Ethylene pathways

The most recent discoveries in the methionine to ethylene pathway are the demonstration of S-adenosylmethionine as the intermediate and the existence of the multigene family of ACC synthases that convert S-adenosylmethionine to ACC (for review see Kende, 1993). The expression of the different genes in different tissues is determined by different stimuli such as ripening, tissue wounding or the status of cell growth responses. The isolation of the oxidase enzyme that converts ACC to liberate the free ethylene molecule in vitro (Ververidis and John, 1991) was another breakthrough, particularly because for many years it was thought that the enzymes concerned would operate only on an intact membrane system in vivo. 

However, it should be pointed out here that not all plants follow the same synthetic pathway for ethylene. Lower plants (liverworts, mosses, ferns, lycopods) do not produce it from methionine, nor from ACC—an alternative ethylene pathway therefore exists (Osborne et al., 1996). In evolutionary terms this is very significant and it remains to be established whether any cells in higher plants still retain this earlier primitive route for ethylene synthesis as part of their metabolic repertoire. 

Ethylene pathway. Ethylene and water can be reacted in the liquid or gas phase at elevated tenqierature and pressure with a catalyst. The gas-phase reaction is used most often. 

Fig. (9). Simplified model of the possible relationships between NO, SA, ROS and ethylene. NO and ROS trigger SA synthesis and SA might also enhance NO and ROS synthesis. SA, NO and ROS may act synergistically by similarly modulating the activity of common targets (for example, aconitase, ACC oxidase and cyclooxygenase). This amplification process favours the SA-dependent pathway. By contrast, NO- and SA-mediated ACC oxidase inhibition prevent ethylene synthesis and consequently the ethylene pathway (SA-independent pathway). SA may also counteract some of the NO effects and may prevent NO deleterious damage (for example accumulation of NO inhibits electron transport through cytochrome c oxidase while SA activates the alternative oxidase [18]).

The PTD setup was first applied to investigate ethylene pathways of a single cherry tomato Lycopersicon esculentum cv. Favorita). The fruit was in the mature orange stage, weight approximately 14 g and diameter about 35 mm. The tomato was detached from the plant and the calyx (often called coronet in the literature) was removed at least 10 h before the start of the experiment. 

Koltai H (2011) Strigolactones ability to regulate root development may be executed by induction of the ethylene pathway. Plant Signal Behav 6 1004—1005. doi 10.4161/ psb.6.7.15501... 

Thermal decomposition of unsubstituted 3,4,5,6-tetrahydropyridazine at 439 °C in the gas phase proceeds 55% via tetramethylene and 45% via a stereospecific alkene forming pathway. The thermal decomposition of labelled c/s-3,4,5,6-tetrahydropyridazine-3,4- f2 affords cfs-ethylene-l,2- f2, trans-ethylene-l,2-if2, c/s-cyclobutane-l,2- f2 and trans-cyclo-butane-1,2- /2 (Scheme 57) (79JA3663, 80JA3863). 

Fluorinaied dienophiles. Although ethylene reacts with butadiene to give a 99 98% yield of a Diels-Alder adduct [63], tetrattuoroethylene and 1,1-dichloro-2,2-difluoroethylene prefer to react with 1,3-butadiene via a [2+2] pathway to form almost exclusively cyclobutane adducts [61, 64] (equation 61). This obvious difference in the behavior of hydrocarbon ethylenes and fluorocarbon ethylenes is believed to result not from a lack of reactivity of the latter species toward [2+4] cycloadditions but rather from the fact that the rate of nonconcerted cyclobutane formation is greatly enhanced [65]... 

In a definitive study of butadiene s reaction with l,l-dichloro-2,2-difluoio-ethylene, Bartlett concluded that [2+4] adducts of acyclic dienes with fluorinated ethylenes are formed through a mixture of concerted and nonconcerted, diradical pathways [67] The degree of observed [2+4] cycloaddition of fluorinated ethylenes IS related to the relative amounts of transoid and cisoid conformers of the diene, with very considerable (i.e., 30%) Diels-Alder adduct being observed in competition with [2+2] reaction, for example, in the reaction of 1,1 -dichloro-2,2-difluoro-ethylene with cyclopentadiene [9, 68]... 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

In summary, the results from the fixed bed reactor study provided evidence as to the effect of Au and KOAc on the performance of the catalyst, though, these experiments did not give any information on the perturbation of the reaction pathways with the addition of Au and KOAc. For this type of information, additional experiments were performed using the TAP reactor with 1,2 C-labeled ethylene used as an isotopic tracer of the kinetics. 

Figure 8 shows the COj (MW=45) response curve for the standard pump-probe experimental conditions. COj would be generated from either the combustion of 1,2 C2H4 or the product 1,2 C-vinyl acetate. Since the curve has a maximum at 4.5 seconds which is before the evolution of vinyl acetate at 5.5-7.5 seconds, ethylene combustion seems to be the pathway that can be attributed to the formation of this peak. Moreover, on interpretation of Figure 7, it is apparent that both the Pd-Au w/KOAc and the Pd w/KOAc catalysts proceed less CO2 than their Pd-Au and Pd counterparts. Therefore, it can be concluded that KOAc impeded the combustion of ethylene. 

The factors involved and mechanistic pathways in the hydrolysis of phosphate esters, particularly those of a cyclic nature, continue to be the source of much speculation. A further study of the simplest cyclic triester, ethylene methyl phosphate, seems only to have served to consolidate already polarized views. The original, experiments of Westheimer s group employed gc and H nmr spectroscopy and demonstrated that ethylene methyl phosphate (26) hydrolyses under alkaline conditions by... 

Y-L He, S Murby, G Warhurst, L Gifford, D Walker, J Ayrton, R Eastmond, M Rowland. Species differences in size discrimination in the paracellular pathway reflected by oral bioavailability of poly (ethylene glycol) and D-peptides. J Pharm Sci 87 626-633, 1998. 

We will now discuss some very recent applications of the soft El ionization method for product detection in CMB experiments. We will first deal with two polyatomic reactions of ground state oxygen atoms with unsaturated hydrocarbons (acetylene and ethylene) these reactions are characterized by multiple reaction pathways and are of great relevance, besides being from a fundamental point of view, in combustion and atmospheric chemistry. 

Colorless, reactive gas. Oxygen was not present in the initial atmosphere of the Earth, although at 50 % it is the most common element in the crust of the Earth (oxides, silicates, carbonates, etc.). The compound with hydrogen is remarkable. The hydrides of all other elements are unpleasant compounds, but H20 is the molecule of life. The 02 found in the air today, of which it makes up 20 %, was formed in the process of evolution by photosynthesis of algae, which then also allowed life on solid land. Oxidation with oxygen became and is still the dominant pathway of life forms for obtaining energy (respiration). Used in medicine in critical situations. Oxidations play a key role in chemistry (sulfuric acid, nitric acid, acetic acid, ethylene oxide, etc.). The ozone layer in space protects the Earth from cosmic UV radiation. Ozone (03) is used in the... 

The pathway and kinetics of the C to S transition have been studied on shear-aligned cylinders of the commercial diblock copolymer of PS and poly(ethylene-co-butylene) (KRATON G 1657 Shell Chemical Company) [143, 144], A complete dissolution of the cylindrical structure before the epitaxial... 

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