Metal-cation substrates

Fig. 10. Unpolarized Raman spectra (T = 300 K) for solid Ceo, KaCeo, RbsCeo, NaeCeo, KaCco, RbeCeo and CseCeo [92, 93], The tangential and radial modes of Ag symmetry are identified, as are the features associated with the Si substrates. From the insensitivity of these spectra to crystal structure and specific alkali metal dopant, it is concluded that the interactions between the Cao molecules are weak, as are also the interactions between the Cao anions and the alkali metal cations.

Polymeric phthalocyanines 3 are available from 1,2,4,5-tetracyanoben ne 2 by deposition from the vapor phase on hot substrate surfaces, or by thermal curing of its tetramer, octacyanophthalocyanine 4 in the presence of metal cations (Scheme 2... 

These results may be viewed in the wider context of interactions between potential ligands of multifunctional xenobiotics and metal cations in aquatic environments and the subtle effects of the oxidation level of cations such as Fe. The Fe status of a bacterial culture has an important influence on synthesis of the redox systems of the cell since many of the electron transport proteins contain Fe. This is not generally evaluated systematically, although the degradation of tetrachloromethane by a strain of Pseudomonas sp. under denitrifying conditions clearly illustrated the adverse effect of Fe on the biotransformation of the substrate (Lewis and Crawford 1993 Tatara et al. 1993). This possibility should therefore be taken into account in the application of such organisms to bioremediation programs. 

The (alkenyl-ethynyl)gold(i) complexes shown in Scheme 13 were prepared with mono- and ditertiary phosphine ligands and used as substrates for the coordination of coinage metal cations. The products are strongly luminescent.60... 

The highly evolved catalyst 20 combines several features that have proved successful in simpler cases. The ionic sulfonate groups make the substrate sufficiently soluble for the reaction to be run in water. (The four hydrophilic cyclodextrins perform the same service for the catalyst.) The target reaction, the seledive oxidation of the steroid skeleton, goes back to the early days of enzyme models,1711 and the choice of porphyrin and of manganese as the metal cation are based on many years experience. The aryl groups are perfluorinated because an earlier version of the catalyst suffered self-oxidation. 

Like aspartic peptidases, metallopeptidases act by activating a H20 molecule, and they do not form a covalent intermediate with the substrate. Here, the activation of a H20 molecule is mediated by a residue that acts as general base (e.g., Glu, His, Lys, Arg, or Tyr), with a divalent cation (usually Zn2+ but sometimes Co2+ or Mn2+) perhaps also contributing. The major role of the metal cation, however, is to act as an electrophilic catalyst by coordinating the carbonyl (or phosphoryl) O-atom in the substrate and orienting the latter for nucleophilic attack by the HO ion generated from H20 by the general base. 

If the dibenzoyl ketyl ion is not included in the ion-pair complex with the alkali cation, it exists in the trans form. The potassium salt exists in the cis form, in which both charged oxygen atoms are in the vicinity of the metal cation (Bauld 1965). On benzoylation, the cis ion-radical pair gives rise mainly to cw-dibenzoyl stilbene. Meanwhile, the reduction of dibenzoyl under conditions, which do not stabilize the contact ion pair, is not stereospecific (Bauld 1965, Thiele 1899). Scheme 3.49 compares the results of these two different reactions of the same substrate. 

Since these early discoveries, xylose isomerases have been isolated from many bacterial species, and these enzymes have been intense investigated, especially those of the genera Streptomyces, Lactobacillus, and Bacillus. The characteristics of substrate specificity (xylose glucose > ribose), divalent metal cation activation (Mg, Mn or Co ), and activity at alkaline pH are properties that most of the enzymes share to a certain extent, but significant variations exist. Some of these em es have been immobilized and patented for commercial use. There are many good reviews in the literature that describe the enzymatic characteristics of the xylose isomerases 9,28,29). 

Early attempts to purify the enzyme brought the quick realization that aconitase is easily inactivated (6,7). In the early 1950 s Dickman and Qoutier (8,9) found that inactivated aconitase could be reactivated by incubation with iron and a reduc-tant. From kinetic analyses of the iron and reductant effects on enzyme activity, Morrison argued that both formed Michaelis-Menten complexes wiA the enzyme (10). This refuted the earlier idea that the sole role of the reductant was to maintain iron in a reduced state (9). Of several metal cations tried, only ferrous ion was found to be capable of this reactivation process (8,11). Because of the absolute requirement for iron in activation, the known chelation properties of citrate, and Ogston s 3-point attachment proposal, Speyer and Dickman proposed that the active site iron provides three coordination sites for substrate binding - one for hydroxyl and two for carboxyl groups (12). 

The stability of the polypyridyl rhenium(I) compounds mentioned above stimulated applications of this coordination chemistry. Thus, new heterotopic bis(calix[4]arene)rhenium(I) bipyridyl receptor molecules have been prepared and shown to bind a variety of anions at the upper rim and alkali metal cations at the lower rim. A cyclodextrin dimer, which was obtained by connecting two permethylated /3-cyclodextrins with a bipy ligand, was used for the preparation of a luminescent rhenium(I) complex. The system is discussed as a model conipound to study the energy transfer between active metal centers and a bound ditopic substrate. The fluorescence behavior of rhenium(I) complexes containing functionalized bipy ligands has been applied for the recognition of glucose. ... 

Aldolases are part of a large group of enzymes called lyases and are present in all organisms. They usually catalyze the reversible stereo-specific aldol addition of a donor ketone to an acceptor aldehyde. Mechanistically, two classes of aldolases can be recognized [4] (i) type I aldolases form a Schiff-base intermediate between the donor substrate and a highly conserved lysine residue in the active site of the enzyme, and (ii) type II aldolases are dependent of a metal cation as cofactor, mainly Zn, which acts as a Lewis acid in the activation of the donor substrate (Scheme 4.1). 

Metal cations as Substrates. The Special Case of Alkali and Alkaline-earth Cations... 

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