Chelation capacity

The differential in chelation capacity between BF3 and SnCl4 was used to control the stereochemistry of the cyclization of the vinyl silane 10." With BF3, the reaction proceeds through a nonchelated TS and the stereochemistry at the new bond is trans. With SnCl4, a chelated TS leads to the cis diastereomer. 

While a great many compounds that have been or might be found in the marine environments have been accused of chelation, this section deals only with the nonspecific measurements of chelation, with what has been called chelation capacity . In general, this capacity is measured by spiking the solution with a transition metal, preferably one that is easily measured, and then determining either how much is complexed or how much is left over. While the principle of all of the methods is the same, the details are different, and often quite ingenious. 

In addition, the reduction of a-11 and (3-11 was s tudied in dimethyl sulfoxide as solvent, in the presence of pyridiniump-toluensulfonate. This medium makes mutarota-tion slower than the redox process.73 The two anomeric forms could reduce Cr(VI) and Cr(V) by formation of a Cr(VI) and Cr(V) ester intermediate. The equilibrium constant for this step and the rate of the redox step were different for each anomer for a-11, the equilibrium constant for ester formation is higher than for (3-11, but the redox process within this complex is faster for the latter anomer. These differences can be explained by the better chelating capacity of the 1,2-cri-diol moiety of a-11. Room-temperature CW-EPR spectra of these mixtures revealed for the a anomer several five-coordinated Cr(V)-bischelates (giso= 1.9820 [crilso 15.9xl(r4cm 1 (=47.7MHz)],... 

Similar to hydrophilic flavonoids, hydrophobic flavonoids can affect membrane permeability. Alterations in this biophysical property of liposome bilayers lead to the release of bulky molecules entrapped into the inner aqueous space. As mentioned in the previous section, a strong correlation was found between flavonoid retention to a hydrophobic matrix and their capacity to induce membrane leakage [Ollila et al., 2002]. Interestingly, hydrophilic flavonoids, such as (—)-epicatechin and related procyanidins (dimer to hex-amer) prevented Fe2 + -mediated liposome permeabilization, although in this case the beneficial effect could be related to both their antioxidant and metal chelating capacities and their membrane stabilizing properties [Verstraeten et al., 2004],... 

Gillespie, P.A. and Vaccaro, R.F., 1978. A bacterial bioassay for measuring the copper chelation capacity of seawater. Limnol. Oceanogr., 23 543-548. 

Tandon, S.K. and Prasad, S., Effect of thiamine on the cadmium-chelating capacity of thiol compounds, Hum. Exp. Toxicol., 19, 523-528, 2000. 

The chelating capacity of copper Lewis acids has been exploited in a variety of selective alcoholysis reactions. Kobayashi used Cu(OTf)2 to affect remote activation of the anomeric carbon for formation of a glycosyl bond (Sch. 13) [34]. The 2-pyridinecarboxylate group was crucial for success of the alcoholysis of 50 to afford disaccharides 51. The 4-pyridyl derivative and simple dialkylamino derivatives were unreactive. A chelating Lewis acid was required, because BF3 -OEt2 or TMSOTf were ineffective, whereas Sn(OTf)2 afforded a level of reactivity similar to that of Cu(OTf)2. 

Other measurements of antioxidant activity include FRAP (ferric reducing-antioxidant power) (123-126), TRAP (total radical-trapping antioxidant parameter) (123, 127), phycoerythrin assay (128, 129), and test of metal chelating capacity (130, 131), among others. Reviews on methods for testing antioxidant activity have been published (9, 12). 

Each of the alkyl triamine ligands has sufficient solubility ( Xl>0.01 ) in the surfactants tested to provide adequate metal chelating capacity. However, the relatively small values of found for DDPA, limit its effectiveness. With DDIEN and HDPA,... 

Evidence Tor a dissociative mechanism has been reported Lanza, S. Minnili, D. Moore. P. Sachinidis. J. Romeo, R. Tube, M. L. Inary. Chem. 1984, 23, 4428-4433. The reaction, which involves substitution of dmso in PtRitdmsofo. proceeds by loss of one dmso ligand. However, because it is possible that the dmso ligand that remains coordinated can function in a chelating capacity, it can be debated, as these workers acknowledge, whether this reaction can be colled a true dissociation. 

The relative chelating capacity of a series of flavonoids was studied spectrophotometrically by measuring the ability to release Fe2+ ions from a Fe2+-EDTA complex and to chelate Fe2+ ions [27]. The flavonoids were classified into four groups (1) flavonoids, such as apigenin, that could release Fe2+ from this complex were ranked as good chelators, (2) flavonoids, such as quercetin, that could not remove Fe2+ from the... 

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