Crystallization activation mechanism

Vol. 8 Thermally Activated Mechanisms in Crystal Plasticity by D. Caillard and... 

The state of polarization, and hence the electrical properties, responds to changes in temperature in several ways. Within the Bom-Oppenheimer approximation, the motion of electrons and atoms can be decoupled, and the atomic motions in the crystalline solid treated as thermally activated vibrations. These atomic vibrations give rise to the thermal expansion of the lattice itself, which can be measured independendy. The electronic motions are assumed to be rapidly equilibrated in the state defined by the temperature and electric field. At lower temperatures, the quantization of vibrational states can be significant, as manifested in such properties as thermal expansion and heat capacity. In polymer crystals quantum mechanical effects can be important even at room temperature. For example, the magnitude of the negative axial thermal expansion coefficient in polyethylene is a direct result of the quantum mechanical nature of the heat capacity at room temperature." At still higher temperatures, near a phase transition, e.g., the assumption of stricdy vibrational dynamics of atoms is no... 

It was possible for two of the systems chosen that the nucleation and crystallization activation energies could be determined separately by distinguishing the induction period and crystal growth period in the overall crystallization process. Of the two hypotheses proposed for zeolite crystallization, in the gel phase or from the solution phase, the data support the latter hypothesis for crystal growth with the crystal-liquid surface enhancing the nucleation process in seeded systems. The precise mechanism of nucleation in unseeded systems remains to be determined. 

The to—1-Fe distance in the low temperature X-ray crystal structure [147], which corresponds to the distal state, is 8.5 A and in the conformation proposed by Jovanovic et al. [151], which corresponds to the proximal state, is 4.5 A. By defining the proximal state to be made of all conformations with w—1-Fe distances less than 6.5 A we obtain the population of the proximal state as a function of temperature shown in Fig. 5.3. The population of the proximal state is 32% at 260 K, increases with temperature and finally plateaus at 318 K with 90% of the population in the proximal state. Both proximal and distal states exist at all temperatures rather than a sharp conformational transition from distal to proximal state at a specific transition temperature, a gradual shift in population from distal to proximal state occurs with increase in temperature. These findings are in agreement with the thermal activation mechanism proposed by Jovanovic et al. [151]. The predicted midpoint of the transition from the distal to the proximal state is 268K (see Fig. 5.3), 20° higher than the observed transition temperature [151]. The increase in population of the proximal state with increasing temperature indicates that the proximal state is stabilized by conformational entropy [80]. 

Since the activation mechanism likely involves displacements of transmembrane helices relative to one another, the helix contacts illuminated by the crystal structure provide a wealth of new information relevant to rhodopsin mechanism. For example, the tilt and central location of TMlll indicates that it can pack against and articulate with TMll and VI, while TMVl is significantly kinked by the presence of Pro267 ° near its extracellular end. These helical interfaces and proline-induced kinks comprise important aspects of current hypotheses of receptor activation (HubbeU et al, 2003 Visiers et al, 2002). Interestingly, TMs 1, IV, and V also contain conserved prolines but are not kinked significantly. The extracellular loops and amino terminus were found to contain antiparaUel / strands, with a strand provided by ECLlf partially buried in the central pore where... 

The first resolution of trioxalatocobaltates was accomplished in 1916 with strychnine. The method of spontaneous crystallization of antipodes from a racemic mixture of a complex was first demonstrated with K3[Co(C204)s] above 13.2°, the optical antipodes may be crystallized and mechanically separated. In practice, however, the standard technique of fractional crystallization of the strychnine diastereoisomers has been used. Partial resolution on optically active quartz has also been reported. Selective decomposition of the antipodes by circularly polarized light... 

Our latest research concerning renal defense against calcifications [72] identified a new and fourth defense mechanism of the kidney. It comprises an active crystal clearing mechanism of already retained crystals involving epithelial crystal overgrowth and interstitial degradation of the crystals. 

Kozlova SA, Kirik SD (2010) Post-synthetic activation of silanol covering in the mesostructured silicate materials MSM-41 and SBA-15. Microporous Mesoporous Mater 133 124-133 Kresge CT, Leonowicz ME, Roth WJ, VartuU JC, Beck JS (1992) Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 359(6397) 710-712... 

As a first step towards the identification of this second activation mechanism, single crystal structures involving bisiodinated halogen-bond donor 29a were obtained directly from the reactimi mixture. X-Ray analysis revealed a complex of 29a with Br4 . The distances between the four bromine atoms hint at the presence of one molecule of elemental bromine (2.43 A) which is coordinated to... 

The product alcohol catalyses its own formation and the reaction shows spectacular asynunetric amplification. If small amounts of product with 5% ee are added at the beginning of the reaction, new product is formed with 55% ee. If this product is used as a catalyst in consecutive reactions, nearly enantiopure product is achieved after a few runs. Even starting with completely racemic material, the reaction product is generally produced non-racemic, with stochastically either one or the other enantiomer in excess (23). Other chiral compoimds can direct the reaction towards the selective production of one particular enantiomer as well. About any form of chiral template has been shown to induce this effect, from helical hydrocarbons to chiral quartz crystals. The mechanism of this remarkable reaction was deduced with the help of kinetic studies and involves catalytically active homochiral dimers and inactive heterochiral ones (24, 25). 

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