相思樹心材抽出成分改善木材之光安定特性及其機制

Abstract

木材三大主成分之一的木質素是參與木材光劣化的主要成分。當木質素吸收紫外光而劣化後，木材表面的機械性質、物理性質及化學性質會改變，進而影響使用壽命。由筆者初步試驗之結果發現，相思樹（Acacia confusa Merr.）心材抽出物能有效減緩木質素光劣化。然而，其有效之抽出成分與確切的木材光安定機制尚未有詳細探討。因此，本研究之目的為探討具光安定能力之相思樹心材抽出成分及其改善木材光安定特性之機制。 由心材抽出物的組成分分析、光安定能力分析及抽出物抑制木質素光劣化之試驗結果得知，心材抽出物中豐富的多酚類化合物與多樣的黃酮類化合物，具有多重光安定作用。在木材光劣化過程中，這些成分能吸收紫外光並與單態氧反應，降低木質素自由基鏈鎖反應及光氧化的機會，進而減緩木材光劣化，延長木材使用年限。進一步分析得知，富含多重光安定能力的乙酸乙酯分離部為心材抽出物中主要的分離部，其中，次分離部1 - 5富含黃酮類化合物及縮合單寧，能有效減緩木材光劣化。此外，當木質素光劣化後，會產生羰基劣化衍生物而黃化。然而，這些分離部的抽出成分在減緩木材光劣化之過程中，亦會因光氧化而產生有色之羰基劣化衍生物，但其處理材的顏色變化趨勢隨縮合單寧與黃酮類化合物組成含量之不同而有所差異。因此，若以減緩木材中木質素光劣化為前提，可視使用者對於色調變化之喜好，選擇適當之次分離部處理木材。 另由光安定能力導向篩選具抑制木材光劣化能力抽出成分之研究結果亦證實，心材抽出物中主要抑制木材光劣化之成分為Okanin與Melanoxetin，能吸收紫外光、淬滅單態氧及清除酚氧自由基。而Okanin更能淬滅激發態木質素，使木質素回復至穩定之基態，進而減緩木材光劣化之程度。若進一步將Okanin與Melanoxetin混合（Melanoka），更能賦予木材多重光安定能力，提升木材光安定之效果。此外，心材抽出物中之黃酮類化合物除了B環為Catechol結構，A環亦多為Catechol結構（如Melanoxetin），能較一般黃酮類化合物（如Quercetin）更有效地淬滅單態氧及清除酚氧自由基。由於這些抽出成分中之Okanin較其他黃酮類化合物不易光氧化，非常值得將其開發成天然光安定劑。 此外，研究結果亦發現，增加心材抽出物含浸處理濃度能提升減緩木材光劣化之效果，且能更有效地減少有色羰基劣化衍生物。由此可知，心材抽出物不僅能透過多重光安定能力減緩木材光劣化，當其含量足夠時，亦能更充分地避免紫外光、單態氧及酚氧自由基所造成的危害，而減緩心材抽出物本身劣化，進一步延長其木材光安定效果之有效期限。此外，由乙酸乙酯分離部、乙酸乙酯次分離部、Okanin、Melanoxetin及Melanoka處理材減緩木材光劣化之效果得知，當處理材中光安定能力化合物越多元，其減緩木材光劣化之能力越佳。由心材抽出物光氧化試驗結果亦得知，不同類別黃酮類化合物之光反應機制亦不同，故不同組成分處理材照光後，其光安定能力便隨組成分光氧化衍生物之差異而有所變化，進而影響減緩木材光劣化之效果及有效期限。 綜合本研究結果得知，相思樹心材抽出物中的縮合單寧及黃酮類化合物能有效抑制木材光劣化，其中，黃酮類化合物中的Okanin及Melanoxetin具有良好的多重光安定能力，尤其以Okanin最佳，極具潛力開發成天然光安定劑，提供給建築、塗料、特用化學品、甚至是醫美化妝等相關產業使用，如此更可擴展相思樹抽出物之用途及提升其經濟價值。此外，本研究所建立之光安定劑篩選方法亦能提供未來篩選林木二級代謝產物及其他木材光安定劑之參考。

Lignin, one of the three main components in wood, is the major component involving in wood photodegradation. As lignin degraded by absorbing UV light, the mechanical, physical and chemical properties of wood surface are changed, decreasing the service life of wood. According to the results of my preliminary studies, the heartwood extract (HWE) of Acacia confusa Merr. can efficiently restrain lignin photodegradation. However, the effective extractives and the exact wood photostabilization mechanisms have not been discussed in detail. Hence, the aims of this study are to investigate the photostabilities of HWE and their mechanisms in enhancing wood photostability. The results obtained from the analyses of the chemical compositions, photostabilities, and inhibition ability of HWE against wood photodegradation demonstrated that the abundant polyphenols and various flavonoids in HWE have multiple photostabilities. During the wood photodegradation processes, these extractives can absorb UV light, quench singlet oxygen and restrain the lignin radical chain reactions and photooxidation, consequently, the photodegradation of wood is inhibited and the service life of wood is also extended. Further investigation revealed that the EtOAc fraction containing extractives with multiple photostabilities is the major part of HWE. In EtOAc fraction, the subfractions 1 to 5 have abundant flavonoids and condensed tannins which can effectively inhibit wood photodegradation. Additionally, after lignin photodegradation, the yellowish carbonyl derivatives formed. However, during the inhibition processes of wood photodegradation by the extractives in these subfractions, the colored carbonyl derivatives formed from the photooxidation of these extractives and the discoloration tendencies of these subfraction-treated specimens varied, depending on the different compositions of flavonoids and condensed tannins. Consequently, under the premise of inhibiting wood photodegradation, these subfractions can be applied on wood by using suitable subfractions with the favorite color tone. Moreover, the results obtained from photostability-guided isolation test demonstrated that okanin and melanoxetin are the major heartwood extractives to inhibit wood photodegradation by absorbing UV light, quenching singlet oxygen and scavenging phenoxyl radicals. Furthermore, okanin can quench excited lignin and return it to the stable ground state, concurrently, the photodegradation of wood is prevented. If wood treated with the combination of okanin and melanoxetin (melanoka), the wood photostabilitiy is enhanced by the multiple photostabilities of this combination. Besides, the flavonoids in HWE not only have catecholic B-ring structure but also have catecholic A-ring structure (such as melanoxetin) which provide themselves the abilities to quchch more singlet oxygen and scavenge phenoxyl radicals than the normal flavonoids (such as quercetin). Due to okanin is less susceptible to photooxidation among these flavonoids, it has great potency to be developed as natural photostabilizers. Furthermore, it was observed that the wood photostability can be enhanced and the colored carbonyl derivatives can be reduced by increasing the impregnation of HWE. It indicated that the multiple photostabilities of HWE can prevent wood from photodegradation; even when the amount is enough, the extractives can be protected by themselves owing to the degradation caused by UV light, singlet oxygen and phenoxyl radicals are sufficiently inhibited, leading to the extension of their service life. On the other hand, according to the results from the evaluation of the inhibition abilities of wood photodegradaton after the treatments with EtOAc fraction, EtOAc subfractions, okanin, melanoxetin and melanoka, the more multiple photostabilites of effective extractives are introduced into wood, the better inhibition ability of wood photodegradation is endowed. Moreover, the results from HWE photooxidation test revealed that the photoreaction mechanisms of flavonoids are varied with different classes of flavonoids. Hence, under irradiation, the efficacy and the effective duration of wood photostabilization are affected by the photostabilities of treated specimens with different constituents and their photooxidation derivatives. In summary, the flavonoids and condensed tannins in HWE can effectively restrain wood photodegrdation. Among them, okanin and melanoxetin belonging to flavonoids have good multiple photostabilities, especially okanin. These extractives have the potential to be developed as natural photostabilizers and to be applied in various industries such as architecture, coating, specialty chemicals, and even, aesthetic medicine. Accordingly, the application and economic value of the extractives from A. confusa can be expanded. In addition, the photostability-guided isolation method for wood photostabilizers established in this study can be applied as the standard procedure for isolating effective photostabilizers from plant secondary metabolites and other wood photostabilizers.