molecules Article Structure Determination of Novel Oxidation Products from Epicatechin: Thearubigin-Like Molecules Kazuhiro Uchida, Kazuki Ogawa and Emiko Yanase * Faculty of Applied Biological Sciences, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan; [email protected] (K.U.); [email protected] (K.O.) * Correspondence: [email protected]; Tel.: +80-58-234-2914 Academic Editor: Derek J. McPhee Received: 29 January 2016 ; Accepted: 23 February 2016 ; Published: 26 February 2016 Abstract: Following the oxidation of epicatechin (EC), three novel compounds and two known compounds were isolated. The chemical structures of these oxidation products were determined by mass spectrometry (MS) and various nuclear magnetic resonance (NMR) experiments, and the A-ring–B-ring linkage that is characteristic of catechin was found in each molecule. Three compounds showed similar ultraviolet–visible (UV-Vis) spectra to EC, whereas two compounds showed different spectral absorption in the region between 300 and 500 nm. A similar spectrum was obtained for the thearubigin fraction prepared from a black tea infusion. This result suggests that the condensation reaction between the A-ring and B-ring is more important than reaction between B-rings for thearubigin formation. Keywords: epicatechin; oxidation; thearubigin; theaflavine; polymeric polyphenol; black tea 1. Introduction Black tea is a globally popular beverage that has attracted attention because of its health benefits, including antioxidative and anticarcinogenic activities. Tea can be classified by differences in processing methods, for example, black tea is a fermented tea. The term “fermented” refers to natural browning reactions induced by oxidative enzymes of the fresh tea leaves. Catechins are one of the components in tea that are drastically changed in this process, and dimers and polymers are generated. The polymer found in black tea is called thearubigin. The name thearubigin was given to an ill-defined group of substances by Roberts in 1962. Thearubigin has a rust-brown color and high water solubility. Many researchers have studied the chemical structure of thearubigin, and the average molecular weight was estimated to be approximately 5000 to 30,000. Thearubigin constitutes approximately 10%–20% of the dry weight of black tea leaves; however, detailed chemical studies have thus far been limited because the numerous components found in the extract leads to a complicated mixture, which makes preparative scale isolation difficult [1–4]. Theaflavins [5–8] are one of the pigments in black tea that have a familiar bright red tone and various bioactivities [9,10]. In the synthesis of theaflavin from epicatechin (1, EC) and epigallocatechin, the synthetic yield increased when an excessive amount of 1 was used for this reaction. However, the recovery of 1 was low after the reaction. This result suggested that another reaction could occur between catechol-type catechins. We consider reactions between catechol-type catechins likely to occur during the fermentation process of black tea and thus be involved in thearubigin generation. Therefore, it is important to clarify the chemical structures of the byproducts from the synthesis of theaflavin. In this study, to increase knowledge of the chemical structure of thearubigin, we investigated the oxidation reaction of 1. Structure determination of the three novel and two known compounds Molecules 2016, 21, 273; doi:10.3390/molecules21030273 www.mdpi.com/journal/molecules Molecules 2016, 21, 273 2 of 9 MoleculesIn 2016this, 21study,, 273 to increase knowledge of the chemical structure of thearubigin, we investigated2 of 9 the oxidation reaction of 1. Structure determination of the three novel and two known compounds isolated showed the importance of condensation reactions between catechin A- and B-rings for isolated showed the importance of condensation reactions between catechin A- and B-rings for thearubigin formation. thearubigin formation. 2. Results and Discussion 2. Results and Discussion Oxidation of 1 was performed using copper chloride, and the reaction mixture was analyzed Oxidation of 1 was performed using copper chloride, and the reaction mixture was analyzed using using high-performance liquid chromatography with ultraviolet detection (HPLC-UV) (Figure 1). As high-performance liquid chromatography with ultraviolet detection (HPLC-UV) (Figure1). As a result, a result, 1 and five unknown peaks, which were determined to be two dimers (peak c and d) and 1 and five unknown peaks, which were determined to be two dimers (peak c and d) and three trimers three trimers (peak a, b and e), were observed. The yield of each peak depended on the reaction (peak a, b and e), were observed. The yield of each peak depended on the reaction conditions, oxidizing conditions, oxidizing agent, reagent equivalents, reaction time, etc. For isolation and structure agent, reagent equivalents, reaction time, etc. For isolation and structure determination of each peak, determination of each peak, the reaction was performed under optimum conditions for that peak. the reaction was performed under optimum conditions for that peak. The reaction solution was The reaction solution was extracted sequentially with ether and ethyl acetate. According to the HPLC extracted sequentially with ether and ethyl acetate. According to the HPLC analysis of each layer, analysis of each layer, the water layer contained peaks a and b, whereas the ethyl acetate layer the water layer contained peaks a and b, whereas the ethyl acetate layer contained peaks c, d, and e. contained peaks c, d, and e. Unreacted 1 was extracted into the ether layer. To collect peaks a and b, Unreacted 1 was extracted into the ether layer. To collect peaks a and b, the water layer was loaded into the water layer was loaded into a column packed with Diaion HP20SS particles and eluted with a column packed with Diaion HP20SS particles and eluted with MeOH. Each peak was separated using MeOH. Each peak was separated using preparative HPLC and characterized using spectroscopic preparative HPLC and characterized using spectroscopic techniques, including 1D and 2D nuclear techniques, including 1D and 2D nuclear magnetic resonance (NMR) and mass spectrometry. magnetic resonance (NMR) and mass spectrometry. FigureFigure 1.1.HPLC HPLC chromatogramchromatogram ofofthe thereaction reactionmixture mixture obtained obtainedfollowing followingoxidation oxidation ofofepicatechin epicatechin (EC)(EC) 2 1 (monitored(monitored atat 280280 nm). Peaks Peaks were were shown shown as as a–d. a–d. a: a:dehydrotricatechin dehydrotricatechin B ( B5),2 b:(5 ),dehydrotricatechin b: dehydrotricatechin B (6), 1 1 Bc1: epicatechin-6(6), c: epicatechin-6′,6-dimer (,6-dimer2), d: epicatechin-6 (2), d: epicatechin-6′,8-dimer (3), e:,8-dimer dehydrotricatechin (3), e: dehydrotricatechin A (4), EC: epicatechin A ( (14).), EC: epicatechin (1). 2.1. Structure Determination for Peaks c and d 2.1. StructureNegative Determination mode electrospray for Peaks io cnization and d mass spectrometry (ESIMS) measurements of peaks c and Negatived both showed mode electrospray a pseudo-molecular ionization massion peak spectrometry at m/z 577 (ESIMS) ([M − measurements H]⁻), suggesting of peaks that cthese and 1 dcompounds both showed are a pseudo-moleculardimers of 1. The ionH-NMR peak atspectram/z 577 of peaks ([M ´ H]c and´), suggestingd were similar that theseand indicated compounds the arepresence dimers of of 1two. The EC1H-NMR units that spectra had oflost peaks onec andaromdaticwere proton similar from and indicatedboth the theA-ring presence and ofB-ring. two ECFurthermore, units that had the lost 1H- one and aromatic 13C-NMR proton spectral from signals both the were A-ring assigned and B-ring. using Furthermore, correlation spectroscopy the 1H- and 13(COSY),C-NMR spectralheteronuclear signals multiple were assigned quantum using cohere correlationnce (HMQC), spectroscopy and (COSY), heteronuclear heteronuclear multiple-bond multiple quantumcorrelation coherence (HMBC). (HMQC), As a result, and the heteronuclear chemical structure multiple-bond of peak correlation c was deduced (HMBC). to be As the a result, EC dimer the chemicalwith a connection structure of between peak c was the deduced6-position to beof the ECA-ring dimer in withone amolecule connection and between the 2′-position the 6-position of the ofB-ring the A-ring in the in other one molecule(compound and 2 the). On 21 -positionthe other of hand, the B-ring peak ind was the otherfound (compound to be the EC2). dimer On the with other a hand,connection peak d between was found the to 8-position be the EC of dimer the A-ring with a connectionand the 2′-position between theof the 8-position B-ring (compound of the A-ring 3 and), as Molecules 2016, 21, 273 3 of 9 Molecules 2016, 21, 273 3 of 9 the 21-position of the B-ring (compound 3), as shown in Figure2. Compounds 2 and 3 have with been previouslyshown in Figure reported 2. Compounds as the products 2 and of 3 radical have with oxidation been previously of 1, and the repo spectralrted as data the products were in agreement of radical publishedoxidation of data 1, and [11 ].the As spectral a result, data compounds were in agreement2 and 3 were published identified data as [11]. epicatechin-6 As a result,1,6-dimer compounds and epicatechin-62 and 3 were 1identified,8-dimer. as epicatechin-6′,6-dimer and epicatechin-6′,8-dimer. Figure 2. Chemical structures of compounds 1–6. 2.2. Structure Determination for Peak e Negative mode mode ESIMS ESIMS measurements measurements of of peak peak e eshowed showed a pseudo-molecular a pseudo-molecular ion ion peak peak at m at/zm 863/z 863([M ([M− H]´⁻),H] suggesting´), suggesting that this that compound this compound is a trimer is a trimer of 1. The of 1 .mass The massnumber number was 2 was Da 2smaller Da smaller than thanthe expected the expected molecular molecular weight weight calculated calculated for a simple for a EC simple trimer EC with trimer connections with connections between betweenEC units ECsimilar units to similarthose in to dimers those 2 in and dimers 3.