Biochemical reactions, epoxides

Although many biochemical reactions take place in the bulk aqueous phase, there are several, catalyzed by hydroxynitrile lyases, where only the enzyme molecules close to the interface are involved in the reaction, unlike those enzyme molecules that remain idly suspended in the bulk aqueous phase [6, 50, 51]. This mechanism has no relation to the interfacial activation mechanism typical of lipases and phospholipases. Promoting biocatalysis in the interface may prove fruitful, particularly if substrates are dissolved in both aqueous phases, provided that interfacial stress is minimized. This approach was put into practice recently for the enzymatic epoxidation of styrene [52]. By binding the enzyme to the interface through conjugation of chloroperoxidase with polystyrene, a platform that protected the enzyme from interfacial stress and minimized product hydrolysis was obtained. It also allowed a significant increase in productivity, as compared to the use of free enzyme, and simultaneously allowed continuous feeding, which further enhanced productivity. 

Laser-aided interfaces find applications in fundamental studies in physical chemistry (see Section 4.2.1). However, they can also be used in the monitoring of environmental matrices as well as chemical and biochemical reactions (MA)LDI-MS is the most prominent example (Section 4.2.2). Moreover, laser beams can be introduced into canonical ion sources such as ESI to enable efficient transfer of microscale aliquots of solid or liquid matrices to the ionization zone. Along these lines, Cheng et al. [143] used electrospray-assisted laser desorption/ionization (ELDI) to monitor epoxidation of chalcone in ethanol, chelation of ethylenediaminetetraacetic acid with copper and nickel ions in aqueous solution, chelation of 1,10-phenanthroline with iron(II) in methanol, and... 

Gardner, H.W. and Kleiman, R. (1981). Degradation of linoleic acid hydroperoxide by a cysteine FeCI catalyst as a model for similar biochemical reactions. II. Specific formation of fatty acid epoxides. Biochim. Biophvs. Acta. 665. 11B-124. 

The existence of isomeric polycyclic aromatic diol epoxide compounds provides rich opportunities for attempting to correlate biological activities with the physico-chemical reaction mechanisms, and conformational and biochemical properties of the covalent DNA adduct8 which are formed. 

N. Chacos, J. Capdevilla, J. R. Falck, S. Manna, C. Martin-Wixtrom, S. S. Gill, B. D. Hammock, R. W. Estabrook, The Reaction of Arachidonic Acid Epoxides (Epoxyeico-satrienoic Acids) with a Cytosolic Epoxide Hydrolase , Arch. Biochem. Biophys. 1983, 223, 639 - 648. 

A number of oxidation reactions of mono- and difluorosteroid compounds has been reported. In some reactions, the specific influence of a fluoro substituent on the reactivity has been observed the presence of a 9a-fluorine in a 11 /i-hydroxy-A4-3-oxo steroid causes completely stcreospecific alkaline epoxidation with hydrogen peroxide in a much slower reaction (4d vs 4 h) in comparison with the nonfluorinated analog.322 Most oxidations are accomplished by the highest selective biochemical (that is, by bacterial enzymatic) transformations. As the biochemical oxidation systems are not discussed in this section, only a list of selected transformations of steroids is presented in Table 21. For additional information see ref 323. 

In natural processes, metal ions are often in high oxidation states (2 or 3), whereas in chemical systems the metals are in low oxidation states (0 or 1). This fact inverts the role of the metal center, such that it acts as a one-electron sink in a natural system, but as a nucleophile in an artificial ones (see other chapters of this book and the review by Aresta et al. [109]). Nevertheless, important biochemical processes such as the reversible enzymatic hydration of C02, or the formation of metal carbamates, may serve as natural models for many synthetic purposes. Starting from the properties of carbonic anhydrase (a zinc metalloenzyme that performs the activation of C02), Schenk et al. proposed a review [110] of perspectives to build biomimetic chemical catalysts by means of high-level DFT or ah initio calculations for both the gas phase and in the condensed state. The fixation of C02 by Zn(II) complexes to undergo the hydration of C02 (Figure 4.17) the use of Cr, Co, or Zn complexes as catalysts for the coordination-insertion reaction of C02 with epoxides and the theoretical aspects of carbamate synthesis, especially for the formation of Mg2+ and Li+ carbamates, are discussed in the review of Schenk... 

Miyamoto T., Yamamoto M., Ono A., Ohtani K. and Ando T. (1999) Substrate specificity of the epoxidation reaction in sex pheromone biosynthesis of the Japanese giant looper (Lepidoptera Geometridae). Insect Biochem. Mol. Biol. 29, 63-69. 

Another relatively simple system for the epoxidation of tri- and c/r-disubstituted olefins is formamide-hydrogen peroxide in an aqueous medium. This reagent has the advantage of being pH-independent, which makes it attractive for biochemically mediated transformations. No reaction was observed in the case of /ran.v-disubstituted and terminal olefins. With bifunctional alkenes, the more reactive double bond is selectively epoxidized [95TL4015]. 

The second type of oxygen transfer reaction is the epoxidation of olefins. This type of reaction is also rare in the plant cell biochemical factory, peroxidases of class III being inefficient catalysts. However, molecular engineered horseradish peroxidase is able to catalyze the... 

Grover, P. L., Sims, P. K-region epoxides of polycyclic hydrocarbons Reactions with nucleic acids and polyribonucleotides. Biochem. Pharmacol. 22, 661 (1973). 

Guengerich, E. P. Cytochrome P450 oxidations in the generation of reactive electrophiles epoxidation and related reactions. Arch. Biochem. Biophys. 2003,409, 59—71. 

Parathion and Paraoxon. Again, this represents a reaction (the sulfur oxidation of a thiophosphate pesticide) that is familiar to most in the pesticide area. Unlike heptachlor epoxide, paraoxon is not a stable compound and its actual presence in a poisoned animal was very difficult to demonstrate. The oxons of other organo-phosphorothioates are not so elusive. In any event, the paraoxon metabolite is an excellent example of where an understanding of metabolic processes and their potential toxicological significance alerted scientists to the likelihood that such a metabolite existed. Many years of work with similar compounds had established that the insecticidal thiophosphates required oxidation to the P=0 form in order to inhibit the neurotrasmitter acetylcholinesterase, the biochemical basis of their toxic action. Paraoxon was eventually isolated in vivo and now consideration of the oxon is a vital part of the overall risk assessment of this group of pesticides. 

Henschler, D., W.R. Hoos, FI. Fetz, E. Dallmeier, and M. Metzler (1979). Reactions of trichloroethylene epoxide in aqueous systems. Biochem. Pharmacol. 28, 543-548. 

Although hydroxylation of phenylalanine to tyrosine looks like a typical electrophilic aromatic substitution, scientists at the U.S. National Institutes of Health discovered that the biochemical pathway combines epoxidation of the benzene ring followed by epoxide ring-opening with rearrangement. This rearrangement, which is the biochemical analog of the pinacol-type reactions described earlier, is known as the NIH shift. ... 

---