Quinonoid constituents as contact sensitisers in Australian blackwood (Acacia melanoxylon RBR)

Australian blackwood (Acacia melanoxylon RBR) is a valuable commercial timber that since 1925 has been incriminated as being injurious to health. In addition to toxic effects numerous cases of allergic contact dermatitis and bronchial asthma have been observed in woodworkers. Several constituents have been identified in recent years, but none of them could be considered as aetiological factors. Sensitizing experiments performed with blackwood heartwood extracts corroborated the described sensitising properties. Chemical studies showed the occurrence of two or possibly three quinones that produced positive skin responses in the sensitised guinea pigs. The main contact allergens were isolated and identified by x-ray analysis. The first, a yellow quinone, was identified as 2,6-dimethoxy-1,4-benzoquinone while the second, a red quinone, has the structure of 6-methoxy-2-methyl-3,5-dihydrobenzofurano-4,7-dion and was named acamelin. Whereas 2, 6-dimethoxy-p-benzoquinone is already known from natural sources, acamelin is new and belongs to the rate group of naturally occurring furanoquinones. Though the obtained sensitising capacity of A melanoxylon RBR in respect of its quinones is not high, it should be considered as a possible source of allergic contact dermatitis, especially as greater amounts of Australian blackwood may enter European countries in the near future.

The histamine amides of Acacia longifolia

Extracts of Acacia longifolia yielded two major histamine aldaloiks which were separated by tlc. Nmr, ms, uv, ir and other physical measurements led to structural assignments of the compounds as N-(2-imidazol-4-yl-ethyl)-trans-cinnamamide and N-(2-imidazol-4-yl-ethyl)-deca-trans-2, cis-4-dienamide. Confirmation of these structures was by comparison of literature data in the case of the cinnamamide and by synthesis of the new decadienamide. Dowex-50 ion exchange resin (H+ form) was found to be highly efficient in the isolation of these compounds from crude plant extracts.

The effect of emulsifier concentration on the rheological properties of acacia emulsions

The effect of potassium arabate concentration on the rheological properties of liquid paraffin emulsions has been examined over a range of emulsifier concentrations and volume fractions. The emulsions were of small particle size. Aggregation of the emulsions occurred when the volume fraction (ø) was in the region of 0·33–0·43, the exact value of ø depending on the emulsifier concentration. The aggregated emulsions demonstrated irreversible shear thinning and hysteresis loops were obtained. The aggregation only occurred during the emulsification process and was due to a macro-molecular bridging mechanism. The emulsifier concentration influenced aggregation and thereby influenced the rheological properties of the emulsions by two opposing mechanisms. It decreased the volume fraction at which aggregation first occurred (øagg), through its influence on particle size, but increased øagg through its effect on the degree of contraction of the acacia molecule.