This study evaluated the methods of grafting commercial catechin with fatty acids, namely capric acid (C10), lauric acid (C12), and myristic acid (C14) through esterification. Specimens of beech wood (
Nowadays, there has been a lot of research interest in developing a new sustainable wood protection system that reduces biocide utilization. It comes as a result of environmental issues and regulations pushing the development of new technologies to create new sustainable wood preservatives [
Catechin is a hydrophylic flavanol possessing antioxidant properties [
Even if it is difficult to find a natural antifungal active against a wide range of wood colonizing fungi, previous study carried out by Malterud et al. [
Clausen et al. [
Beech wood is one of the most important commercial tree species in Europe that has been universally used for furniture, plywood, particleboards or bentwood industry [
(+)-Catechin hydrate, capric acid, myristic acid, lauric acid, and all reactants were obtained from Sigma–Aldrich Chimie, France.
To a stirred solution of catechin (2 g, 6.89 mmol) in 25 mL acetonitrile, lauric acid 1 equivalent is added directly, and after 10 min DCCI 1 equivalent is added drop by drop to the reaction mixture at 0°C, which is stirred for 3 h at 0°C and 24 h at room temperature under N2 condition. Precipitated urea is then filtered off and the filtrate evaporated down
Products were identified using 1H and 13C Nuclear Magnetic Resonance (1H NMR and 13C NMR) performed on Bruker AC-200 and 400 MHz. FTIR spectra were recorded on a Perkin Elmer Spectrum 2000. All the products and their description are presented in
Conditioned sapwood portion of beech wood (
Oven-dried wood specimens were placed in a 250 mL beaker inside a desiccator equipped with a two-way tap and subjected to a 90–110 mbar vacuum for one hour. Afterward, the samples were taken out from the beaker and oven-dried at 103 ± 2°C for 24 h until reaching the constant weight (m1). Sixteen replicates were used for each impregnation. The weight percentage gain (WPG) was calculated by the difference between the oven-dried weight of the samples before and after impregnation (
Leaching procedure was carried out according to the standard NF X41–568 [
where m0 is the initial anhydrous weight before impregnation, m1 is the anhydrous weight after impregnation, and m2 is the anhydrous weight after leaching.
Treated and untreated wood samples were exposed to the white-rot fungus
Treatment | Yield (%) | |
---|---|---|
Monoester 4’/monoester 3’ | Diester | |
Catechin-C10 | 45/25 | 5 |
Catechin-C12 | 45/25 | 8 |
Catechin-C14 | 42/28 | 6 |
Regioselectivity of the reaction can be explained based on the acidity of the different phenolic groups. First of all, phenolic groups of catechol moiety (ring B) are more acidic due to the proximity of the two electronegative oxygen atoms compared to the two phenolic groups of ring A. Then, phenol in position 4’ is believed to present a higher acidity compared to phenol in position 3’ due to its distance from the electron donor inductive alkyl group in position 1’. Consequently, reaction take place mainly at the 4’ position due to the easier deprotonation of hydroxyl group by DCCI acting as weak base. Once formed, the monoester may be stabilized by intramolecular hydrogen bonding with hydroxyl group in 3’ position limiting its reactivity. Formation of the diester occurred as a by-product under the reactional conditions used in this study. Such reactivity has already been described in the literature [
Modification of catechin with fatty acids results in products with an amphiphilic behavior possessing both hydrophilic and hydrophobic properties. Hydrophobic part reduces the release of the compound impregnated in wood, while at the same time, the hydrophilic part allows formation of a stable suspension in water [
The results on WPG, WPGAL, and PL are illustrated in
Treatment | WPG (%) | WPGAL (%) | PL (%) |
---|---|---|---|
5% catechin | 3.02 ± 0.35 (a)* | −1.14 ± 0.20 (a)* | 4.04 ± 0.42 (a)* |
5% catechin + C10 | 4.18 ± 0.15 (b)* | 1.87 ± 0.10 (c)* | 2.22 ± 0.16 (b)* |
5% catechin + C12 | 3.92 ± 0.35 (b)* | 1.57 ± 0.29 (b)* | 2.26 ± 0.42 (b)* |
5% catechin + C14 | 4.00 ± 0.26 (b)* | 2.25 ± 0.16 (d)* | 1.68 ± 0.14 (c)* |
Note:
Statistically, the leaching process has a significant effect on all treatments related to the WPGAL results. Samples treated with catechin showed the highest leachability compared to the modified ones. A negative value on WPGAL is possibly caused by some water-soluble extractives that were leached during the test. Results indicated that the leachability decrease evidently when treating catechin with myristic acid, possessing the highest WPGAL and lowest PL. It can be understood because this type of fatty acid has the longest chain, which represents the hydrophobic properties (more resistant to water). This also confirmed that the increase of hydrophobicity with longer chain length might reduce their solubility in water [
The results of weight loss are evident for the untreated beech wood (51.49%), showing the vigorous fungal activity of
Samples | Treatment | Weight Loss (%) |
---|---|---|
5% catechin | Leached | 40.93 ± 5.34 (ab)* |
Unleached | 37.06 ± 1.42 (bc)* | |
5% catechin−C10 | Leached | 30.25 ± 1.66 (bc)* |
Unleached | 30.19 ± 3.70 (bc)* | |
5% catechin−C12 | Leached | 30.78 ± 3.25 (bc)* |
Unleached | 28.77 ± 4.91 (c)* | |
5% catechin−C14 | Leached | 29.62 ± 2.28 (bc)* |
Unleached | 28.79 ± 4.17 (c)* | |
Untreated beech wood | 51.49 ± 4.97 (a)* |
Note:
Samples treated with catechin alone presented lower weight losses, 40.93% and 37.06% for the leached and unleached samples respectively, compared to untreated beech wood, which presented weight loss of 51.49%. In a previous experiments conducted by Laks et al. [
While the modification of catechin and fatty acids seem to improve their leachability, it can be noted that the decay weight loss does not differ significantly between the samples that leached and not. The specimens treated with catechin and C10 fatty acid experience the highest weight loss (30.25% and 30.19%), while specimens treated with catechin and C14 fatty acid have the lowest weight loss (29.62% and 28.79%). However, there is no significant difference between them.
Literature studies reported that the hydrophobic groups of saturated fatty acids possess an important role in bioactivity, with an increase of the chain length led to increased antifungal efficiency [
The potential of catechin for wood protection can be optimized by selectively change its properties through chemical modification, notably to reduce its leachability. The combination of antifungal and hydrophobic properties may improve efficiency of catechin derivatives. Results suggested that catechin modified with myristic acid (C14) provides better resistance towards leaching. Moreover, no significant differences were observed in the decay weight loss for the samples treated with modified catechin with fatty acids before or after leaching. It is believed that the addition of fatty alkyl chain increased resistance to leaching. However, it is recommended to increase the concentration of modified catechin to obtain significant effect on decay resistance. It would also be interesting to consider the combination of these compounds with already used wood preservatives in order to increase the effectiveness of the treatment solution through a potential synergistic effect due to the antioxidant effect of catechin derivatives.
The authors acknowledge support of “LERMaB” by the “Impact Biomolecules” Project of the “Lorraine Université d’Excellence” (Investissements d’avenir–ANR 15-004) and of the French Ministery of Agriculture and the Lorraine-FEDER for the support of “EXTRAFOREST” Project.