Antimicrobial properties of volatile phenylpropanes

The examination of antimicrobial structure-activity relationships of 93 volatile phenylpropanes (VPs) and 21 related aromatic compounds revealed a dependence of antimicrobial activity from the kind and number of substituents on the aromatic ring, their substitution pattern and microbial characteristics, such as Gram coloring and strain specific factors. Eugenol isomers were predominantly inhibitory in a concentration range from 25 to 2000 mg/L against all microorganisms tested, which were three strains of Escherichia coli and Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, Bacillus subtilis, Listeria monocytogenes, and Candida albicans. Etherified VPs were either less active or inactive depending on the type of side chain and/or substitution pattern. Differences in the antimicrobial activity of cis- and trans-isomers were observed. Species specific structure-activity relationships exist as was demonstrated with the Gram-negative bacteria (inactivity of E-ortho-eugenol) C. albicans (activity of di- and threefold methoxylated 1-propenylbenzenes), S. aureus and B. subtilis (activity of di-ortho methoxylated phenolic allylbenzenes and hydroquinone derivatives). With regard to the variety of observed specific effects and natural variation of susceptibility towards VPs according to literature reference data, the chances for successful prediction by computational analysis (QSAR) appear to be limited.