Selective preservation pathways

PATHWAYS OF HUMIC SUBSTANCE FORMATION 2.4.1. Selective Preservation Pathways of Humification... 

Environmental organic matter is a composite of humic and nonhumic substances, which is formed through operation and interactions of various biotic and abiotic processes. Humic substances are formed through both selected preservation (residue) and catalytic synthesis mechanisms. Both enzymatic and mineral catalyses contribute to the formation of humic substances in the environment. The relative importance of these catalytic reactions would depend on vegetation, microbial population and activity, enzymatic activity, mineralogical composition and surface chemistry of environmental particles, management practices, and environmental conditions. Selective preservation pathways would play a more important role in humification processes in poorly drained soils and lake sediments, compared with more aerated environmental conditions. 

Fig. 4.10 The selective preservation pathway model of kerogen formation (after de Leeuw and Largeau 1993 and Tegelaar et al. 1989).

Lichtfouse E, Chenu C, Baudin F, Leblond C, DaSilva M, Behar F, Derenne S, Largeau C, Wehrung P, Albrecht P (1998a) A novel pathway of soil organic matter formation by selective preservation of resistant straight-chain biopolymers chemical and isotope evidence. Org Geochem 28 411-415... 

In contrast to the selective preservation theory, the condensation pathway proposes that humic substances are derived from the polymerization and condensation of low-molecular-weight molecules that are products of the partial microbial degradation of organic residues (Kogel-Knabner, 1993). Under this scheme of increasing complexa-tion, fulvic acids would be the first humic substances synthesized, followed by humic acids and then humin (Stevenson, 1994). The two commonly accepted condensation models are the polyphenol theory and the sugar-amine or mela-noidin theory. 

Ross NS and Hansen TPB (1992) Riboflavin deficiency is associated with selective preservation of critical flavoenzyme-dependent metabolic pathways. Biofactors 3 185-190. 

The use of promoters in formulating a catalyst is often critical to the performance. Promoters can provide an extra edge to the performance of a catalyst by improving its operation. Promoters can be many and varied. They can be additives to stabilize a particular oxidation state of the catalyst, to optimize a particular phase or stmcture of the active ingredient(s), to provide additional pathways for facilitating reactions, to alter the concentration of a particular oxidation state in the active phase of the catalyst, to increase the activity or selectivity (chemical), to preserve mechanical strength and limit sintering (stmctural), to increase the surface area of the catalyst or to alter the... 

Homoallylic type alcohols (67), on the other hand, give predominantly cyclopentenones independent of substitution (equation 37). In Ae 3-hydroxyalkyl-substituted systems, presumably allene oxide (68) is the intermediate. Thus it would appear Aat the initial site of allene oxidation is not critical to the success of the reaction. Either precursor (58) or (59) is expected to give the observed stereochemical relationships of the newly formed stereocenters by the concerted mechanism. Finally, Cha has noted that the two intermediates may lead to different stereochemical relationships by the zwitterionic mechanism. This assumes a specific pathway for breakdown of (58) or (59). That stereochemical information is preserved in the reaction is shown by the selective transformations in equation (38). 

Mismatch-optimized IS transfer Nuclear Overhauser enhancement Nuclear Overhauser effect spectroscopy Numerically optimized isotropic-mbdng sequence Preservation of equivalent pathways Pure in-phase correlation spectroscopy Planar doubly selective homonuclear TACSY Relayed correlation spectroscopy Radiofrequency... 

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