Abstract

Objectives. Tannins with complex structures are important plant resources, which are abundant in the genus Terminalia. Various Terminalia species have been playing an important role in traditional medicine system. A systematic scoping review of Terminalia Linn. research literature for tannins was conducted to summarize the structures of tannins and analysis fragmentation pathway characteristics, which could provide references for the structural analysis of tannins from Terminalia Linn. Methods. After an update of the literature search up to September 2018, the terms of Terminalia in all publications were analyzed. Electronic searches were conducted in scifinder and PubMed, and the information from 197 articles in all with regard to the tannin structure study was extracted. Results. The compounds of 82 tannins from the genus Terminalia were reviewed. According to the structural differences, they can be divided into three categories, hydrolysable tannins, condensed tannins, and complex tannins, respectively. The fragmentation pathways of 46 identified tannins were analyzed, and the fragmentation rules of tannins were speculated according to different types. Conclusion. This review has attracted attention to the active substances in this species such as the tannins summarized in further study. How to improve the extraction and purification technology of tannins from genus Terminalia is an urgent problem to be solved.

1. Introduction

Plants of the genus Terminalia (family Combretaceae) are widely used in traditional medicine all over the world [1]. There are about 250 Terminalia species, of which at least 50 are used as food [2]. Many species have biological activities including antitumor, anti-inflammatory, wound healing, antifungal, antibacterial, and antiviral activities [3–7]. In particular, Terminalia chebula, an Indian species, is well noted as the king of plants in Ayurveda for its extensive medicinal use [8]. The plants mainly include tannins, polyphenols, triterpenoids, flavonoids, aliphatic compounds, and other active ingredients, among which tannins and polyphenols are the main constituents [9].

Tannins are a kind of polyphenolic compounds with complex structures in plants. They are classified into three groups on the basis of their structures: hydrolysable tannins, condensed tannins, and complex tannins. Usually, their molecular weights are greater than 500 Da. Tannins are widely distributed in various plants, and they are considered defensive molecules to protect plant tissues from herbivorous attacks because of their astringent taste [10]. It has been reported that several natural tannins and related compounds have various biological activities, including antioxidant, antitumor, hypolipidemic, hypoglycemic, and antibacterial activities [11–14]. Takashi Tananka isolated terflavin A and B, tercatain, and tergallagin from the leaves of Terminalia catappa Linn. in 1986 [15]. Since then, more than 82 tannins have been isolated from the fruits, barks, leaves, and galls in the plants of the genus Terminalia. The mass spectrometric data of these tannins and the structure analysis of the compounds are discussed. This review aims to provide references for the structure identification of tannin constituents in the plants of Terminalia Linn. In the further study of phytochemistry, the research field of medicinal activity of this important genus should be highlighted and guided.

2. Methods

2.1. Data Sources and Searches

Electronic searches were conducted in scifinder and PubMed for articles up to September 2018, using terms related to tannins, Terminalia, and MS. Searches were conducted with no date or language restrictions.

2.2. Eligibility and Selection

The titles and abstracts of 197 articles were screened, respectively, and the full text of the article was reviewed to obtain sufficient information. Any disagreements regarding the inclusion of articles were resolved through discussion and consensus.

2.3. Data Extraction

The final data extraction included the following five categories: (1) general characteristics (compound name, source, structure, and journal name); (2) MS data (compound name, ion Source, ion mode, fragments, and journal name); and (3) MS fragmentation pattern (fragmentation rules and journal name).

3. Results and Discussions

3.1. Tannins

Tannins are widely distributed in plants. They can be classified into three types according to their structural differences. Hydrolysable tannins are a group of compounds formed by phenolic acids and their derivatives through glycoside bonds or ester bonds with glucose or polyols. They are further divided into gallotannins containing only galloyl groups, ellagitannins containing hexahydroxydiphenoyl group(s), and hydrolysable tannin oligomers divided into dimers, trimers, and tetramers according to the number of glucose nuclei [16]. Condensed tannins are a class of compounds formed by the carbon-carbon bond polymerization of flavane-3-ol such as catechins or their derivative gallocatechin. Complex tannins are a class of compounds composed of flavane-3-ol, the unit of condensed tannins, and hydrolyzed tannins, which are partially linked by carbon-carbon bonds.

On the basis of the structural differences, they are divided into different types. Compounds 1–74 are hydrolysable tannins. Among them, compounds 1–11 (Figure 1) having only galloyl groups are gallotannins and compounds 12–71 (Figure 2) having hexahydroxydiphenoyl group(s) are ellagitannins. In addition, compounds 72 and 73 (Figure 3) possess two glucose nuclei, and compound 74 (Figure 3) possesses three glucose nuclei. Therefore, they are thought to be hydrolysable tannin dimers and hydrolysable tannin trimers, respectively. Meanwhile, compounds 75–79 (Figure 4) are condensed tannins. Compound 79 is further classified into condensed tannin trimers, and the others are condensed tannin dimers. Compounds 80–82 (Figure 5) possess the unit of condensed tannins and the unit of hydrolyzed tannins which are thought to be complex tannins. The names, corresponding plant resources, and related references of the compounds have been listed in Tables 1–5.


Figure 1 
Structures of compounds 1–11.



Figure 2 
Structures of compounds 12–71.

Figure 3 
Structures of compounds 72–74.

Figure 4 
Structures of compounds 75–79.

Figure 5 
Structures of compounds 80–82.
Table 1 
Gallotannins 1–11 in Figure 1.
Table 2 
Ellagitannins 12–71 in Figure 2.
Table 3 
Hydrolysable tannin polymers 72–74 in Figure 3.
Table 4 
Condensed tannins 75–79 in Figure 4.
Table 5 
Complex tannins 80–82 in Figure 5.
3.2. MS Data of Tannins

The MS data of the tannins from the genus Terminalia (family Combretaceae) are shown in Table 6 as summarized. According to the compiled MS data, this review provides a useful and fast way for the identification of tannins.

Table 6 
The MS spectral data of compounds 1–82 except those which have no reported MS data.
3.3. Fragmentation Pattern
3.3.1. Gallotannins

Most gallotannins produce major fragment ions [M-H-170]− and [M-H-152]−, which indicate the loss of gallic acid and galloyl residue. In addition, other fragment ions such as [M-H-170]−, [M-H-170-152]−, and [M-H-170-152-152]− are produced owing to the sequential losses of galloyl group and gallic acid.

Compound 7 (Figure 6) gave the [M-H]− ions at m/z 787 and displayed a fragmentation pattern similar to the successive neutral losses of gallic acids (170 Da) and galloyl radicals (152 Da). Due to the limited mass spectrometry information, it was difficult to distinguish the link position between galloyl groups and glucosyl unit [96].


Figure 6 
Fragmentation of compound 7.

Compound 8 (Figure 7) is characterized by fragment ions at m/z 635, corresponding to the loss of a galloyl residue ([M-H-152]−) and at m/z 617 owing to the loss of a gallic acid group ([M-H-170]−) [97].


Figure 7 
Fragmentation of compound 8.

Compound 11 (Figure 8) with the [M-H]− ion at m/z 939 and m/z 469 [M-2H]2−, showed typical fragments at m/z 769 [M-H-170]−, m/z 617 [M-H-170-152]−, m/z 465 [M-H-170-152-152]−, and m/z 313 [M-H-170-152-152-152]− which corresponded to the sequential losses of galloyl group and gallic acid [108].


Figure 8 
Fragmentation of compound 11.
3.3.2. Ellagitannins

Most ellagitannins produce major fragment ions [M-H-170]−, [M-H-170-162]−, and [M-H-302]−, which indicate the loss of gallic acid, galloylglucose group, and HHDP group. In addition, other fragment ions such as 151, 169, and 301 confirm the existence of galloyl group, gallic acid, and HHDP group, respectively.

Compound 18 (Figure 9) presented [M-H]− at m/z 633.0762 and MS2 fragments at m/z 463.0793 [M-H-152-H2O]−, which is consistent with sequential losses of galloyl and H2O and at m/z 300.9986 [M-H-152-180]− owing to the loss of a galloyl unit with a hexose [117].


Figure 9 
Fragmentation of compound 18.

Compound 37 (Figure 10) displayed [M-H]− at m/z 933 and MS2 ion at m/z 631 resulting from the loss of HHDP and presented MS3 ions at m/z 451 owing to the loss of glucosyl moiety and at m/z 301 which corresponded to the loss of galloyl-glucosyl moiety from the parent MS2 ion at m/z 631 [144].


Figure 10 
Fragmentation of compound 37.

Compound 39 (Figure 11) had an [M-H]− ion at m/z 933 and three mass fragments: one at 601 ([M-H-332]−) which corresponded to the loss of a galloylglucose unit and two others at m/z 781 ([M-H-152]−) which corresponded to the presence of a galloyl group and at m/z 721 after the cross-ring fragmentation of glucose ([M-H-152-60]−) [124].


Figure 11 
Fragmentation of compound 39.

Compound 42 (Figure 12) displayed molecular anions at m/z 935 and produced fragments at m/z 633 ([M-H-302]−), corresponding to the loss of an HHDP group and at m/z 301 ([M-H-634]−), indicating the presence of HHDP (302 Da), gallic acid (170 Da), and glucosyl (162 Da) groups [129].


Figure 12 
Fragmentation of compound 42.

Compounds 62 and 63 (Figure 13) are isomers, which had the same fragmentation behaviors, presented a same [M-H]− ion at m/z 1083, and further produced ions at m/z 781 ([M-H-302]−), m/z 601 ([M-H-302-180]−), and m/z 301, demonstrating the existence of HHDP and gallagic acid groups [165].


Figure 13 
Fragmentation of compounds 62 and 63.
3.3.3. Condensed Tannins

Structurally significant product ions were produced by cleavages between monomeric subunits, which contain quinone methide (QM), heterocyclic ring fission (HRF), and retro-Diels–Alder (RDA) fragment ions.

QM fragmentation cleaves the single bond between subunits in B-type procyanidins to form a single quinone resulting in two possible product ions.

A second important structural fragmentation pathway for deprotonated procyanidins is heterocyclic ring fission (HRF), which results in the elimination of 1,3,5-trihydroxybenzene ([M-H-126]−).

Retro-Diels–Alder (RDA) fragmentation was distinguished by elimination of hydroxyvinyl benzenediol ([M-H-152]−), an extra water molecule ([M-H-152-18]−) simultaneously.

The dimeric procyanidins occur as the B-type procyanidins in nature, which contain four major isomers such as B1, B2, B3, and B4. We have sorted out compounds 75–77 which presented in Terminalia Linn. The three compounds presented the specific fragments of m/z 425 and 407, which corresponded to the characteristic fragmentations of procyanidin B-type dimmers [166].

Compound 76 (Figure 14) presented an [M-H]− ion at m/z 577, with fragment ions at 425 ([M-H-152]−), originated from Retro Diels–Alder (RDA) fragmentation of the heterocyclic ring. The fragment at m/z 407 ([M-H-170]−) resulted from both RDA rearrangement and loss of water molecule [155].

(a)
(a)
(b)
(b)
(c)
(c)
Figure 14 
Fragmentation of compound 76. (a) QM, (b) RDA, and (c) HRF.

Compound 77 had an [M-H]− ion at m/z 577 which presented a Retro-Diels–Alder (RDA) product with a neutral loss of 152 ([M-H-152]−) and subsequently loss of a water molecule [M-H-152-18]− [158].

Compound 79 (Figure 15) gave the [M-H]− ion at m/z 865 and showed fragment ions at m/z 287/577 and m/z 575/289 due to QM fragmentation. The fragment at m/z 713/695 corresponded to RDA fragmentation and at m/z 425/407 owing to RDA fragmentation of the QM product ion of m/z 577. It also formed ions of m/z 739, m/z 451, and m/z 413 through HRF fragmentation [155].

(a)
(a)
(b)
(b)
(c)
(c)
Figure 15 
Fragmentation of compound 79. (a) QM, (b) RDA, and (c) HRF.
3.3.4. Complex Tannins

Compound 81 (Figure 16) had an [M-H]− ion at m/z 1205 and other fragments at m/z 915, due to the loss of the substituent at C-1 of the vescalagin-derived nuclei structure and at m/z 613 resulting from the loss of the 4,6-hexahydroxybiphenoyl unit from the latter fragment and at m/z 301 which corresponded to the existence of ellagic acid [143].


Figure 16 
Fragmentation of compound 81.

4. Biological Activity

Natural compounds are important sources of drugs. More and more attention has been paid to the scientific investigation of natural bioactive compounds which may yield new compounds or leading compounds that can overcome the limitations of currently used drugs. At present, some achievements have been made in the study of tannins, but there are still some deficiencies. Tannins extracted from plants are often a collection of monomers of different kinds of tannins mentioned above. Their bioactivities are closely related to the action of these tannin monomers which need further studies. The reported biological activity of these tannins from the genus Terminalia (Family Combretaceae) was summarized briefly.

4.1. Antioxidant Activity

Ellagitannins such as compounds 18, 48, and 70 were found to be the major components in Terminalia bellirica, which exhibited the antioxidant and hepatoprotective activities [167]. Compounds 11, 20, 30, 33, and 43 exhibited great antioxidant activity in both chemical-based and cellular-based antioxidant assays, and compound 11 showed the highest cellular antioxidant activity [168]. Compound 11 has the highest potency for DPPH-, NO-, and ONOO-scavenging activity with IC50 ranging from 5 to 20 μM, 0.20, and 0.06 μM, respectively [169]. Compounds 33 and 43 showed the highest increase in GSH, and compound 30 produced the highest increase in SOD among four tannins [170]. Compounds 28 and 62 had in vitro antioxidant activity and in vivo antioxidative stress effects [171]. A lot of research showed that antioxidant compounds are related to a variety of oxidative stress-related diseases, such as cardiovascular diseases, neurodegenerative diseases, and cancer [172].

4.2. Anticancer Activity

It was confirmed that compound 18 could induce autophagy, apoptosis and ROS accumulation in gastric cancer cells in vitro [173]. IC50 values of HepG2, Molt-3, HL-60, NPC-BM1, HT 1080 and K562 were 1.42, 0.35, 0.12, 0.81, 1.02, 1.53 mg/mL in vivo, respectively [174]. A molecular mechanism study showed that the inhibition of the proliferation of ovarian cancer cells by compound 18 is mediated by blocking the TGF-beta/AKT/ERK/Smad signaling pathway [175]. Compound 11 could induce autophagy of HepG2, MCF-7, and A549 by activating MAPK 8/9/10 and JNK signaling pathways [176]. Compound 11 could also enhance GNMT promoter activity by downregulating MYC expression in hepatocellular carcinoma [177]. Compounds 70, 62, 63, 42, and 19 were isolated from Terminalia calamansanoi with the IC50 values of 65.2, 74.8, 42.2, 38.0, and >100 μM, respectively, for HL-60 cells [178]. It was confirmed that protective effects of compound 20 against DNA damage are induced by different mutagens [179]. The chemopreventive effect of compound 62/63 on H-ras-induced transformation may be due to inhibition of intracellular redox status and activation of JNK-1/p38 [180]. Compounds 30, 33, 49, and 68 could inhibit MCF-7/wt cell viability, and the inhibition ability is stronger with the number of functional units: hexahydroxydiphenoyl (HHDP) group [181]. Compound 50 was proven to have antiproliferative, proapoptotic, and antimigratory effects which are related to the PI3K/AKT and MAPK/ERK pathways [182].

4.3. Antimicrobial and Antivirus Activity

Compound 18 could inhibit biofilm formation, quorum sensing, and toxin secretion. This indicated that corilagin might be an effective antibacterial compound [183]. Compound 11 efficiently blocked entry of HCV of all major genotypes and also of the related flavivirus Zika virus [184]. Compound 11 could effectively inhibit the replication of RABV by the miR-455-5p/SOCS3/STAT3/IL-6-dependent pathway [185]. Compounds 28, 44, and 62 reduced the HCV replication [186] via a dual mechanism through preventing the formation of cccDNA and promoting cccDNA decay [187].

4.4. Antidiabetic Activity

It was confirmed that compound 18 can regulate diabetes, by exhibiting antidiabetic, antihyperlipidemic, and antioxidant properties in STZ-induced diabetic rats [188]. Compound 11 could maintain normal glycemia through the inhibitory action on alpha-amylases [189]. Compounds 22, 48, and 50 with the IC50 values of 690 μM, 97 M, and 361 μM could inhibit activity of mammalian intestinal maltase [53].

4.5. Other Therapeutic Activities

Compounds 20, 30, and 33 which have HHDP moiety decreased the ratio of MMPs/TIMPs to develop skin ageing [190]. Compound 48 was confirmed to inhibit TGF-beta 1-induced antifibrotic activity in choroid-retinal endothelial cells (RF/6A) [191] and inhibit TNF alpha induced proangiogenic and proinflammatory activities in retinal capillary endothelial cells [192].

The study of nanoparticles plays an important role in tannin activity and application. Bioavailability and bioactivity of a component are often altered once it is embedded into nanoparticles. Zheng Li fabricated the PPE with 16.6% (w/w) of punicalagin A, 32.5% (w/w) of punicalagin B, and a small amount of ellagic acid-hexoside and ellagic acid (1%, w/w). PPE-gelatin nanoparticle suspension had similar effects in inducing late stage of apoptosis and necrosis compared to PPE [193]. Guo-Bin Song fabricated a natural promising protein protective film through soluble dietary fiber (SDF)-tannin nanocluster self-assembly which characterized to possess a broad spectrum of antimicrobial properties and are beneficial to food preservation [194]. The field of nanoparticles plays an important role in the utilization of tannin activity with great development potential.

There is a lack of research on the interaction between proteins and tannins from Terminalia Linn., but the tannin extracted from persimmon fruits has been reported to have a high affinity to pancreatic lipase and possessed pancreatic lipase inhibition with IC50 of 0.44 mg/mL. Molecular docking showed that this interaction is mainly caused by the hydrogen bonding and π-π stacking [195]. It has been demonstrated that the very simple tannin methyl gallate was able to stack with itself or with caffeine [10]. The self-association of tannins should also take into account the interaction between tannins and proteins, as it governs their bioavailability. The interactions between tannin-tannin and tannin-protein are still unclear. Changes in protein bioactivity and structure induced by tannin binding need further studies.

Current limited metabolic studies showed that tannins are mainly metabolized as urolithins in the gut [196]. Urolithins are characterized to possess antitumor, antioxidative, and anti-inflammatory activities in vitro, which can be isolated and purified by high-speed counter-current chromatography. Urolithin A, a major punicalagin metabolite, could result in autophagy in SW620 colorectal cancer (CRC) cells at submicromolar concentrations [197]. It is very helpful for drug design to clarify the biotransformation of tannins in vivo.

Therefore, it is necessary to accelerate the development of the technical means for the analysis of bioactive compounds of natural medicines, so as to realize the large-scale development and utilization of tannin monomer compounds. The physiological activity of tannins has been fully confirmed, but the physiological mechanism of its various pharmacological effects is still not clear, limiting the development and utilization of tannins.

5. Conclusion

Terminalia species have been widely used in various traditional medical systems such as Siddha, Traditional Chinese Medicine (TCM), and Western, Southern, and Central African medicinal systems [8]. Apart from reports on the ethnopharmacological uses of many Terminalia species, few studies have carried out rigorous studies on the medical properties, mechanisms, and phytochemistry of this important genus. This may be due to the fact that tannins are the main active constituents in many Terminalia species. Tannins have strong polarity, high molecular weight, complex structure, active chemical properties, and are extremely difficult to crystallize which make them difficult to extract, separate, purify, and identify, and the quality standard is not easy to control. Therefore, they are so complex that they are not suitable for drug design and often overlooked as potential for drug discovery. Thus, how to improve the extraction and purification technology of tannins from genus Terminalia is an urgent problem to be solved. Researchers need to further determine the structure-activity relationship between tannins and their functions, clarify the mechanism of action, and carry out safety toxicological evaluation to ensure safety and stability, so as to make tannins hopeful to become new drugs on the market.

The structures of 82 tannins from the genus Terminalia were reviewed in this paper. The fragmentation pathways of identified tannins were analyzed, and the fragmentation rules of tannins were speculated, which could provide references for the structural analysis of natural medicines and their analogues. In further research, researchers may need to pay more attention to the species and the active substances such as the tannin summarized above.

Conflicts of Interest

No competing financial interest exists.

Authors’ Contributions

Zihao Chang and Qiunan Zhang contributed equally to this work.