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    • Tanaka and his team were also

    2018-10-29

    Tanaka and his team were also focused on the chemical examination of theasinensin production using in vitro experiments to mimic black tea production process in particular on the fermentation step. EGCG and 5-lipoxygenase from homogeneous pear were applied at room temperature for 2h. The study elucidated the structure of dehydrotheasinensin A and mechanism of theasinensin formation, and successfully obtained dehydrotheasinensin A, a theasinensin precursor from enzymatic oxidation of EGCG. The results also revealed that dehydrotheasinensin A produced from coupling of EGCG quinone monomers is equivalent to a hydrated ortho-quinone of the theasinensin A, which is readily converted to theasinensin A and D by oxidation reduction dismutation. Previous studies from Takana et al. [35,37] suggested that several chemical reactions involved in EGCG oxidation subsequently produced theasinensins (A and D). EGCG was initially oxidized to the ortho-quinone, and subsequently an unstable dimer, theasinensin quinone was formed after stereoselective dimerization (Fig. 2). This unique quinone was equivalent to dehydrotheasinensin A, generated by both enzymatic and non-enzymatic reaction [28,33,35,37,43,45,46]. To further understand the chemical mechanism of theasinensin formation, many studies had concentrated on the enzymatic oxidation of catechins such as EGCG oxidation. However, several studies also focused on the non-enzymatic oxidation of catechins. The non-enzymatic oxidation of EGCG providing theasinensin quinones can be developed from not only chemical oxidation with either potassium ferricyanide or copper salts, but also autoxidation in a phosphate buffer (pH 7.4 or 6.8) in which the oxidized product is unstable even at a neutral pH [37]. Therefore, it can be implied that the production of theasinensin A and D derived from dehydrotheasinensin A during black tea and oolong tea manufacturing may possibly be a non-enzymatic process which occurs spontaneously when tea leaves are heated and dried. Shii further showed that non-enzymatic oxidation of EGCG by copper salts with the optimum pH of 4–5 at room temperature yielded dehydrotheasinensin A and the product increased with the elevated temperature [43]. Initial oxidation reaction was carried out at room temperature and elevated temperature increased byproduct generation. The difference between enzymatic oxidation in tea leaves and in vitro chemical oxidation by copper salt is that catechol-type catechins prefer to enzyme catalyzed oxidation, such as EC and ECG, whereas pyrogallol-type catechins, such as EGC and EGCG, are less reactive [43,45]. By enzymatic oxidation (banana homogenate) of catechin, EGC was oxidized to the corresponding catechin quinone but the reaction was accelerated in presence of ECG. The oxidation of ECG was much faster than EGC due to its higher redox potential than pyrogallol-type catechin (EGCG, EGC) [35,43]. Technically, the catechol-type catechins were not chemically oxidized by cupper salt. Therefore, the chemical oxidation of catechins does not yield theaflavins which are generated from the oxidative couplings between the quinones produced from catechol-type and pyrogallol-type catechins [47–49]. Enzymatic oxidation of EGC generates a new quinone dimer with a hydrated cyclohexenetrione structure, which may be equivalent to dehydrotheasinensin C whereas theasinensins C and E were produced through oxidation–reduction dismutation [39]. In addition, the enzymes isolated from pear homogenates catalyzed the oxidation of EGC at the pyrogallol B-ring, yielding predominantly unstable quinone products known as dehydrotheasinensin C trapped with o-phenylenediamine [45]. Non-enzymatic coupling reaction of dehydrotheasinensin C was subsequently occurred to give theasinensins A and C [37,38,45,46]. From commercial back tea, an interesting derivative of theasinensin, N-ethylpyrrolidinonyl theasinensin has been isolated and identified [44,50]. Tea leaves contain an amino acid named -theanine (N-ethy--glutamine) accounting for over 50% of the total amino acids. Consequently, the catechin quinones generated from the beginning of tea process maybe react with -theanine to generate theanine Strecker aldehyde [51,52] as demonstrated in Fig. 3. The Strecker aldehyde of theanine subsequently reacts with theasinensin to yield N-ethylpyrrolidinonyl theasinensin.