Paracrystal

In the classical treatment of the paracrystal, Hosemann [5] refers to the quantity oc/dc as g-factor . 

Figure 8.43. In all ID lattice models (including the paracrystal) the higher length distributions of lattice constants are formed by repeated convolution of the fundamental distribution hi (.x)... 

Figure 8.46. Construction of observable distances (dal, Lf) in a paracrystal from its fundamental distances, L (not observable pseudo long-periods), and dcl (observable crystallite thicknesses). Advancing to the bottom the relations to the observable thicknesses of amorphous layers, dm, and the long periods, L , are sketched...

For special purposes, tubulin may be prepared by vinblastine-induced paracrystal formation and subsequent centrifugal manipulation (Wilson et al, 1976). Dimethyl sulfoxide has been useful in promoting microtubule assembly (Himes, 1977), and deuterium oxide is also effective as a promoter of tubule assembly (Borisy et al, 1975). An example of the latter is provided by the isolation of microtubule proteins from neuroblastoma cells (Olmsted and Lyon, 1981). 

Treatment of cells with vinblastine or vincristine can result in the formation of paracrystals, complexes containing the alkaloid molecules and tubulin dimers in a 1 1 ratio. Paracrystal formation in neuronal tissue of a freshwater snail has been proposed as a model for the neurotoxic effects of Catharanthus alkaloids and derivatives 44). Vincristine is approximately 10-fold more active than vinblastine as an inducer of paracrystal formation when snail neuronal tissue is treated with high concentrations (150 lM) of the alkaloids. 

It is an unusual problem to construct a homogeneously distorted two-dimensional domain structure which allows us to cover the range of the hexagonal closely packed crystal up to a two-dimensional gas. For example, the concept of the paracrystal is based on the philosophy that every state of condensed matter has at least a micro-paracrystalline arrangement of segments within a distorted lattice of arbitrary symmetry with a defined coordination number. 

Any homogeneously distorted two-dimensional coordination scheme should be based upon the distance correlation statistics between next neighbors at least. In the case of a two-dimensional lattice construction, this distance correlation principle has been used by Hosemann and coworkers [13] to generate micro-paracrystals of finite size with the help of a computer. The construction procedure (known as spiral-paracrystal ) terminates if a coordination point cannot be assigned to a lattice point. 

Zannetti, R., Celotti, G. C., Fichera, A. and Francesconi, R. The structural effects of annealing time and temperature on the paracrystal-crystal transition in isotactic polypropylene. Makromol. Chemie 128, 137 (1969)... 

Bennett, P. M. (1981). The structure of spindle-shaped paracrystals of light meromyosin. / Mol. Biol. 146, 201-221. 

Cohen, C., and Lonaley, W. (1996). Tropomyosin paracrystals formed by divalent crystals. Science 152, 794—796. 

The variance of the lattice parameters in the 111 direction for this Ni/Al203 catalyst, and the paracrystal size in this direction were found to be very similar to those that had been found earlier for an NH3 synthesis catalyst. It was suggested that there could be a common mechanism for the structural perturbation in each catalyst involving solution of a spinel phase in the host metal. Since the ratio of the volume of a Ni atom in bulk Ni to that of an oxygen anion in nickel aluminate is 1.5 1, it was suggested that the substitution could involve one (A102) group for three nickel atoms. 

Weber K, et al. Specific visualization of tubulin-containing structures in tissue culture cells by immunofluorescence. Cytoplasmic microtubules, vinblastine-induced paracrystals, and mitotic figures. Exp Cell Res 1975 95,111-20. 

In the present concept of the structure of crystalline polymers there is only room for the fringed micelle model when polymers of low crystallinity are concerned. For polymers of intermediate degrees of crystallinity, a structure involving "paracrystals" and discrete amorphous regions seems probable. For highly crystalline polymers there is no experimental evidence whatsoever of the existence of discrete amorphous regions. Here the fringed micelle model has to be rejected, whereas the paracrystallinity model is acceptable. 

Figure 9-62. Paracrystal lattice with one atom per unit cell adapted from Guinier [130], Used with permission.

LMM, after dialysis against a solution of 0.05 M KC1-0.005 M tris-maleate buffer (pH 6.2), also exhibited a decreased capacity to form well-ordered paracrystals or tactoids as examined by electron microscopy (Figure 5). It is interesting that several half-reconstituted paracrystals (Figure 5b) were found after 6 weeks of frozen storage. 

Figure 5. Electron micrographs of reconstituted paracrystals of carp LMM before and after frozen storage in 0.6M KCl at —20°C. A and B, no additives C and D, 0.2M sodium glutamate added. A and C, reconstituted before freezing B and D, reconstituted after 8 weeks of frozen storage. Negatively stained. Frozen storage in 0.05M KCl gave similar results (82).

According to this concept, the stabilizer function of alumina reduces to paracrystal-line lattice defects an analogous effect is to be expected with Cr203, Sc203, etc. [262], [296]. Another theory is based on the observation that during reduction part of the alumina precipitates with other promoters into the surface of the iron crystallite in a molecularly dispersed distribution [258], [297] or in small islands [254]. This patchy monolayer of alumina acts like a spacer between iron atoms of neighboring crystallites and prevents sintering by means of a skin effect [263]. (See also [101], [110], [253].)... 

For purposes of display, the intensity of each calculated reflection was assumed to be distributed about its mean position according to Gaussian distributions in 20 and p. The breadths of these distributions represent the expected peak broadening due to finite crystallite sizes, the crystal mosaic, and paracrystal distortions, which are not available a priori from the model calculations. Additionally, we require an estimate for p1/2 for the Lorentz correction of meridional reflection intensities. The peak broadening distributions were selected of the form... 

Various PFs [61-63] and their pyrrole analogues [64] functionalized with chiral side chains exhibit NLO character and it is not clear how the chirality, helicity and NLO character are related. Because PF2/6 has become a model test system in the study of helical PFs, fully understanding its intramolecular structure can serve as a reference point for further comparisons. The molecular level heterogeneity intrinsic in the single chain structure of PF2/6 insures a high level of frustration and thus it represents a model polymer for paracrystals and paracrystallinity [24]. 

The interaction of troponin T with tropomyosin was first shown by viscosity measurements of mixtures of the two components (Yamamoto and Maruyama, 1973) as well as by measurements of the electron microscopic density increase of specific regions in tropomyosin paracrystals or crystal nets (Greaser et al., 1973 Margossian and Cohen, 1973). 

When tropomyosin solutions are dialyzed against divalent cations such as Mg " or Ca +, fibrous paracrystals are formed. Paracrystalline tactoids under these conditions have a characteristic axial repeat period of 40 nm (Cohen and Longley, 1966). The axial periodicity is formed by the alternating arrangement of wide bright (27 nm) and narrow dark (13 nm) bands, each of which is surrounded by clear boundary lines. Two sets of oppositely directed tropomyosin filaments are arranged in the paracrystalline structure and each boundary line corresponds to the position of... 

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