Abstract
The action of many antimicrobial agents is dependent upon their ability to interact strongly with biological membranes. As part of a study of the surface activity of these types of compounds, the properties of the cyclic decapeptides gramicidin S and tyrocidine A were examined. Both molecules were found to have a significant effect on the surface tension of aqueous systems. Solutions of Tyrocidine A in 10% acetic acid and 20% methanol exhibited a critical micelle concentration in a manner similar to that portrayed by a typical detergent. Aqueous solutions of gramicidin S revealed a phenomenon never reported before for any surfactant, synthetic or biological. Upon dilution, a minimum in surface tension was found at a concentration below the critical micelle concentration. The minimum in surface tension was concomitant with a sharp change in the extinction coefficient of the solution at 257.7 nm. The effect is attributed to the time-dependent breaking of intramolecular hydrogen bonds and a subsequent conformational change in the molecule.
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