3.11). Some twin planes do not change the composition of the crystal while at others atoms are lost and a composition change can result. If faults that alter the composition are introduced in considerable numbers, the crystal will take on the aspect of a modular material and show a variable composition. 

Modular structures are those that can be considered to be built from slabs of one or more parent structures. Slabs can be sections from just one parent phase, as in many perovskite-related structures and CS phases, or they can come from two or more parent structures, as in the mica-pyroxene intergrowths. Some of these crystals possess enormous unit cells, of some hundreds of nanometers in length. In many materials the slab thicknesses may vary widely, in which case the slab boundaries will not fall on a regular lattice and form planar defects. 

This author has always distinguished between the terms synthon and building block and has rarely used the latter, because it implies modularity and a chemical insensitivity that is scarcely known in molecular crystals. 

STRUCTURE FAMILIES RECOMBINATION STRUCTURES , MODULAR ASPECTS OF CRYSTAL STRUCTURES... 

Emphasis to a similar approach has been given by Zvyagin (1993). He pointed out that many crystal structures can be represented as a composite of certain standard construction modules and various combinations, distributions and arrangements of these. The simplest example of a modular structure is the densest packing... 

Schultz, S. C., Shields, G. C. and Steitz, T. A. (1990). Crystallization of Escherichia coli cataboUte gene activator protein with its DNA binding site. The use of modular DNA. /. Mol Biol 211,159-166. 

The three-state RIS model of conformer statistics is used to analyze the 16 independent dipole coupling constants measured in a proton NMR study of n-hexane in a nematic liquid crystal solvent. The orientational ordering of the n-hexane molecule is treated in the context of the modular formulation of the potential of mean torque. This formulation gives an accurate description of alkane solute orientational order and conformer probabilities in the nematic solvent. Consequently, substantially more accurate calculated diplar couplings are obtained, and this is achieved without the need to resort to unconventionally high values of the trans-gauche energy difference E(g) in the RIS model. 

In these tissues the distribution of vesicles correspond closely to the patterns of matrix mineralization. Furthermore, it has been suggested that crystals of HA are deposited within the vesicles. Subsequently apatite is deposited within the vesicles and upon their surfaces to produce typical modular clusters of mineral. Such morphological observations strongly implicate the matrix vesicles in the formation of apatite crystals (Fig. 9). Once the first crystals are formed, they mineralize further by epitactic crystal growth458. ... 

The Modular Approach to Building Crystals From Molecules to Networks. . 242... 

THE MODULAR APPROACH TO BUILDING CRYSTALS FROM MOLECULES TO NETWORKS... 

Diamondoid networks represent just one example of the types of architecture that can be generated in a rational manner using the modular self-assembly approach. The remainder of this chapter will focus upon the diversity of simple architectures that are accessible using crystal engineering strategies, many of which have no precedent in minerals yet are afforded via one-pot reactions and commercially available components. 

Rao et al. recently reported a related structure that is based upon modular self-assembly 59 an organic network formed by trithiocyanuric acid (TCA) and bipy. Adjacent layers are aligned parallel to each other and there is no interpenetration. The resulting open framework structure exhibits channels with an effective diameter of 10 A. An interesting feature of this compound is that the cavities in the layers, and therefore the channels, can vary in size depending on the solvent of crystallization that is used to template the modular self-assembly process. (Scheme 8). It should be noted that the two architectures are not simply distorted or stretched variants of one another, they have distinct hydrogen-bonding patterns. Another salient feature is that there are sulfur atoms accessible in the cavities, which could... 

These principles of crystal engineering and supramolecular synthesis have thus far been used to design, isolate, and characterize network structures from relatively small molecular components. In the context of coordination polymer networks, a recent review indicates how wide the range of chemical components and accessible network motifs has become.48b However, the scale of these structures is such that cavities and channels are on the order of 1 nm and, to date, each cavity is identical. Careful selection of appropriate substrates or components and ever more control over crystal packing will offer the potential for rational design of an even more extensive array of modular (i.e. binary, ternary, or even higher order) structures than those that are currently available. In particular, judicious choice of supermolecules... 

Stapleton, D. et al. (1999). The crystal structure of an Eph receptor SAM domain reveals a mechanism for modular dimerization. Nature Struct. Biol. 6, 44—49. 

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