Characterization and Analysis of 2-(2-Phenylethyl)-chromone Derivatives from Agarwood (Aquilaria crassna) by Artificial Holing for Different Times

Chemical analysis of different parts from agarwood columns by artificially agarwood-inducing method based on GC-MS and UPLC-TOF-MS

Agarwood is resin-containing wood produced by plants that have been injured. It is widely used in herbal medicine, incense, decorative items, and so on. In this study, we conducted resin area statistical analysis, determined starch particle and reducing sugar contents, and performed multivariate statistical analysis of chemical composition by GC-MS and UPLC-Q-TOF-MS to explore the different components in sections cut from an agarwood column, designated as A1-A4. The results showed that after stimulation by Agar-Bit inducer, the internal phloem parenchyma cells of the column started to form agarwood, and then starch granules were converted into soluble reducing sugars and agarwood resin. Section A1 showed rapid loss of starch granules, resulting in higher contents of reducing sugars and resin. The resin areas of agarwood in the respective sections were different, gradually decreasing on going from A1 to A4. Total numbers of metabolites of 87 and 63 were identified by GC-MS and UPLC-Q-TOF-MS, respectively. Of these, 10 and 16 metabolites with significant differences (variable importance projection >1) were selected through multivariate statistical analysis. These metabolites included chromones, sesquiterpenes, alkanes, and fatty acids. Among them, 6-methoxy-2-(2-phenylethyl)chromone and 6,7-dimethoxy-2-(2-phenylethyl)chromone were significant markers detected by both GC-MS and UPLC-Q-TOF-MS, which may be essential substances responsible for differences in the agarwood-forming capacities of the cut sections. In conclusion, there has been limited research on the different agarwood-forming capacities of agarwood columns. Here, we explored the differences in various sections of agarwood through chemical analysis to provide a more comprehensive and in-depth understanding of its constitution.

Dynamics of Physiological Properties and Endophytic Fungal Communities in the Xylem of Aquilaria sinensis (Lour.) with Different Induction Times

Xylem-associated fungus can secrete many secondary metabolites to help Aquilaria trees resist various stresses and play a crucial role in facilitating agarwood formation. However, the dynamics of endophytic fungi in Aquilaria sinensis xylem after artificial induction have not been fully elaborated. Endophytic fungi communities and xylem physio-biochemical properties were examined before and after induction with an inorganic salt solution, including four different times (pre-induction (0M), the third (3M), sixth (6M) and ninth (9M) month after induction treatment). The relationships between fungal diversity and physio-biochemical indices were evaluated. The results showed that superoxide dismutase (SOD) and peroxidase (POD) activities, malondialdehyde (MDA) and soluble sugar content first increased and then decreased with induction time, while starch was heavily consumed after induction treatment. Endophytic fungal diversity was significantly lower after induction treatment than before, but the species richness was promoted. Fungal β-diversity was also clustered into four groups according to different times. Core species shifted from rare to dominant taxa with induction time, and growing species interactions in the network indicate a gradual complication of fungal community structure. Endophytic fungi diversity and potential functions were closely related to physicochemical indices that had less effect on the relative abundance of the dominant species. These findings help assess the regulatory mechanisms of microorganisms that expedite agarwood formation after artificial induction.

2-(2-Phenylethyl)chromones increase in Aquilaria sinensis with the formation of agarwood

Obtained from Aquilaria Lam. and Gyrinops Gaertn., agarwood is a prestigious perfume and medicinal material in the world. Its primary chemical constituents and indicators of agarwood's development are 2-(2-phenylethyl)chromones (PECs). However, how PECs affect its quality, accumulation, and transformation pattern is still unclear. The present study investigated this issue by monitoring resin filling in agarwood generated by the whole-tree agarwood-inducing technique over a span of a year, observing the ethanol extract concentration at different sampling times, and statistically examining PECs in agarwood from each sampling period. In agarwood, the resin accumulated over time, except during the 4th-6th month due to the creation of a barrier layer. The relative content of total PECs demonstrated an overall increase throughout the year but a decrease from the 4th month to the 6th month, and the relative content of 19 PECs that persisted throughout the year was positively correlated with the content of ethanol extracts. In addition, the process of chromone accumulation was accompanied by the production and transformation of different types of chromones, with flindersia type 2-(2-phenylethyl)chromones, epoxy-2-(2-phenylethyl)chromones, and diepoxy-2-(2-phenylethyl)chromones being the major chromone components; in addition, the content of 5,6,7,8-tetrahydro-2-(2-phenylethyl)chromones kept increasing after 6 months of agarwood formation. Three main trends were identified from 58 analogs of PECs, each with notable variation. The first type had the highest content at the beginning of resin formation. The second type had the highest content at 6 months and then started to decrease, and the third type had a slowly increasing content. As a whole, this study systematically investigated the accumulation of PECs during injury-induced agarwood production in A. sinensis, which is of scientific significance in resolving the transformation of PECs and revealing the secret of agarwood formation.

Quality Similarity between Induced Agarwood by Fungus and Wild Agarwood

To prevent the exploitation of wild agarwood, the development of artificial agarwood through fungal inoculation is a promising method, but finding species that produce efficient high-quality agarwood remains difficult. In this study, a fungal inducer was prepared using wild agarwood containing fungi and high-throughput sequencing was performed to determine its species makeup. Subsequently, it was used to inoculate Aquilaria sinensis(Lour.) Spreng. The induced agarwood (IA), wild agarwood (WA), and nonresinous whitewood (WW) were analyzed for the extract content. In addition, liquid and gas chromatography-mass spectrometry was used to determine the chemical composition of the samples. The results were used to evaluate the quality of the IA. Mortierella humilisLinnem. ex W.Gams, Oidiodendron maius(Barron), and Tolypocladium album(W. Gams) Quandt, Kepler, and Spatafora were the fungal inducers that were discovered to produce agarwood. The extracts from the IA and WA contained 64 and 69 2-(2-phenylethyl)chromones, respectively, while there were none in the WW. Furthermore, 20 (relative content 36.19%) and 27 (relative content 54.92%) sesquiterpenes were identified in the essential oils of the IA and WA, respectively, and none were identified in the WW. The fungal inducer that was prepared from the WA effectively improves the quality of the agarwood, which is extremely similar to that of the WA.

Rapid screening and characterization of 2-(2-phenylethyl)chromones in agarwood by UHPLC-Q-Exactive Orbitrap-MS

The purpose of this study is to develop an improved comprehensive data filtering strategy, which was implemented primarily through the Microsoft Office platform's Excel software for rapid screening of potential 2-(2-phenylethyl)chromone (PEC) monomers and their dimers (PEC dimers) obtained from agarwood. A total of 108 PEC monomers and 30 PEC dimers in agarwood were characterized. In conclusion, the results obtained in this work could provide useful information for the future utilization of agarwood. In particular, it is the first time to conduct an in-depth analysis of the MS/MS fragmentation behavior of a large number of PEC monomers and PEC dimers, including the identification of substituent positions of them. The proposed data filtering strategy could improve the comprehensive characterization efficiency of complex components in spices.

Chemical and Bioactivity Diversity of 2-(2-Phenylethyl)chromones in Agarwood: A Review

2-(2-Phenylethyl)chromone derivatives are regarded as key components in agarwood. An oxygen-containing heterocycle with a benzoannelated γ-pyrone moiety form the bioactive core of 2-(2-phenylethyl)chromones. With different substituents and positions, 2-(2-phenylethyl)chromone derivatives exhibit diverse biological properties, such as antioxidant, antimicrobial, neuroprotective, anti-inflammatory, and acetylcholinesterase inhibitory activities. In this review, we summarized the studies (from January 1976 to September 2021) on phytochemistry, bioactivity and quality control of 2-(2-phenylethyl)chromones. These studies aimed to clarify the chemical specificity, diversity and structure-activity relationship of 2-(2-phenylethyl)chromones. In addition, we assumed that diverse factors such as tree species, induction methods and formation time contribute to the chemical diversity of 2-(2-phenylethyl)chromones. Furthermore, this review contends that different types of 2-(2-phenylethyl)chromones should be utilized in the quality control methods of agarwood.

New 2-(2-Phenylethyl)chromone derivatives from agarwood originating from Aquilaria sinensis

Two new dimeric 2-(2-phenylethyl)chromones, aquilasinenones L and M (1 and 2), and one new monomer analogue, 5S, 6 R, 7S, 8 R-tetrahydroxy-[2-(3-methoxy-4-hydroxyphenyl)ethyl]- 5,6,7,8-tetrahydrochromone (3), together with two known compounds, were isolated from the artificial agarwood originating from Aquilaria sinensis. Compound 1 was the first structure found with C8-O-C4"' linkage among 2-(2-phenylethyl)chromone dimers. Their structures were unambiguously elucidated based on 1 D and 2 D NMR spectroscopy, as well as by comparison with the literature. The absolute configuration was determined by ECD calculation. None of the compounds exhibited acetylcholinesterase inhibitory activity.

Factors affecting 2-(2-phenylethyl)chromones in artificial agarwood

Recently, "artificial agarwood" manufactured by the artificial treatment on cultivated agarwood trees is popular in agarwood-producing countries. Although there are various treatment methods, they are not standardized. Moreover, factors that may affect the generated chemical compounds have not been investigated. In this research, the effects of different treatment methods and individual differences on the quantities and types of 2-(2-phenylethyl)chromone in agarwood were investigated to experimentally produce artificial agarwood using Aquilaria sinensis. Each solvent-extracted agarwood sample was analyzed using Liquid Chromatography/Tandem Mass Spectrometry (LC-MS/MS), and peaks were identified by comparing ten types of 2-(2-phenylethyl)chromone with reference standards. The composition and 2-(2-phenylethyl)chromone content of each agarwood sample were observed based on the type of chemical compound, and results indicated that when the treatment method was different, the accumulation pattern of the 2-(2-phenylethyl)chromones differed even when the number of resinification years was the same. Furthermore, the findings of this study showed that additional treatment on a single branch produced more 2-(2-phenylethyl)chromones. Moreover, market products composed of artificial agarwood pieces derived from different tree species and collected from different location were analyzed.

Rapid authentication of agarwood by using liquid extraction surface analysis mass spectrometry (LESA-MS)

INTRODUCTION

Agarwood is a highly valuable fragrant resinous wood which is widely used as traditional Chinese medicines, perfumes, incense and decorations. Due to its high economic value and excessive demand, this leads to a rising price and proliferation of fake commodities. Thus, strict authenticity identification and quality evaluation of agarwood are of great significance.

OBJECTIVE

To establish a simple, rapid and non-destructive technique for identifying the authenticity of agarwood.

METHODS

Liquid extraction surface analysis mass spectrometry (LESA-MS) was firstly proposed to identify the authenticity of 62 agarwood samples without sample preparation. In addition, multivariate statistical models and thin-layer chromatography (TLC) method were used to analyse and verify the results of LESA-MS.

RESULTS

Representative compounds of agarwood were detected by LESA-MS. A characteristic 2-(2-phenylethyl)chromone compound (m/z 319.1) was treated as a key chemical marker to identify agarwood and its counterfeits rapidly. Several other chromones ions were identified and used as additional evidence for authentic samples. A total of 62 samples were visually discriminated as two groups by principal component analysis (PCA) and orthogonal projection to latent structures discriminant analysis (OPLS-DA), and the specific characteristic marker was highlighted. Moreover, the qualitative results of the conventional TLC method were in agreement with the LESA-MS approach.

CONCLUSION

The proposed LESA-MS method was successfully applied in the direct qualitative analysis of agarwood from different sources. This study indicated great feasibility and practicality of LESA-MS in the rapid identification of agarwood, and provided a non-destructive and meaningful preliminary screening tool for the agarwood industry.

Natural products in agarwood and Aquilaria plants: chemistry, biological activities and biosynthesis

Covering: Up to the end of 2019.Agarwood is a resinous portion of Aquilaria trees, which is formed in response to environmental stress factors such as physical injury or microbial attack. It is very sought-after among the natural incenses, as well as for its medicinal properties in traditional Chinese and Ayurvedic medicine. Interestingly, the chemical constituents of agarwood and healthy Aquilaria trees are quite different. Sesquiterpenes and 2-(2-phenethyl)chromones with diverse scaffolds commonly accumulate in agarwood. Similar structures have rarely been reported from the original trees that mainly contain flavonoids, benzophenones, xanthones, lignans, simple phenolic compounds, megastigmanes, diterpenoids, triterpenoids, steroids, alkaloids, etc. This review summarizes the chemical constituents and biological activities both in agarwood and Aquilaria trees, and their biosynthesis is discussed in order to give a comprehensive overview of the research progress on agarwood.

Agarotetrol in agarwood: its use in evaluation of agarwood quality

Agarwood, which is used as medicine and incense, contains sesquiterpenes and chromones. Agarotetrol is a chromone derivative found in high concentrations in the water-extract fraction of agarwood and thus may be present in pharmaceutical products made from decoctions of agarwood. Agarotetrol has been reported to be present at the early stages of cell death in calli. We therefore examined the presence of agarotetrol in medical- and incense-grade agarwood, in agarwood-source plants lacking resin deposits, and in artificially made agarwood. Agarotetrol appeared as a large peak in the HPLC chromatograms of all samples of medical-grade and artificially made agarwood, and in most incense-grade agarwood samples. In contrast, agarwood samples lacking resin deposits did not contain agarotetrol. These results show that agarotetrol is characteristic of resin formation. Agarotetrol was also detected in decoctions of agarwood. A newly developed TLC method for the detection of agarotetrol in agarwood is described.

The Scent of Stress: Evidence From the Unique Fragrance of Agarwood

Agarwood (Aquilaria spp.) fragrance and its origin in stress make it probably the most suitable model to study stress-induced aroma. Production being confined only to certain small pockets of South and Southeast Asia, agarwood is arguably the costliest wood in the world. Formation of fragrant agarwood resin is the outcome of complex biotic, abiotic, and physical stress on the Aquilaria trees. The intricate mechanism by which some 150 odd fragrant molecules that constitute agarwood aroma is formed is still not clearly understood. The present review therefore aims to bring to focus this less known but highly valuable stress-induced aroma from Asia. Discussions on agarwood species, occurrence, distribution, formation, and products have been included as foundation. Although global trade in agarwood and its products is estimated at US$6 billion to US$8 billion, no reliable data are readily available in literature. Therefore, an effort has been made to review the current status of agarwood trade. The element of stress and its correlation to agarwood aroma is discussed in the subsequent sections. Natural agarwood formation as well as technologies and interventions in agarwood induction are stress-based (natural and artificial injury, insect and fungal attack, chemical induction). The molecular triggers are gradually coming to light as new studies are implicating jasmonate, LOX signaling, and other stress reaction routes as the source of agarwood aroma. This review therefore has strived to compile the information that is scattered across scientific as well as other authentic literature and update the reader on the current status. More information about the specific roles of other vital stressors like insects, abiotic, and genetic factors is eagerly awaited from ongoing and future research to further understand the unique fragrance of agarwood.

Mass spectrometry imaging guided molecular networking to expedite discovery and structural analysis of agarwood natural products

Structural analysis of biomolecules is essential to natural product discovery, especially for precious biomaterials such as agarwood. However, one of the greatest challenges to the characterization of natural products is the profound cost in time and manpower to the structural elucidation of these highly diverse compounds. Here, we demonstrate a multi-modal mass spectrometric strategy, integrating matrix-assisted laser desorption ionization (MALDI) mass spectrometry imaging (MSI) and mass spectral molecular networking, to uncover agarwood natural products of Aquilaria sinensis trees. A simple workflow for preparing wood sections for MALDI-MSI analysis was demonstrated. Notably, tens of natural products in the agarwood region in wood stem section of A. sinensis were spatially revealed by MALDI-MSI. For the first time, such a great number of plant specialized metabolites is obtained by a single wood section MSI. Guided by the spatially resolved features, mass spectral molecular networking was subsequently applied for structural analysis of the agarwood natural products, in which three major classes of 2-(2-phenylethyl)chromones and their analogues were putatively characterized. These results suggest an efficient strategy to the dereplication of plant natural products.

Chemical Profiles of Cultivated Agarwood Induced by Different Techniques

Agarwood is the resinous wood produced in some Aquilaria species and is highly valued for wide usages in medicine, incense, and perfume. To protect the threatened Aquilaria species, the cultivation of Aquilaria sinensis and artificial agarwood induction techniques have been effectively established in China. To evaluate the quality of agarwood induced by different techniques, patterns of chemical constituents in artificial agarwood by four methods (wounding using an axe, burning-chisel-drilling, chemical inducer, and biological inoculation) were analyzed and compared by UPLC-ESI-MS/MS and GC-EI-MS in this study. Results of GC-MS gave a panorama of chemical constituents in agarwood, including aromatic compounds, steroids, fatty acids, sesquiterpenoids, and 2-(2-phenlyethyl)-chromones (PECs). Sesquiterpenoids were dominant in agarwood induced by wounding using an axe. PEC comprised over 60% of components in agarwood produced by biological inoculation and chemical inducers. PECs were identified by UPLC-ESI-MS/MS in all artificial agarwood and the relative contents varied in different groups. Tetrahydro-2-(2-phenylethyl)-chromones (THPECs) in wounding by axes induced agarwood were lower while 2-(2-phenylethyl)-chromones (FPECs) were higher than other groups. The results showed that methods used for inducing agarwood formation in Aquilaria sinensis affect the chemical constituents of agarwood.

A full solution for multi-component quantification-oriented quality assessment of herbal medicines, Chinese agarwood as a case

The quality of herbal medicines (HMs) is the prerequisite for their pronounced therapeutic outcomes in clinic, and multi-component (also known as quality markers, Q-markers) quantification has been widely emphasized as a viable means for quality evaluation. Because of the chemical diversity, the quality control practices are extensively dampened by four principal technical bottlenecks, including the lack of authentic compounds, large polarity span, extensive concentration range, and signal misrecognition for those potential Q-markers. An attempt to promote the potential of LC-MS/MS is made herein to cope with those obstacles and Chinese agarwood was employed as a case study. Firstly, a home-made fraction collector was introduced to automatically fragment the entire extract into a panel of fractions-of-interest. Secondly, quantitative ¹H-NMR was deployed to offset the LC-MS/MS potential towards in-depth chemical profiling each fraction, and those well-defined fractions were then pooled and combined with some accessible authentic compounds to generate the pseudo-mixed standard solution. Thirdly, serial improvements were conducted for LC-MS/MS measurements. Reversed phase LC and hydrophilic interaction LC were serially coupled in respond to the large polarity window, and online parameter optimization, response tailoring, as well as RRCEC (relative response vs. collision energy curve) matching were integrated in MS/MS domain to advance the quantitative confidences. Simultaneous determination was conducted for 26 components, in total, in Chinese agarwood after method validation. In particular, authentic compound-free quantification was achieved for eight 2-(2-phenylethyl)chromone derivatives. Above all, the strategy is a promising solution to completely tackle with the technical barriers toward Q-marker quantification-oriented quality control of Chinese agarwood, as well as other HMs.