Crystals dimensions from microscopy

A similar ordering is found within each of the perovskite layers in the reduced n=3 Ruddlesden-Popper phase SrjMn Og, derived from the fuUy oxidised Sr Mn O,. Under normal preparation methods the ordering is confined to the perovskite layers and does not extend to three dimensions in the macroscopic crystal, although electron microscopy suggests that microdomains of such three-dimensionally ordered stmctures do exist. These structures are similar to those of Mn-containing brownmiUerite-related compounds (Section 2.5.1). 

One of the most classic examples of chiral expression in thermotropic liquid crystals is that of the stereospecific formation of helical fibres by di-astereomers of tartaric acid derivatised either with uracil or 2,6-diacylamino pyridine (Fig. 9) [88]. Upon mixing the complementary components, which are not liquid crystals in their pure state, mesophases form which exist over very broad temperature ranges, whose magnitude depend on whether the tartaric acid core is either d, l or meso [89]. Electron microscopy studies of samples deposited from chloroform solutions showed that aggregates formed by combination of the meso compounds gave no discernable texture, while those formed by combinations of the d or l components produced fibres of a determined handedness [90]. The observation of these fibres and their dimensions makes it possible that the structural hypothesis drawn schematically in Fig. 9 is valid. This example shows elegantly the transfer of chirality from the molecular to the supramolecular level in the nanometer to micrometer regime. 

To many of us, a crystal structure is the most beautiful conceivable representation of a MIM because of its high content of truth nothing can be more accurate about the way a molecule looks - in the solid state at least - than an X-ray crystal structure, save perhaps for some very recent advances in single molecule imaging provided by atomic-resolution microscopy [81]. Although we cannot see the molecule itself, a crystal structure elicits the visualization of the exact positions of every atom and bond in a molecule relative to one another in the solid state. The ability to rotate and examine the structure from any angle in three dimensions supplies a satisfying sense of connection with the molecular world. 

Fig. 6, left shows an end view of a type-I crystal formed by stacking two-dimensional crystal layers, ordered sheets of proteins. Many proteins, but not all, can form such a two-dimensional crystal layer, in which the hydrophobic regions of the proteins interact with the hydrocarbon tails of the lipids, the two-dimensional structure being stabilized by both hydrophobic and polar interactions. In each two-dimensional crystal layer no detergent is present and only the polar domains are exposed at the surface. These two-dimensional crystal layers then stack up to form a three-dimensional crystal through polar attractions between the layers. In three-dimensional crystals, the successive two-dimensional crystal layers need to be ordered in the third dimension with respect to translation, rotation and up-down orientation. Examples of type-I crystals which have been prepared are mitochondrial cytochrome oxidase, chloro-plastChl-a/ proteins, and a protein from the purple membrane ofhalobacteria. Two-dimensional crystals are usually rather small and useful only for examination by electron microscopy. 

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