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The antimicrobial activity of the homologous series of N,N-dimethylalkylamine oxides (DMAO) was found to be quasi parabolically dependent on alkyl chain length with a maximum at n approximately 15 and n approximately 12 for Staphylococcus aureus and Escherichia coli, respectively. The physiochemical properties of DMAOs as characterized by critical micelle concentrations, retention times of 1-alkenes generated from DMAOs by gas-liquid chromatography, Rm values in reversed phase chromatography, and bacterial lipid/aqueous phase partition coefficients were found to correlate with the alkyl chain length. The effect of DMAOs on the structure of the model membrane prepared from isolated lipids from Escherichia coli as detected by a spin probe method was maximal for the alkyl chain length n approximately 10-12 coinciding with the maximum in the antimicrobial activity observed with Escherichia coli. It is suggested that the cut-off in the DMAO antimicrobial activity is caused by the cut-off in the DMAO perturbing effect on the membrane structure.

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