Peroxidase-catalyzed dimerization

The use of europium chelates, with their unusually long fluorescence decay times, as labels for proteins and antibodies has provided techniques that are referred to as time-resolved fluoroimmunoassays (TRFIA). Fluorophores as labels for biomolecules will be the topic of Sect. 3. Nevertheless, TRFIAs always have to compete with ELISA (enzyme-linked immunosorbent assays) techniques, which are characterized by their great versatility and sensitivity through an enzyme-driven signal amplification. Numerous studies have been published over the past two decades which compare both analytical methods, e.g., with respect to the detection of influenza viruses or HIV-1 specific IgA antibodies [117,118]. Lanthanide luminescence detection is another new development, and Tb(III) complexes have been applied, for instance, as indicators for peroxidase-catalyzed dimerization products in ELISAs [119]. 

Hordatines (XXXIII) are antifungal compounds, which are synthesized in barley Hordeum vulgare) in response to powdery mildew infection [119]. These compounds are the result of the peroxidase-catalyzed dimerization of p-coumaryl-agmatine (XXXIV) (Scheme XV) and p-coumaryl-hydroxyagmatine. 

Chemical ionization mass spectrometric detection has been explored for the detection of methyl hydroperoxides However, fluorometry has dominated the current detection schemes for the organic peroxides. Typically, a nonfluorescent substrate is oxidized by the peroxide to generate a fluorescent product. These methods are sufficiently sensitive for accurate measurement of the peroxides in the low ppt by volume. For example, the peroxidase-catalyzed dimerization of p-hydroxyphenylacetic acid (POPHA) occurs in the presence of a peroxy group at elevated pH. The formation of the fluorescent dimer, detected by excitation at 310 nm and emission at 405 nm, is proportional to the concentration of the peroxide. The most common peroxidase catalyst used for this reaction is horseradish peroxidase (HRP). Cost and stability issues with the use of HRP led to the use of other catalysts, such as metalloporphyrins or phthalocyanine complexes. Another fluorescent reaction scheme involves the oxidation of the nonfluorescent thiamine (vitamin Bi) to the fluorescent thiochrome by the peroxide group. This reaction is catalyzed by bovine hematin. This reaction is 25-fold more sensitive for H2O2 than for the organic peroxides. 

Peroxidases have been used very frequently during the last ten years as biocatalysts in asymmetric synthesis. The transformation of a broad spectrum of substrates by these enzymes leads to valuable compounds for the asymmetric synthesis of natural products and biologically active molecules. Peroxidases catalyze regioselective hydroxylation of phenols and halogenation of olefins. Furthermore, they catalyze the epoxidation of olefins and the sulfoxidation of alkyl aryl sulfides in high enantioselectivities, as well as the asymmetric reduction of racemic hydroperoxides. The less selective oxidative coupHng of various phenols and aromatic amines by peroxidases provides a convenient access to dimeric, oligomeric and polymeric products for industrial applications. 

Yu, J., K. E. Taylor, H. Zou, N. Biswas, and J. K. Bewtra, Phenol conversion and dimeric intermediates in horseradish peroxidase-catalyzed phenol removal from water , Environ. Sci. Technol., 28, 2154-2160 (1994). 

Perhaps the most well-known peroxidase-catalyzed reactions are those involving electron transfer, in which an aromatic substrate is oxidized in a mono-electronic oxidation up to its mono-radical, Eq. (1), which is capable of participating further in a variety of non-enzymatic reactions such as disproportionation, polymerization and electron transfer. These types of reactions are very common during the peroxidase-catalyzed oxidation of phenols and, in some cases, during the oxidation of alkaloids. For example, peroxidase is capable of dimerizing jatrorrhizine (IV) to 4,4 -bis-jatrorrhizine (V) in the presence of H2O2 (Scheme III) [50]. 

Catharanthine (LIV) and vindoline (Lin) are regarded as the monomeric precursors of the dimeric alkaloids vinblastine and vincristine, via a-3 ,4 -anhydrovinblastine. C. roseus peroxidase catalyzes the coupling reaction of catharanthine and vindoline (Scheme XXVI) to lead to a-3 ,4 -anhydrovinblastine (XLVH) or, more properly, to an iminium intermediate (LVI) from which a-3 ,4 -anhydrovinblastine is directly derivated [52,74,166]. a-3 ,4 -Anhydrovinblastine is then converted to vinblastine (XLIX, R = CH3) and vincristine (XLIX, R = CHO) in C. roseus plants [167-169], a-3 ,4 -Anhydrovinblastine (XLVn), or the unstable iminium intermediate (LVI) formed during the coupling reaction, is then assumed to be the precursor of all dimeric alkaloids in C. roseus. 

Thus, peroxidases may potentially effect direct free radical coupling reactions between aniline and humic substances or create additional substrate sites within the fiilvic or humic acid molecules for nucleophilic addition by aniline. A model for the latter pathway can be found in the work with guaiacol and 4-chloroaniline by Simmons et al. (19). Peroxidase catalyzed the coupling of guaiacol, itself not a substrate for nucleophilic addition, into the extended quinonoid dimer which subsequently underwent nucleophilic attack by the chloroaniline ... 

Typically, peroxidase-catalyzed polymerization of phenol is carried out in the presence of H2O2, which acts as an oxidizing agent. The free radicals of monomers (substrates) formed initially undergo coupling to produce dimers, and successive oxidation and coupling eventually results in the formation of polymers. The peroxidase-catalyzed polymerization of phenols and substituted phenols usually produce the polymer with complicated structures. The main structure was estimated to be of phenylene units or a mixture of phenylene and oxyphenylene units (5). 

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