The behavior of the tyrocidines at interfaces

The tyrocidines A, B and C, which are cyclic decepeptides, can be adsorbed layers 150–250 Å thick on metallized glass slides coated with a few monolayers of the barium salts of high homologs of straight-chain fatty acids. There is a sharp transition temperature for each fatty acid below which no adsorption occurs and above which a thick layer of tyrocidine can be adsorbed. The transition temperatures increase with the length of the chain of the acid by 3–3.5° per additional carbon atom in the chain. The range of the transition temperatures covers an interval of roughly 30°, from a few degrees centigrade for myristic acid to over 30° for ligoceric acid. A transition point can be considered as a pseudo melting point. Tyrocidine can be adsorbed in spite of a protective membrane deposited on the fatty acid layers. If the membrane consists of two layers of a longer fatty acid whose transition temperature is higher than the temperature at which the adsorption is carried out, an adsorption nevertheless takes place above a certain temperature which is intermediate between the temperature of transition of the underlying shorter acid and that of the longer acid used for the membrane. Cooperative phenomena involving many of the successive layers deposited on the slide play a role in these interactions.