Crystal formation kinetic pathways

The preceding experiments prove that there is an intermediate on the reaction pathway in each case, the measured rate constants for the formation and decay of the intermediate are at least as high as the value of kcat for the hydrolysis of the ester in the steady state. They do not, however, prove what the intermediate is. The evidence for covalent modification of Ser-195 of the enzyme stems from the early experiments on the irreversible inhibition of the enzyme by organo-phosphates such as diisopropyl fluorophosphate the inhibited protein was subjected to partial hydrolysis, and the peptide containing the phosphate ester was isolated and shown to be esterified on Ser-195.1516 The ultimate characterization of acylenzymes has come from x-ray diffraction studies of nonspecific acylenzymes at low pH, where they are stable (e.g., indolylacryloyl-chymotrypsin),17 and of specific acylenzymes at subzero temperatures and at low pH.18 When stable solutions of acylenzymes are restored to conditions under which they are unstable, they are found to react at the required rate. These experiments thus prove that the acylenzyme does occur on the reaction pathway. They do not rule out, however, the possibility that there are further intermediates. For example, they do not rule out an initial acylation on His-57 followed by rapid intramolecular transfer. Evidence concerning this and any other hypothetical intermediates must come from additional kinetic experiments and examination of the crystal structure of the enzyme. 

Kinetics and mechanisms complex formation involving rate expressions, rate laws, dissociative and associative pathways, techniques used in probing reaction mechanisms, crystal field effects are discussed in the following chapter. 

Because of the importance of microstructure on dielectric and ferroelectric properties, the transformation pathway associated with conversion of the amorphous film into the crystalline state has been studied extensively. The basic mechanism involved is one of nucleation and growth, although the formation of intermediate phases that can impact the thermodynamic driving forces associated with the transformation frequently occurs. " Another key aspect of CSD films is that crystallization occurs well below the melting point of the materials. Therefore, compared to standard mixed-oxide processing of bulk materials, the thermodynamic driving forces associated with the transformation are much greater and the kinetics of mass transport are much less. 

On the basis of these in vitro observations, it seems probable that the immature bacterial crystals develop through phase transformation processes involving a solution interface between the crystalline and amorphous phases. Initially, the amorphous phase is the kinetically favored product resulting from iron(II) oxidation. Continual flux of iron(II) across the magnetosome membrane will result either in additional ferric oxide formation or reaction of iron(II) with the preexisting iron(III) phase to give magnetite within the vesicle. The second pathway becomes competitive with a continual increase in iron(II) influx. 

Size-dependent structure and properties of Earth materials impact the geological processes they participate in. This topic has not been fully explored to date. Chapters in this volume contain descriptions of the inorganic and biological processes by which nanoparticles form, information about the distribution of nanoparticles in the atmosphere, aqueous environments, and soils, discussion of the impact of size on nanoparticle structure, thermodynamics, and reaction kinetics, consideration of the nature of the smallest nanoparticles and molecular clusters, pathways for crystal growth and colloid formation, analysis of the size-dependence of phase stability and magnetic properties, and descriptions of methods for the study of nanoparticles. These questions are explored through both theoretical and experimental approaches. 

Microporous framework solids are synthesised via solvent-mediated crystallisations from mixtures of reactive precursors. The reaction pathway is controlled by kinetic as well as thermodynamic considerations so that equilibrium phase diagrams, so relevant in the high-temperature preparation of ceramics, are not useful here. Rather, synthetic routes have been developed empirically via a major synthetic effort that continues today. The continuing industrial and academic interest in these materials provides a powerful incentive to understand the principles underlying their formation through the processes of gel formation and evolution, nucleation and crystal growth. 

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