High temperature treatment allows the detection of episesamin in paulownia wood extractives

Abstract The composition and the relative variation of secondary metabolites of Paulownia tomentosa S. wood under thermal effect is a little explored area. Wood material was previously thermo-treated at 210 °C for 3 hours using a press vacuum technology. Extractives of untreated and thermo-treated wood material achieved with Soxhlet extraction techniques were obtained. Then the extracts were chromatographed by using thin layer chromatography. Component groups in extracts were determined by gas chromatography in combination with mass spectrometry. In terms of wood change the thermo-treatment of wood induces a darkening of wood color surface (ΔL* = 28.3), an increase of mass loss (3.5%) and an increase of the amount of extractives and lignin content as well as an increase of the chloroform soluble fraction. This work mainly describes the chemical exploration of the extract from paulownia wood, leading to the isolation and identification of episesamin. Graphical Abstract


Introduction
Paulownia species belonging to Scrophulariaceae family are largely cultivated in the world as a fast growing bioenergy crop and for wood industry (Akyildiz and Kol 2010). Among these plants Paulownia tomentosa S., a hardwood species widely used in Chinese medicine and pulping industries, shows a very fast growth (Olson and Carpenter 2007).
Thermal-treatment processes have been known for a long time including several different methods finalized to obtain, among other, an increased dimensional stability and resistance to biological degradation. In case of Paulownia elongata wood, the effect of heat treatment (160, 180, and 200 C) with three duration time (3, 5, and 7 h) on the changes in some physical properties (swelling, density, color, and equilibrium moisture content), were studied by Kaygin et al. (2009a). Another research (Candan et al. 2013) revealed that the hardness values of the wood boards were significantly improved by the hot pressure method.
Wood can be used as a source of valuable chemical compounds. The extractives of wood allow isolating some important chemical compounds such as vanillin or ferulic acid (Calvo-Flores and Dobado 2010;Li et al. 2015). Recent studies demonstrate the importance of thermal treatment in detecting new compounds in the extractives of wood (Lovaglio et al. 2017) taking into account different solvent and extraction techniques . Some studies conducted on Paulownia tree highlight several types of secondary metabolites in bark (Si et al. 2011) or fruits (Smejkal et al. 2007). However, knowledge of the chemical constituents after thermal modification and the enormous potential of these trees in the field of bio-based chemicals are still underevaluated, although Schneiderov a and Smejkal (2015) and He et al. (2016) showed detailed researches summarizing the main information about the isolated compounds, their bioactivities, and nature of sources such as fruits, bark, leaves and wood.
Thermo-treatment is recognized to alter the microstructure and cell-wall components of wood, among other changes and the most volatile extractives may leave the wood or be degraded, with new chemical compounds being obtained (Lovaglio et al. 2017). This author stated also that extractives can be drawn from wood by different approaches, and specific methods could be applied to influence the yield and type of extracted compound. A classic solvent extraction and fractionation followed by high performance liquid chromatography, or gas chromatography combined with off-line derivatization could help to improve the characterization of wood extractives.
Though a substantial amount of research has been conducted in the several field of the tree (fruits, bark, leaves and wood) to the best of our knowledge this is first attempt to assess the effect of thermo-vacuum treatment on the chemical potential of this multipurpose plant.
This work aims to contribute to extend the knowledge of paulownia wood through chemical study of the extract of both untreated and thermo-treated wood material.

Results and discussion
Mass loss is considered an important indicator of the severity of the thermal process (Ferrari et al. 2013). It is affected by the degradation of the chemical components of wood, mainly hemicellulose, with direct effect on extractives and mechanical properties Kaygin et al. (2009b). The mass loss found in this study for thermotreated Paulownia wood (5.1%) was lower to that reported (7.7) by Kaygin et al. (2009b) for Paulownia treated at 200 C for 3 hours. This difference could be due to the dissimilar thermo processes or geographical provenance of wood.
In term of color the darkening (DL Ã ¼ 28.3) increased significantly with treatment while Da Ã (6.1) and Db Ã (8.1) showed a slight rise. Kaygin et al. (2009a) investigated the color variation of Paulownia wood treated at 160, 180, and 200 C for 3, 5, and 7 h. According to the results obtained, the samples treated at 210 C were found to be darker than the other samples.
The composition of this thermo-treated wood in comparison with that of native wood is reported in Table 1. The thermo-treatment of wood induces an increase of the amount of extractives as well as an increase of the chloroform soluble fraction.
A portion corresponding to 15% of the total amount of extractives was analyzed by using a gas chromatographymass spetrometer apparatus showing the presence of a single component identified, on the basis of comparison of the mass spectra, with episesamin (1) (Figure 1). The registered mass spectrum is reported in the Figure 1S. The mass spectrum is completely superimposable with that of episesamin, as reported in literature (Kasiotis et al. 2017). In particular it is noteworthy the peaks at m/z 354 (M þ ), 203, 161, 149 (100%), and 135. The 1 H NMR spectrum (500 MHz, CDCl 3 ) of this compound showed a multiplet at d 3.07 ppm (1H, ArCHCHCH 2 ), two doublets at d 3.93 (1H, ArCHCHCHH, J ¼ 9.0 Hz), and 4.06 ppm (1 H, ArCHCHCHH, J ¼ 9.0 Hz), a double doublet at d 4.52 ppm (1H, ArCHCH, J 1 ¼ 9.0 Hz, J 2 ¼ 8.5 Hz), a doublet at d 4.85 ppm (1H, J ¼ 8.5 Hz), and a doublet at d 5.98 ppm (2H, OCH 2 O, J ¼ 12.0 Hz); other signals are due to the aromatic protons. This spectrum is in agreement with the identification of the compound as episesamin (Venkataraman and Gopalakrishnan, 2002). The 13 C NMR spectrum showed peaks at d 135. 4, 134.5, 134.3, 132.8, 122.8, 106.7, 104.5, 96.5, 95.7, 95.4, 92.8, 92.3, 91.2, 80.3, 78.2, 66.5, 60.1, 49.2, and 46.5 ppm. In this study episesamin is present at a rate of 1.33% compared to the initial wood quantity used. The isolation of this compound in Paulownia species has not been reported until now. The presence of episesamin in wood has been described in very few cases (Whiting 1985;Vaquette et al. 1979;Tillekertne et al. 1982).
In two detailed reports the presence of sesamin was recognized. He et al (2016) reported in a review the presence of D-sesamin in xylem while Schneiderov a and  Smejkal (2015), citing several authors, stated that (þ)-sesamin, isolated from the wood of Paulownia tomentosa, has various biological activities, such as antioxidant, angiogenic, antiphlogistic, antihypertensive, insecticidal, neuroprotective, and anticarcinogenic As stated before, episesamin showed interesting biological properties. The inhibitory effect on endothelial C receptor has been reported (Ku et al. 2013a). Episesamin inhibits HMGB-1 (high mobility group box-1) release, a protein that acts as a late mediator of severe vascular inflammatory conditions (Lee et al. 2013). Finally, episesamin showed a marked antithrobotic activity (Ku et al. 2013b).
Since Paulownia wood is widespread in several world countries, the improvement of the knowledge on technological properties of its wood and the chemical characteristics of their extracts may be of high interest from the economic point of view. Considering that Paulownia wood shows light density, and is easy to be treated at high temperatures (undesired collapses are not frequent), this research confirmed the results of previous papers and strengthen the idea that pretreatment at high temperature change some aesthetical characteristics dealing important new insight on the secondary metabolite characteristics.

Disclosure statement
No conflict of interest was reported by the authors.