Metals, activated compounds, mechanism

The difficulty to transform CO2 into other organic compounds lies in its high thermodynamic stability. Typical activation energies for the dissociation and recombination ofC02 are of 535 and 13 kJ/mol, respectively [5], The activation can occur by photochemical or electrochemical processes, by catalytic fixation or by metal-ligand insertion mechanisms. As documented in different reviews, organometallic compounds, metallo-enzyme sites and well defined metallic surfaces are able to activate carbon dioxide [6-16],... 

In the absence of mechanical activation, the XPS spectrum is different from that obtained in the presence of mechanical activation. Thus, sulfur compounds that are formed on the surface (162 eV) appear in the spectra that are not present in the spectra for static immersion. Cutting produces a change in the surface chemistry and the reaction product formed appears to be similar to that obtained by mechanical activation. Thus, mechanical activity plus increased surface temperatures on a solid surface tend to promote surface chemical reactions. A lubricant interacting with metal oxide can produce entirely different reaction products from that of a lubricant interacting with rubbing metal surfaces. 

Copperphthalocyanine and phthalocyanine itself were shown to catalyze H/D exchange as well as O/P conversion, but deuterium was built into the molecule 19>2°). Later studies showed copperphthalocyanine to be active for O/P conversion, but the metal-free compound did not show reaction for 63 hours at -80 °C 21>. At working temperatures up to 120 °C no H/D exchange was observed and energies of activation were negative, which is in accordance with the physical adsorbtion mechanism. 

Latent forms of MMPs can be activated by mechanisms which cause the dissociation of the intramolecular complex between a particular cysteine residue and the required zinc metal ligand (a complex that blocks the active site) [47], This occurs because the cysteine of the latent enzyme is coordinated to the active site in a particular way that blocks the MMP active site. Collectively, the activation of MMPs occurs through a process which has been termed the cysteine-switch . Activators of the MMPs include proteases (e.g. plasmin), conformational perturbants (SDS, NaSCN), heavy metals and organomercurials (e.g. Au(I) compounds, Hg(II)), oxidants (e.g. OC1-), disulfide compounds (e.g. GSSG) and sulfhydryl alkylating agents (e.g. V-ethylmaleimide) [47 and refs, therein]. 

Materials science associated with fracture mechanics has mainly been confined to composite materials such as concrete, ceramics and metals. Much of the emphasis of the research has been on preventing fatigue and failure rather than designing for it to occur. The way a structure deforms and breaks under stress is crucial for properties such as flow and fracture behaviour, sensory perception of structure, water release and the mobility and release of active compounds. In the case of foods, the ability to break down and interact with the mouth surfaces provides texture and taste attributes. The crack propagation in a complex supramolecular structure is highly dependent on the continuous matrix, interfacial properties and defects and the heterogeneity of the structure. Previous structure-fracture work has dealt with cellular plant foods, and it has been demonstrated that the fracture path differs between fresh and boiled carrots due to cellular adhesion and cell wall strength as well as cell wall porosity and fluid transport (Thiel and Donald 1998 Stoke and Donald 2000 Lillford 2000). 

There has been a great deal of recent interest in the reactions of metal chelate compounds. In most reactions of this type, the metal chelate ring remains intact throughout the course of the reaction, particularly when inert chelates of metals such as Cr(III), Co(III), or Rh(III) are used. In such systems, therefore, it is reasonable to assume that the kinetically active species is the metal chelate and this assumption has been verified in a few instances. Although in the course of the reactions of the labile chelates of metal ions such as Cu(II), Zn(II), or Mn(II) the chelate ring undoubtedly remains intact during some portion of the reaction, it is often difficult to determine the species that is kinetically important. Until detailed studies on the mechanisms of these reactions have been carried out, the role ascribed to the metal ion in such reactions is entirely speculative. 

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