EDTA cleavage

The presence of ascorbic acid as a co-substrate enhanced the rate of the Ru(EDTA)-catalyzed autoxidation in the order cyclohexane < cyclohexanol < cyclohexene (148). The reactions were always first-order in [H2A]. It was concluded that these reactions occur via a Ru(EDTA)(H2A)(S)(02) adduct, in which ascorbic acid promotes the cleavage of the 02 unit and, as a consequence, O-transfer to the substrate. While the model seems to be consistent with the experimental observations, it leaves open some very intriguing questions. According to earlier results from the same laboratory (24,25), the Ru(EDTA) catalyzed autoxidation of ascorbic acid occurs at a comparable or even a faster rate than the reactions listed in Table III. It follows, that the interference from this side reaction should not be neglected in the detailed kinetic model, in particular because ascorbic acid may be completely consumed before the oxidation of the other substrate takes place. 

This means that either /r 2 or k is 0.21 M order redox process which (1.117) represents, but is very unlikely for k 2 since formation of the bridged adduct must involve Co "-0 bond cleavage in Co(edta) and such a process would be expected to be much slower (Ch. 4). For this, and other reasons mechanism (b) is strongly preferred. 

The intron group I ribozymes feature common secondary structure and reaction pathways. Active sites capable of catalyzing consecutive phosphodi-ester reactions produce properly spliced and circular RNAs. Ribozymes fold into a globular conformation and have solvent-inaccessible cores as quantified by Fe(II)-EDTA-induced free-radical cleavage experiments. The Tetrahy-mem group I intron ribozyme catalyzes phosphoryl transfer between guanosine and a substrate RNA strand—the exon. This ribozyme also has been proposed to use metal ions to assist in proper folding, to activate the nucleophile, and to stabilize the transition state. ... 

Such a species cannot be ruled out in reactions of iron-EDTA complexes with hydroperoxides recently described by Bruice and coworkers (27). On the other hand, a hydroperoxide complex that reacts with the substrate such that bond formation fiom O to substrate is concerted with 0-0 bond breaking, as proposed by Klinman for dopamine P-monooxygenase (18), could provide compensation for the cost of 0-0 bond cleavage in the transition state. In fact, it is interesting to speculate that for each of these enzymes, the mechanism by which the substrate is oxidized may be dependent on the reactivity of the substrate. One could envision certain substrates that would react with the metal-bound hydroperoxide ligand prior to or concerted with 0-0 bond cleavage. This possibility is difficult to assess because of our lack of information concerning the reactivity of HQ2" when complexed to different metal ions. 

Fig. 14. Initial interval of cleavage of HPMA copolymer based polymeric substrates by lysosomal cysteine proteinase cathepsin B (isolated from bovine spleen). Only the cleavage of the bond between the distal amino acid residue and p-nitroaniline was monitored. Conditions of cleavage [Cathepsin B] = 1.9 x 10 7 M [NAp] = 1.2 x 1(T3 M [EDTA] = 1 x 10 3 M [Cys] = 2.5 x 10 2 M 0.1 M phosphate buffer pH = 6.0 40 °C. Data from [249]...

One critical exception is the sapphyrin-ethylenediaminetetraacetic (EDTA) conjugate (23). Previous studies of small molecule-EDTA conjugates have shown that these species, in the presence of iron(II), Oj, and a suitable reducing agent, can effect the cleavage of the DNA backbone. In turn, so-called affinity cleavage can be used to elucidate the specificity of the small molecule-DNA interactions. It was thus considered worth making 23. ... 

Fig. 19.7 Sensitization of nanosized Ti02 films in water dissolving Ru(bpy)32+ and EDTA. a sacrificial reducing agent, as a trial for future visible light cleavage of water.

Citrate lyase catalyzes the cleavage of citrate to oxaloacetate and acetate in the presence of Mg2+ or Mn2+, but in the presence of EDTA it catalyzes its synthesis. The enzyme is a complex of three subunits. The y-subunit functions as an acyl carrier protein (ACP). The a-subunit is an acyl transferase involved in citryl-ACP formation and the release of acetate, and the /8-subunit catalyzes the cleavage of the citryl-ACP intermediate to oxaloacetate and acetyl-ACP. The enzyme from Klebsiella aerogenes has been purified, and binds 18 Mn2"1 in a cooperative manner. 

The Fenton reaction may also be used site-specifically, e.g. for the sequence-specific cleavage of DNA with the help of benzopyridoindole-EDTA intercalator forming triple-helical structures (Marchand et al. 2000). 

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