Chelate formers

Figure 5-1 Examples of Metal Chelates. Only the relevant portions of the molecules are shown. The chelate formers are (A) thiocarbamate, (B) phosphate, (C) thioacid, (D) diamine, (E) o-phenantrolin, (F) a-aminoacid, (G) o-diphenol, (H) oxalic acid. Source From K. Pfeilsticker, Food Components as Metal Chelates, Food Sci. Technol., Vol. 3, pp. 45-51, 1970.

Pharmacons (glucocorticoids, contraceptives, antimetabolites, chelate formers)... 

Calcium is bound to the chelate former BAPTA [(l,2-bis(o-aminophenoxy)-ethane-A. Af.A. A -tetraacetic acid] and will not interfere with the reaction. 

The ligand-promoted dissolution is illustrated by the effect of bidendate chelate formers seen in Figure 5. An example of the acid-promoted dissolution of A1203 and the dependence of some types of oxides on surface protonation is given in Figures 6 and 7. 

Membrane-active microbicides include alcohols - phenols - acids- saHcylanilides - carbanOides - dibenzamidines -biguanides - quaternary ammonium salts and other active ingredients with cationic character, e.g. azole fungicides which also act as chelate formers. As many of the antimicrobial agents which are able to complex metal cations display membrane-activity, they will be considered here as membrane-active microbicides. 

The antimicrobial activity of chelate formers bases partly on their ability to compete for the complexion of metal cations necessary for a functional cell metabolism. However, membrane-activity of the compounds also plays a role. Antimicrobial agents which form chelates include besides the already mentioned azole fungicides 2-mercaptopyridine-N-oxide (pyrithione) [II, 13.1.3.] (Cooney and Felix, 1972 Chandler and Segel, 1978 Khattar et al., 1988), 8-hydroxyquinoline (oxine) [II, 13.3.](Albert et al, 1947 Albert, 1968), and dithiocarbamates [II, 11.11.], e.g. zineb, thiram (Ludwig and Thorn, 1960). 

Yarborough et aL studied the effect of temperature upon the absorbance of simple compounds and found temperature coefficients ranging from 0.0%/deg C (acetone) to 0.74%/deg C (xylene). Absorbance decreased in a linear manner as temperature increased from 5 to 33° C. The effect was greatest for aromatics and chelate formers, least for oxygenated molecules. 

Soil organic substances are also comprised of biochemical compounds, such as simple aliphatic and aromatic acids, phenols and phenolic acids, hydroxamate siderophores, sugar acids, and complex polymeric phenols, which are synthesized by living organisms. Biochemical compounds are common to all soils and natural waters and would be expected to play a dominant role in zones where microbial activity is intense. However, biochemical chelating agents normally have only a transitory existence in the pedosphere, and the amounts found at any one time represent a balance between synthesis and destruction by microorganisms (Stevenson 1967). The amount of potential chelate formers present in soils is normally low and variable (Stevenson and Fitch 1986). Table 1 lists examples of different environments and the various types of biochemical compounds present within them. 

Chem. Descrip. Org. chelate former based on polycarboxylic acids Ionic Nature Anionic... 

Thermal stability is enhanced in chelates thus dimethyl-2-methy1pentane-2,4-dio1titanium [23916-35-0] (22) is much more stable than (CH2)3Ti(OCH(CH2)2)2 (68)- The stmcture of the former has been shown by x-ray diffraction to be dimeric and five-coordinate through oxygen bridges. The more highly substituted the six-membered ring, the mote thermally stable the compound. 

Heteroleptic complexes of uranium can be stabilized by the presence of the ancillary ligands however, the chemistry is dominated by methyl and benzyl ligands. Examples of these materials include UR4(dmpe) (R = alkyl, benzyl) and U(benzyl)4MgCl2. The former compounds coordinate "soft" chelating phosphine ligands, a rarity for the hard U(IV) atom. 

Chelated oxo structures were assigned to 5-hydroxy-4-acyl-l,3-oxazoles on the basis of their NMR spectra, the preference being given to the con-former 249a with a six-membered chelate ring (Scheme 86) (75BSB845). 

A current area of interest is the use of AB cements as devices for the controlled release of biologically active species (Allen et al, 1984). AB cements can be formulated to be degradable and to release bioactive elements when placed in appropriate environments. These elements can be incorporated into the cement matrix as either the cation or the anion cement former. Special copper/cobalt phosphates/selenates have been prepared which, when placed as boluses in the rumens of cattle and sheep, have the ability to decompose and release the essential trace elements copper, cobalt and selenium in a sustained fashion over many months (Chapter 6). Although practical examples are confined to phosphate cements, others are known which are based on a variety of anions polyacrylate (Chapter 5), oxychlorides and oxysulphates (Chapter 7) and a variety of organic chelating anions (Chapter 9). The number of cements available for this purpose is very great. 

The reaction shows a dependence on the E- or Z-stereochemistry of the enolate. Z-enolates favor anti adducts and E-enolates favor syn adducts. These tendencies can be understood in terms of an eight-membered chelated TS.299 The enone in this TS is in an s-cis conformation. The stereochemistry is influenced by the s-cis/s-trans equilibria. Bulky R4 groups favor the s-cis con former and enhance the stereoselectivity of the reaction. A computational study on the reaction also suggested an eight-membered TS.300... 

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