抗氧化劑及金屬螯合劑對木材耐腐朽性之影響

Abstract

本論文旨在探討低毒性之抗氧化劑及金屬螯合劑對木材耐腐朽性之影響。首先以瓊脂平板試驗法及液態試驗法篩選商業用抗氧化劑及金屬螯合劑等之抑菌活性，結果顯示除了沒食子酸辛酯（Octyl gallate）可以抑制多種木材腐朽菌外，大多數藥劑並不具抑菌活性。沒食子酸辛酯之抑菌活性主要來自其適當比例的疏水端與親水端之結構，與其良好的自由基捕捉能力無關。透過DNA及RNA電泳分析及抑制扣除雜合技術（Suppression substraction hybridization, SSH）之應用，推測沒食子酸辛酯因干擾真菌細胞膜，影響細胞膜上諸多蛋白質之功能及流動性，致使DNA及RNA結構破壞。此外，與真菌生理基本機能有關之反應如細胞膜上離子的結合與傳輸、糖類代謝、能量產生與細胞內訊息傳遞等及對抗外界壓力或解毒等之自救能力，皆因添加沒食子酸辛酯受到抑制。 後續分別以土壤木塊試驗（ASTM D1413-07）及土床試驗（AWPA E14-07）評估不同配比之抗氧化劑及金屬螯合劑處理材於單純菌種及與土壤接觸動態變化菌種環境下之耐腐朽性能。由土壤木塊試驗結果顯示，木材單獨含浸5%之沒食子酸丙酯（Propyl gallate）、沒食子酸辛酯或金屬螯合劑EDTA（Ethylenediaminetetraacetic acid）等皆獲得良好且長效之耐腐朽效果，顯示藉由添加不具殺菌活性之抗氧化劑或金屬螯合劑，降低木材內自由基及金屬之含量，進而阻礙真菌對木材的非酵素降解機制而達保護木材的效果是可行的。其中，沒食子酸辛酯與鐵布可唑（Tebuconazole）或肉桂醛（Cinnamaldehyde）混用，更產生促進效應，此意謂在應用上藉由添加沒食子酸辛酯可以降低鐵布可唑或肉桂醛等殺菌劑的使用量。此外，由土床試驗結果顯示，抗氧化劑如沒食子酸丙酯、沒食子酸辛酯、沒食子酸十六酯（Hexadecyl gallate）、BHT（Butylated hydroxytoluene）及抗壞血酸（Ascrobic acid）等仍具一定保護木材的效果，唯效果不如土壤木塊試驗，推測係處理材在複雜微生物之土床環境可能遭遇到自由基以外的降解模式所致；而金屬螯合劑EDTA處理材在與土壤接觸的過程，則因藥劑大量流失而失去保護木材的效果。 傳統木材保存藥劑往往以毒殺方式保護木材，但同時其毒性對其他生物及環境亦造成相當之危害與負擔。本研究證實藉由阻斷腐朽菌降解木材所涉及之氧化路徑，選擇以非毒殺機制之低毒性化學藥劑應用在木材保存藥劑之研發是可行的，此外抗氧化劑或金屬螯合劑尚具有安全且低成本等優點，因此極有潛力作為室內或不與土壤接觸之環保型木材保存藥劑或添加劑。

The objective of this research was to evaluate the effects of antioxidant and metal chelator on the fungal decay resistance of wood. Antifungal activity was screened by agar plate test and broth test. Octyl gallate was found to possess wide antifungal spectrum, while most of antioxidants and metal chelator showed weak or no antifungal activity. Antifungal activity of octyl gallate was related to its hydrophobic/hydrophilic structure but not from its high free radical scavenging activity. Results from DNA and RNA electrophoresis and suppression substraction hybridization suggested that octyl gallate interfered with the surfaces of the membrane of fungal cell leading to the disorder of protein, degradation of DNA and RNA. Besides, gene related to fungal basic physiology such as ion channel activity on membrane, carbohydrate metabolic process, energy production, signal transduction, response to stress, and detoxification were all inhibited. Fungal decay resistance of the treated wood was evaluated according to soil block test (ASTM D1413-07) and soil bed test (AWPA E14-07). Results from pure cultured soil block test revealed that deterioration of wood caused by decay fungi (Gloeophyllum trabeum and Trametes versicolor) could be effectively reduced by impregnation of propyl gallate, octyl gallate, and EDTA, indicating that non-fungacidal antioxidants and metal chelator endowed wood with good resistance against wood rot fungi by reducing the amount of free radicals and metal in wood. Besides, octyl gallate also showed enhanced efficacy when it co-added with fungicides like tebuconazole. Results from soil bed test revealed that wood treated with antioxidants such as propyl gallate, octyl gallate, hexadecyl gallate, butylated hydroxytoluene, and ascrobic acid could alleviate fungal attack to some extent even most of them have no antifungal activity. EDTA showed no protective effect in siol bed test which may be ascribed to its high leachability. A major problem of traditional wood preservatives is that they pose a serious threat to the environment. According to the above results, this study verified that in addition to fungicidal activity of traditional wood preservatives, non-biocidal properties like free radical scavenging activity or metal chelating activity might provide protection for wood by blocking one of the pathways of fungal attack. In addition, antioxidant and metal chelator possess the advantages like safety and low-cost, they could be a feasible alternative or additives to develop environmentally-benign wood preservatives for sheltered or non-ground-contact purposes.