Evaluation of tropolone as a wood preservative : activity and mode of action
P Gérardin, M Baya, N Delbarre, P N Diouf, D Perrin, P Soulounganga, E Gelhaye,
J P Jacquot, C Rapin
The fungicidal activity of 2-hydroxy cyclohepta-2,4,6-trienone (tropolone) analogue of b-thujaplicin a natural product responsible for the durability of heart wood of several
cupressaceous trees was investigated in vitro on growth of white and brown rot fungi. Results obtained show that tropolone, easily prepared from commercially available products,
possesses strong fungicidal activity similar to that of fungicides currently used for wood preservation. In addition, laboratory malt-agar block tests indicate that tropolone, like b-thujaplicin, is able to protect wood blocks against brown rot fungi like Poria placenta but not against white rot fungi like Coriolus versicolor.
These differences were investigated on the basis of the mechanism of action of tropolone. Fungal growth inhibition on malt-agar could be prevented by adding iron salts in the medium, which indicates that chelating properties of tropolone are important on its mode of action. Determination of the stoechiometry of the reaction between tropolone and ferric ion shows the formation of a strongly insoluble precipitate involving 3 equivalents of tropolone for 1 equivalent of Fe3+ with a solubility product (Ks) of approximately 10-28 which creates metal limitation conditions inhibiting fungal growth. Moreover, tropolone possesses weak antioxidative properties and is able to inhibit ferric iron reduction by catecholates lowering the redox potential of the iron couple.
All these data are consistent with the hypothesis that tropolone inhibits wood degradation by Poria placenta by chelating iron present in wood thus avoiding initiation of Fenton reaction, while Coriolus versicolor which produces several lignolitic enzymes like laccases and peroxidases able to degrade rapidely tropolone is unaffected by tropolone.