Novel Synthetic Applications

Using various mediators, laccase may drive new, "green" synthetic applications in addition to those discussed in Section 2.2.1.5 [2].

Laccase-oxidized N—O^ mediators may abstract H from allylic C—H to form allylic O, which may result in the homolysis of adjacent C—O, C—H, or C—C bonds, or in the rearrangement of C=C bonds. Under laccase oxidation, benzylic or aliphatic allylic hydrocarbons may be converted to benzylic or allylic alcohol, alkene, or cyclized products [2, 145, 156, 208, 222]. Toluene, benzyl alcohol, or benzyl amine may be oxidized to benzaldehyde; N-hydroxyamine may be oxidized to oximes, nitrones, nitriles, or carboxylic acids; and propargyl alcohol may be oxidized to the corresponding carbonyl [141, 142, 145, 209-211]. Unsaturated fatty acids or lipids may be converted to hydroperoxide or epoxide derivatives amenable for further functionalization [200, 212]. Similarly, some alkenes (such as 1-(3,4-dimethoxyphenyl)-prop-1-ene) may be transformed by laccase-driven epoxida-tion, hydroxylation, or other reactions [213].

Laccase-oxidized TEMPO (oxoammonium form) may oxidize primary alcohols via an ionic oxidation mechanism, as discussed in Section 2.3.1.2 [107, 145]. This process could be of special interest for carbohydrate targets. For instance, the 6-OH of glucose may be oxidized to an aldehyde or even a carboxyl by TEMPO-mediated laccase catalysis [214, 215], a reaction potentially useful for modification/functionalization of oligo/polysaccharides (like cellulose or amylose), as well as synthesis of bioactive glycosides or other carbohydrate-based compounds [107, 216, 217].

For these biocatalytic syntheses, laccase basically serves as a generator of diffusible and active radical or cationic species, which then act on targets chemically to carry out or induce peroxygenation, hydroxylation, epoxidation, C—C cleavage, decarboxylation, C=C rearrangement, or other reactions. The mediation may impart a higher degree of regioselectivity to the laccase-catalyzed process (such as acting on primary alcohols instead of secondary alcohols). But the enantiose-lectivity, often sought after for high-value syntheses, is in general low for mediated laccase biocatalysis, although the TEMPO mediation may have a moderate stereo-preference due to the involvement of the tetrahedral transition-state intermediate (cf. Section 2.3.1.2). Future designer mediators might be able to address this shortcoming [141, 142, 145, 209]. Nevertheless, as laccases are able to use inexpensive O2 as the cosubstrate/electron acceptor, instead of expensive NAD(P) + or other cofactors, and to generate harmless H2O as the coproduct, they should be highly desirable as synthesis catalysts. TEMPO, N-hydroxyphthalimide, and other compounds are well known for their function in organic synthesis [180]. The ability of laccases to regenerate these compounds for catalytic turnover may allow the exploitation of novel mediated laccase biooxidations.

Mediated laccase oxidation has recently been applied to reoxidize/turnover fla-voenzymes, such as cellobiose dehydrogenase and pyranose-2-oxidase [138, 218], during their catalysis. Replacing the flavoenzymes' native, H2O2-producing interaction with O2, laccase-oxidized mediators may take the electrons from the reduced flavin, shuttle them to laccase, which then transfer them to O2 to produce H2O. Although the reported studies were aimed primarily at reducing the flavoen-zyme-destabilizing H2O2 production, the approach may be applied to other oxido-reductases dependent on NAD(P) +, cytochrome c, or other non-O2 cofactors. Some of the enzymes have desirable stereoselective biocatalysis, but are difficult to develop as industrial biocatalysts due to their dependence on expensive cofactors.

Heal Yourself With Qi Gong

Heal Yourself With Qi Gong

Qigong also spelled Ch'i Kung is a potent system of healing and energy medicine from China. It's the art and science of utilizing breathing methods, gentle movement, and meditation to clean, fortify, and circulate the life energy qi.

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