Metal chelation studies relevant to wood preservation. 1. Complexation of propyl gallate with Fe2+

We previously found that a combination of various organic biocides with low-cost non-biocidal antioxidants gave enhanced (synergistic) efficacy against wood-decaying fungi. The best laboratory results were obtained with propyl gallate, perhaps due to its dual antioxidant/metal chelating properties. In this study we report on potentiometric titration experiments for the complexation of Fe2+ with propyl gallate. The results suggest four coordination compounds are present in the pH range from 3 to 12. These are [(H(2)PG)Fe](+), (HPG)Fe, [(PG)Fe](-), and [(HPG)(2)Fe](2-) {PG = O3C6H2CO2CH2CH2CH33-}. Equilibrium constants for the formation of all complexes are given. A plot of species present versus pH clearly indicates that propyl gallate strongly complexes with Fe2+ at the mildly acidic pH levels normally present in wood, with some complexing still possible in relatively acidic environments present in decaying wood. Propyl gallate complexes more strongly than catechol at the pH values of normal wood. Propyl gallate may also interfere with the proposed fungal redox cycles. Metal complexation may thus be an important part in the overall mechanism by which propyl gallate and organic biocides synergistically protect wood. Furthermore, the metal chelating properties of heartwood extractives, such as the hydrolysable tannins from which propyl gallate is made, may be an important factor in natural durability.