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Tentative synthesis

Concerning the estimation of deep groundwater movement using noble gases  [Pg.592]

It does not appear possible to assume a priori a He accumulation rate in aged groundwaters. The studies presented in this section show that the He accumulation rates are highly variable, from virtually no external He contribution required, to He accumulation rates apparently exceeding the He flux from the whole continental crust. Hence the expectation that the He flux from the crust into aquifers could be used to estimate groundwater residence time is not valid. [Pg.592]

Noble gases studies suggest that flow rates in basins are discontinuous and variable in time and space. For example, the water velocity in the Triassic aquifer of the Paris basin decreases drastically with distance from the recharge area and therefore with depth (Dewonck et al. 2001). Stute et al. (1992) also noted a similar decrease of water movement with depth in the case of the Great Hungarian Plain. It has also been suggested that deep fluid movement is controlled by tectonic events affecting the basins (Ballentine et al. 1991, 2001 Pinti and Marty 1995). [Pg.592]

Unique information about groundwater residence times and cross formational flow can be derived from noble gases. The occurrence of He excesses that cannot be accounted for by production in the crust or in basins and are attributed to a mantle origin demonstrate without ambiguity the occurrence of cross formational flows. He/ Ne / °Ar ratios close to the production values found in shallow aquifers can hardly be established in situ because it would require temperatures much in excess of those prevailing in these aquifers (Ballentine et ah, 1994 Ballentine and Bumard 2002, this volume). They instead provide evidence for a contribution of a fluid component coming from much deeper regions of the basins or of the crust. [Pg.593]

Noble gas paleotemperatures coupled with stable isotope data indicate that basins can host waters for geologically significant periods of time. In two studies of Northern Poland (Zuber et al. 1997) and the Paris basin (Pinti et al. 1997), stable isotope data and NGT up to 25 °C require waters in deep aquifers to have been recharged in much warmer periods than the Quaternary, implying residence times of several Ma at least. This would translate to groundwater (Darcy) velocities on the order of a cm/yr. It is probable that in these conditions, the water flows cannot be modeled simply using [Pg.593]


In 1952, it was reported that a constituent of excretions from female American cockroaches of the species Periplaneta ameri-cana is an extraordinarily potent sex pheromone.1 Early attempts to isolate and characterize the active compounds were hampered because individual cockroaches store only minute amounts of the pheromone ( 1 pg), and a full 25 years elapsed before Persoons et al. reported the isolation of two extremely active compounds, periplanones A and B.2 The latter substance is present in larger relative measure and its germacranoid structure (1, without stereochemistry) was tentatively assigned on the basis of spectroscopic data. Thus, in 1976, the constitution of periplanone B was known but there remained a stereochemical problem of a rather serious nature. Roughly three years intervened between the report of the gross structure of periplanone B and the first total synthesis of this substance by W. C. Still at Columbia.3... [Pg.211]

T he synthesis of an alkali-stable highly potent insect toxicant, tentatively called Compound 118, has been announced recently (2). Structurally Compound 118 is 1,2,3,4,10,-10-hexachloro-l,4,4a,5,8,8a-hexahydro-l,4,5,8-dimethanonaphthalene (I). The chemistry and applications of this material have been discussed by Lidov, Bluestone, Soloway, and Kearns (5). [Pg.190]

Now, this tentative description of the development of a correlation, later to become information from bases to the synthesis of proteins, by no means solves the problem of the origin of this code nor does it bring into focus the fact that the very proteins which were produced are responsible for the synthesis of the basic metabolic units, formaldehyde and acetic acid and then the amino acids and bases and finally the polymers by catalysts which are the polymers themselves. We do state, however, that the set of reactions quite probably give the most kinetically stable products. Now, the amounts of the different amino acids, lipids, saccharides... [Pg.148]

The first synthesis of 9-azabicyclo[6.1.0]nona-2,4,6-triene (235) has been achieved by irradiation of l//-azonine (236) at 0°C.179 An oxonin structure (237) has tentatively been assigned to the labile photoproduct of the azonine 1,2-oxide 238.180... [Pg.276]

Proof of Constitution.—From the steps in its synthesis from methyl 3-tosyl-2,4,6-triacetyl-jS-D-glucopyranoside, Reeves, Adams and Goebel88 tentatively postulated that the crystalline product was methyl 2,4-di-methyl-6-D-glucopyranoside. They exercised commendable caution,... [Pg.177]

The 4-coumarate CoA ligase (4CL EC 6.2.1.12) enzyme activates 4-coumaric acid, caffeic acid, ferrulic acid, and (in some cases) sinapic acid by the formation of CoA esters that serve as branch-point metabolites between the phenylpropanoid pathway and the synthesis of secondary metabolites [46, 47]. The reaction has an absolute requirement for Mg " and ATP as cofactors. Multiple isozymes are present in all plants where it has been studied, some of which have variable substrate specificities consistent with a potential role in controlling accumulation of secondary metabolite end-products. Examination of a navel orange EST database (CitEST) for flavonoid biosynthetic genes resulted in the identification of 10 tentative consensus sequences that potentially represent a multi-enzyme family [29]. Eurther biochemical characterization will be necessary to establish whether these genes have 4CL activity and, if so, whether preferential substrate usage is observed. [Pg.73]

Citrus species are well-known for their accumulation of flavone- and flavanone-glycosides, and thus should contain all of the enzyme activities necessary for the synthesis of these compounds. Two tentative consensus sequences for FNS-II have been identified by in silico analysis of the CitEST database, apparently representing the first identification of putative FNS-II genes in this genus [29]. Biochemical determination of function and analysis of the proteins encoded by these genes will be an important step toward elucidating flavone synthesis in Citrus. [Pg.77]

Bacteria represent a promising source for the production of industrial enzymes. Bacterial cellulases are an especialfy interesting case in point. Many thermophilic bacterial species produce cellulases that are stable and active at high temperature, resistant to proteolytic attack, and stable to mechanical and chemical denaturation. However, cellulase productivities in bacteria are notoriously low compared to other microbial sources. In this paper bacterial enzyme production systems will be discussed with a focus on comparisons of the productivities of known bacterial cellulase producers. In an attempt to draw conclusions concerning the regulation of cellulase synthesis in bacterial systems, a tentative model for regulation in Acidothennus cellulofyticus has been developed. [Pg.331]

Lastly, early work on polyoxycation clusters of aluminium led to tentative claims of the synthesis of AI24O72 [66]. More recently, it has been established, by Al MAS NMR spectroscopy, that the sulfate salt of this species has a radius which, at 7 A, agrees well with that (of 7.2 A) calculated for a dimer of AI12 clusters [67]. Thermal treatment of the complex reveals not only 4- and 6-coordinate aluminium centres, but also the formation of a metastable, 5-coordinate metal site. [Pg.78]

Results analogous to the above have been obtained in the Ullmann synthesis of polychlorobiphenyls from polychloroiodobenzenes at 220 C when by-product polychlorodibenzofurans ( 0.1%) are produced. Routes (i) and (iii) appear to op>erate under these conditions. Thus the dichloroiodo-benzene 76 gave the tetrachlorodibenzofuran 78 by route (i), and in addition and in contrast to the results of Moron et a/., a pentachlorodibenzofuran, tentatively assigned structure 79 (Scheme 19), was detected. ... [Pg.29]

Moreover, according to this general schematic mechanism, a more detailed picture of the particle formation has been tentatively given in two particular cases (1) synthesis of colloidal silver particles, and (2) formation of bimetallic ferromagnetic metal particles. [Pg.490]


See other pages where Tentative synthesis is mentioned: [Pg.138]    [Pg.592]    [Pg.214]    [Pg.138]    [Pg.592]    [Pg.214]    [Pg.8]    [Pg.604]    [Pg.110]    [Pg.103]    [Pg.735]    [Pg.73]    [Pg.6]    [Pg.140]    [Pg.53]    [Pg.735]    [Pg.247]    [Pg.15]    [Pg.101]    [Pg.158]    [Pg.280]    [Pg.36]    [Pg.68]    [Pg.237]    [Pg.242]    [Pg.58]    [Pg.85]    [Pg.300]    [Pg.89]    [Pg.357]    [Pg.479]    [Pg.21]    [Pg.334]    [Pg.344]    [Pg.64]    [Pg.309]    [Pg.8]    [Pg.476]    [Pg.684]    [Pg.297]    [Pg.493]   


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