Interfacial behaviour and micelle formation of novel amphiphilic sequential lipid-lysine oligomers

As part of work on the design and synthesis of new supramolecular carrier systems for drugs, a series of novel linear oligomers of alternating alpha-amino tetradecanoic acid and lysine having positively charged groups and lipid chains was synthesised. The smallest member of the series (n=2) is insoluble in water and diluted acid solutions, but the larger members are soluble in acid conditions and poorly soluble in alkaline conditions. Hence, in one series, one can conduct experiments both on the determination of micelle formation and spread monolayer behaviour. The surface pressure-area isotherms revealed limiting surface areas at the air/water interface ranged from 0.04 to 0.9 nm(2) according to the oligomer size, and a linear correlation between the observed area per molecule and that projected by computer-generated molecular models was demonstrated. The surface tension of the soluble members in dilute acid solution fell as the concentration of the oligomers was increased, indicating that all of these polymers were surface active with quite clearly defined critical micelle concentrations. The fluorescence intensity ratio of third to first band in the emission spectra of pyrene as a function of the polymer concentrations demonstrated that, even after normalising the data for the amount of lipid chains in the system, the (n=3) oligomer had fewer accessible hydrophobic sites for pyrene, and the forces of the repulsion between the charged head groups was crucial on the formation of micelles, especially in the case of the n=3 oligomer. Supramolecular fibre-like structures were observed in aqueous solution only when n=3 by transmission electron microscopy (TEM). Cryogenic TEM observation of the (n=3) solution also revealed that the micelles might elongate to form long cylindrical or fibrous structures. The diameter of these structures was estimated to be 6.0-13 nm, although their length varied.