Bridged pathway

The products of the heterolytic H-H bond cleavage show some minor differences for the two pathways. The Ni-Fe distance is 2.76 A for the bridging pathway, while it is as short as 2.66 A for the terminal pathway. The reason is that Cys492 remains bridging for the terminal pathway. The reaction energies are still quite similar. The product of the bridging pathway is 12.8 kcal/mol below the molecular minimum, and 11.4 kcal/mol below this minimum for the terminal pathway. 

Experimental estimates of <5r/c , are relatively difficult to obtain. While they can, in principle, be extracted from temperature-dependence studies, this approach is complicated by uncertainties in the entropic term (Sect. 4.3). An alternative method has recently been described for some Cr(III) reductions which involves comparing the work-corrected rate constants, kco , with unimolecular rate constants, ket, for structurally related reactants that reduce via ligand-bridged pathways [30]. Provided that the corresponding outer- and inner-sphere pathways involve the same activation barrier (Sect. 4.6) and the latter also follow adiabatic pathways, we can write [30]... 

Maji T. K., Mukherjee P. S., Mostafa G., Mallah T., Cano-Boquera Joan and Chaudhuri N. R., First observation of a ferromagnetic interaction through and end-to-end azido bridging pathway in a ID copper(II) system, J. Chem. Soc., Chem. Commun. (2001) pp. 1012-1013. 

Other pathways are also conceivable for the initially formed oxo-bridged pathway. For example, a 1,3-shift of a fluoride provides quick access to the neutral intermediate F4P-O-IF6 ... 

We aim to show below how an explicit coding of the chemical structures of the starting materials and products of biochemical reactions and their reaction centers might allow us to achieve progress in our understanding of biochemical pathways. Furthermore, it will be shown how a bridge between chemoinformatics and bioinformatics can be built. 

CH2SH + 1/2 O2 -CH2-S-S-CH2 + H2O This reaction requires an oxidative environment, and such disulfide bridges are usually not found in intracellular proteins, which spend their lifetime in an essentially reductive environment. Disulfide bridges do, however, occur quite frequently among extracellular proteins that are secreted from cells, and in eucaryotes, formation of these bridges occurs within the lumen of the endoplasmic reticulum, the first compartment of the secretory pathway. 

The dipyrrylacelylenedicarbaldehyde 18, which already contains, along with the acetylene moiety, two of the two-carbon bridges of the final macrotetracycle, can be prepared by a Heck-type coupling of 5-iodopyrrole-2-carbaldehydes with acetylene.8a,b The main conjugation pathway with 22n electrons is already present in the acetylene-cumulate systems 19. The expected planarity of this chromophore has been confirmed by X-ray structure analysis.8a b... 

The scheme in Fig. 9-5 above illustrates the case in which the bridging ligand, X, is transferred from metal center Mi to M2 in the course of the reaction. Although this is not a necessary consequence of an inner-sphere pathway, it is often observed, and provides one method for establishing the mechanism. 

Some years ago, we tackled (ref. 7) the particular question of bromine bridging, related mainly to stereochemistry, postulating that bromonium ions and bromo-carbocations are formed in separate pathways as shown in Scheme 3. The relative rates of reaction by these pathways depend on the olefin structure. As demonstrated later... 

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