Identification of Three Dalbergia Species Based on Differences in Extractive Components

Dalbergia cultrate, Dalbergia latifolia, and Dalbergia melanoxylon are precious and valuable traded timber species of the genus Dalbergia. For chemotaxonomical discrimination between these easily confused species, the total extractive content of the three wood species was determined using four different organic solvents. Fourier transform infrared (FTIR) spectroscopy was used to analyze functional group differences in the extractive components, inferring the types of principal chemical components according to characteristic peak positions, intensities, and shapes. Gas chromatography-mass spectrometry (GC-MS) was carried out a detailed characterization of the extractive components. The relative content of individual chemical components was determined by area normalization. Results revealed differences in the chemical components and total and individual extract contents of the three Dalbergia species, indicating that FTIR and GC-MS spectroscopy can be applied to identify and discriminate between Dalbergia cultrate, Dalbergia latifolia, and Dalbergia melanoxylon.


Introduction
Wood extractives are non-structural wood molecules that represent a minor fraction in wood, specifically defined as compounds that can be extracted by polar, non-polar, or neutral solvents [1,2]. Wood extractives account for approximately 2% to 5% of wood content, however, relatively high amounts of extractives can be found in some tropical woods, especially chemical extractives that are highly concentrated in heartwood [3][4][5]. Studies have demonstrated that many extractive components exhibit various biological activities and are important in medical applications [6,7]. Wood extractives include an array of compounds, usually aliphatic, terpenoid and phenolic in nature. According to the literature [8], Dalbergia spp. are rich in aromatic compounds, however, significant differences have been found in the types and contents of wood extractives, even within the same genus [3].
Dalbergia is a genus of trees belonging to the Fabaceae (pea) family, that includes approximately 250 species. All species of Dalbergia spp. were listed in the 17th Convention on International Trade in Endangered Species of Wild Fauna and Flora [9]. Dalbergia cultrate (Benth.), Dalbergia latifolia (Roxb.), and Dalbergia melanoxylon (Guill. & Perr.) are high-profile species renowned for their use in high-quality products worldwide [10,11]. These species are used in luxury furniture, artwork, and musical instruments due to their refined colors and excellent hardness and intensity properties [12,13]. Chemical and physical properties are greatly influenced by extractives, with characteristic colors and textures generating extensive market demand and heavy deforestation. Hence, these species are protected by Appendix II of CITES, D. cultrate is included in the list of national key preserved wild plants in the People's Republic of China (first batch). However, any species appearing in Appendix II of CITES are banned from international commercial trade except for those with an import and export license or re-export certificate [14]. Therefore, a more persuasive method for distinguishing closely related species is needed.
Traditional wood anatomy identification methods based on macroscopic and microscopic characteristics have been well-established for several years [15]. However, wood anatomy is too specialized for legal identification, especially when considering leaves, flowers, fruits, and other information to provide the extent of identification required by CITES [16]. By contrast, chemotaxonomical and genetic methods are useful in wood identification [17]. Previous investigations have found chemotaxonomical identification based on the analysis of extractive compounds to be an effective method for distinguishing extremely similar wood species [18][19][20].
To the best of our knowledge, Fourier transform infrared (FTIR) spectroscopy is a fast, simple, non-destructive method and a powerful technology to determine functional groups in the fingerprint region. This approach has been widely used in identification of complex systems such as traditional Chinese medicines and is suitable for analyzing woody materials [21,22]. In recent years, research on infrared spectroscopy in wood extractive studies has grown in popularity [23][24][25]. Additionally, gas chromatography-mass spectrometry (GC-MS) offers clear advantages when analyzing complex mixtures; the combination of an ideal separation technique (GC) with a sensitive identification technique (MS) constitutes a reliable and commonly used method for qualitative and quantitative analysis of compounds [26][27][28]. GC-MS has been widely applied in establishing chromatographic fingerprints for quality evaluation of herbal medicines [29]. It has also been well-established in the characterization and identification of wood extractive compounds [30][31][32].
Little research has been carried out to identify the three similar Dalbergia species Dalbergia cultrate, Dalbergia latifolia, and Dalbergia melanoxylon based on chemical taxonomy. This paper examines differences in extractive contents and components using FTIR and GC-MS approaches to discriminate chemotaxonomically between the extractive components in Dalbergia spp. Functional group analysis using FTIR spectroscopy and detailed analysis of their components by GC-MS allow one to classify similar species by their chemical nature. These two techniques also facilitate comprehensive analysis of chemical components to support the chemotaxonomical classification of wood. GC-MS data provide a persuasive basis for specific compounds identified by infrared absorption spectra, as the two methods are complementary and valuable in chemotaxonomical identification.

Contents Analysis of Extractives
The extractive contents of the three Dalbergia species are listed in Table 1. Dalbergia cultrate, Dalbergia latifolia, and Dalbergia melanoxylon were extracted with four different polar organic solvents, the extractive contents were calculated, and the average value of three experiments was taken. The findings indicated that the non-polar solvent n-hexane was not suitable for wood extraction, indicating that the extractable components of these three kinds of wood are mostly polar. After successfully extracting samples with ethanol/water, benzene/ethanol, ethyl acetate respectively, the extract contents were clearly different. Among the benzyl alcohol and ethyl acetate extracts of the three species Dalbergia cultrate had the lowest content, whereas the content of Dalbergia melanoxylon was highest. With ethanol/water (9:1, v/v), the content of Dalbergia cultrate was slightly higher than that in Dalbergia latifolia but clearly lower than in Dalbergia melanoxylon. Therefore, Dalbergia melanoxylon contained more types of main compounds than Dalbergia cultrate, and Dalbergia latifolia had the fewest kinds of principle compounds, likely because it contained fewer polar components. In general, the effect of ethanol/water (9:1, v/v) was better than benzyl alcohol and ethyl acetate, and more components could be obtained. Therefore, to analyze the primary extractive components in Dalbergia cultrate, Dalbergia latifolia, and Dalbergia melanoxylon, ethanol/water (9:1, v/v) was used as the final solvent in this experiment.

FTIR Analysis
The FTIR spectra of the Dalbergia cultrate, Dalbergia latifolia, and Dalbergia melanoxylon extractives are illustrated in Figure 1. Detailed peak positions are summarized in Table 2. The characteristic infrared absorption peaks of the functional groups of the main extractive components were reflected in the FTIR spectra. In the region of 3401-2839 cm −1 , all spectra exhibited strong, broad peaks attributable to the stretching vibration of O-H or N-H along with a moderate-intensity peak ascribed to methyl and methylene stretching vibrations. Benzene ring characteristic peaks (1602, 1510, and 1450 cm −1 ) of the three species were clear and strong, indicating a large proportion of aromatic substances [33]. However, the relative intensity of these three peaks was distinctly different. Especially for Dalbergia cultrate, the intensity of peaks at 1602 cm −1 and 1445 cm −1 was stronger, suggesting that the number, location, and properties of substituents on aromatic rings varied between the three species. Moreover, Dalbergia cultrate showed strong absorption bands around 1691 cm −1 for C=O, a peak around 1555 cm −1 with medium intensity for C=C of the aromatic ring, and strong absorption peaks at 1286 cm −1 and 1153 cm −1 for C-O-C [34]; these findings were presumably due to skeletal stretching vibration of the aromatic rings A and B and the functional group C-O-C of ring C of flavonoids [24,35]. These peaks were clearly visible only in Dalbergia cultrate; the C=O was weak for Dalbergia melanoxylon and was essentially invisible for Dalbergia latifolia, indicating an abundance of flavonoid compounds in Dalbergia cultrate.
Dalbergia cultrate also generated a stronger sharp absorption peak at 1378 cm −1 . In general, methyl groups have two absorption peaks around 1375 cm −1 and 1450 cm −1 , corresponding to symmetric bending vibrations and asymmetric bending vibrations, respectively [36]; this finding explains why the peak strength of Dalbergia cultrate at 1445cm −1 was so strong. Then, the absorption peaks at 1366 cm −1 of Dalbergia melanoxylon and 1352 cm −1 of Dalbergia latifolia were ascribed to either CH 3 symmetrical bending or in-plane C-OH bending, as the spectral band shape was low and wide.
Peaks in the region ranging from 910 cm −1 to 1300 cm −1 were mainly due to C-O single bond stretching vibrations. The extractives of the three species exhibited clear, strong peaks at 1200 cm −1 primarily due to C-O-C stretching; however, the band intensity of the three species included typical vibrations around 1270 cm −1 (Ar-O) and 1023 cm −1 (R-O) for C-O vibrations, although the peak intensity at 1270 cm −1 in Dalbergia melanoxylon was stronger than that in Dalbergia latifolia. Dalbergia melanoxylon likely contained more aromatic ether functional groups; a peak appeared at 1130 cm −1 for C-OH stretching of the alcohol groups [36]. C-OH stretching near 1114 cm −1 was found exclusively in Dalbergia cultrate. A C-H out-of-plane bending peak was seen around 999 cm −1 in the spectrum of Dalbergia cultrate and around 917 cm −1 in the spectrum of Dalbergia latifolia. Unique vibrations near 700 cm −1 were ascribed to the stretching vibration of C-S, which was obvious in Dalbergia cultrate and Dalbergia latifolia.

GC-MS Analysis
The GC-MS chromatograms of Dalbergia cultrata, Dalbergia latifolia, and Dalbergia melanoxylon extractives are presented in Figure 2. The results show clearly the different characteristic peaks of the three species. The peak area was chosen as the analytical signal for the relative amount. The relative content of each chemical component was calculated by area normalization and the average value of the three experiments. Identified chemical components (peak area above 1.0%) and the relative content of these compounds are listed in Table 3. The main chemical components of the three species were determined to be aromatic compounds. In general, these compounds were classified into flavonoids, miscellaneous, quinones, phenols, esters, stilbenoids, and amide compounds. The components of the extracts of Dalbergia cultrata, Dalbergia latifolia, and Dalbergia melanoxylon are detailed below.
According to the GC-MS analysis results, the principal components of the three species varied. Seven constituents were identified as having higher relative contents in Dalbergia cultrate: 3,7,3′,4′tetrahydroxyflavone (peak 4, 10.78%), 7-methoxy-1-thioflavone (peak 5, 25.55%), and parietin (peak 7, 24.81%). 7-Methoxy-1-thioflavone is a flavonoid derivative. Some thioflavones have been reported to function as novel neuroprotective agents and exhibit antiviral activities. 3,7,3′,4′-Tetrahydroxyflavone is a natural flavonol in foods and plants and has been identified as showing various biological activities. Parietin is an anthroquinone and has been identified in the traditional Chinese herbal medicines Polygoni multiflora and Eryngium foetidum L.  Table 3 reveals there were only two common compounds among the three species and each species also contained unique chemical components.

Wood Samples
The Latin names, trade names, and places of origin of the Dalbergia species are presented in Table  4. The three Dalbergia heartwood samples were obtained from the Research Institute of Wood Industry at the Chinese Academy of Forestry, China. Figure 3 shows tangential sections of the three kinds of Dalbergia heartwood. Three replicates were analyzed per sample.

FTIR Analysis
Two mg of the 90% ethanol extract were mixed with 100 mg of KBr powder in a smooth agate mortar. Then, the mixture was ground and pressed into a transparent pellet. FTIR spectra were

FTIR Analysis
Two mg of the 90% ethanol extract were mixed with 100 mg of KBr powder in a smooth agate mortar. Then, the mixture was ground and pressed into a transparent pellet. FTIR spectra were obtained within a scanning range of 4000-400 cm −1 at a resolution of 4 cm −1 and 16 total scans using a Spectrum ONE spectrometer (Perkin Elmer, Waltham, MA, USA) equipped with a DTGS detector at room temperature. The instrument was free of H 2 O and CO 2 .

GC-MS Analysis
GC/MS analysis was carried out by a triple quadrupole GC-MS system 450GC-320MS, (Bruker Billerica, MA, USA). Separation was achieved using a DB-5MS column (30 m × 0.25 mm × 0.25 µm; Agilent Technologies, Santa Clara, CA, USA) with a temperature program from 50 • C (5 min) to 290 • C (12 min) at 10 • C/min with helium as the carrier gas (1 mL/min). The solvent was delayed 4.5 min; the injection volume was 1 µL at a split ratio of 10. The mass spectrometer was operated in electron impact mode (70 eV), and masses were scanned over a range of 40-800 m/z. The transmission line temperature was 250 • C, and the ion source temperature was 200 • C. Peak assignment was accomplished by comparing the MS spectra to the National Institute of Standards and Technology (NIST 2010) library.

Conclusions
This study reveals that Dalbergia cultrate, Dalbergia latifolia, and Dalbergia melanoxylon can be successfully distinguished based on extractive analysis. Initially, the extract content of the three species was found to be different, with Dalbergia melanoxylon demonstrating the highest extractive content. By comparing different solvents, the extraction components were found to be mostly polar. Then, FTIR spectra effectively revealed additional information about the functional groups of the extractive components. We also identified the unique and primary components using GC-MS. The main chemical components of the three species varied, and 7-methoxy-1-thioflavone, 4,2 ,3 ,4tetramethoxy-5 -methyl-6-methylaminomethyl-1,1 -biphenyl and 10,11-dihydro-10-hydroxy-2,3,6trimethoxydibenz(b,f)oxepin exhibited the highest relative content of the three species, respectively. Furthermore, each species contained its own characteristic components, a useful finding for distinguishing between the three species. In summary Dalbergia cultrate, Dalbergia latifolia, and Dalbergia melanoxylon can be distinguished successfully according to differences in extractive content, functional groups, and chemical composition.