Nitrous acid deaminative depolymerisation was used to prepare three chitosan oligomer (CO) mixtures from high-molecular weight chitosan. These mixtures of chitosan oligosaccharides were analysed by electrospray ionisation mass spectroscopy, potentiometric titration and gel permeation chromatography. A method based on potentiometric titration of the amino groups of the oligomers gave an average degree of polymerisation (DP) for the three preparations of 5 (CODP5), 9 (CODP9) and 14 (CODP14). Chitosan acetate and the chitosan oligomer mixtures were assayed against Leptographium procerum, Sphaeropsis sapinea and Trichoderma harzianum on nutrient media. Leptographium procerum and S. sapinea growth was prevented by chitosan acetate and chitosan oligomers at concentrations of 0.3–0.4% (w/v), whereas T. harzianum was able to overcome the fungistatic action of these compounds. The oligomer preparation CODP14 exhibited superior specific activity to both CODP5 and chitosan acetate, suggesting an optimum molecular weight for bioactivity. All oligomer preparations were more effective at pH 4 than at pH 6. This result, in combination with the inactivity of N-acetylated CODP14, indicated that amino group protonation was an important factor for fungistatic activity. The CODP14 preparation was reduced with sodium borohydride and fractionated by alkali precipitation and ion exchange chromatography. Bioassays of these fractions pointed towards DP and degree of deacetylation (DD) as key factors in chito-oligosaccharide bioactivity. Conversely, the terminal aldehyde groups generated by depolymerisation did not contribute to the activity observed.
Essential oils were screened for their antifungal activity against common wood inhabiting fungi. Subsequently, two pure essential oil extracts, eugenol and cinnamaldehyde, were evaluated for their efficacy on radiata pine wood durability. During the in vitro screening trial, variability in the tolerance of the tested fungi towards the selected essential oil was apparent. However, wood durability test on wood confirmed the antifungal activity of eugenol and cinnamaldehyde but highlighted the leaching (when exposed to wet conditions) of these compounds from treated wood. Blocks treated with 3% w/v eugenol without exposure to wet conditions had ,1% weight loss when exposed to all three test fungi, Oligoporus placenta, Coniophora puteana and Antrodia Xantha. However, blocks which were exposed to water showed weight losses in the range of 13?4-23?1%. This study identified eugenol and cinnamaldehyde as potentially benign wood preservatives for treatment of timber not exposed to wet conditions.
Chitosan, a polymer of D-glucosamine, is known for its antimicrobial activity. However, the physicochemical properties of chitosan depend upon three principal factors, i.e., source of raw material, molecular weight and degree of deacetylation. Here, synthesis and characterization of chitosan oligomers is reported, which were prepared by the nitrous acid deaminative depolymerization of chitosan obtained from Sigma Aldrich and China (industrial grade). Subsequently, their antifungal activities were evaluated against selected basid-iomycetes. The nutrient medium bioassay results show that chitosans and chitosan oligomers affected mycelial growth to different magnitudes. Nevertheless, two of the low-MW chitosan oligomers completely inhibited the growth of all the tested fungi as compared to high-MW chitosan oligomers. Accordingly, antifungal activity increases by decreasing degree of polymerization of chitosan oligomers, which is divergent to literature data. The wood decay trial confirmed the antifungal activity of chitosan oligomers against basid-iomycetes. Chitosan is leachable by water.
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