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 s ystems for drugs, a series of novel linear oligomers of alternating alpha - amino tetradecanoic acid and lysine having positively charged groups and li pid chains was synthesised. The smallest member of the series (n = 2) is in soluble in water and diluted acid solutions, but the larger members are sol uble in acid conditions and poorly soluble in alkaline conditions. Hence, i n one series, one can conduct experiments both on the determination of mice lle formation and spread monolayer behaviour. The surface pressure-area iso therms revealed limiting surface areas at the air/water interface ranged fr om 0.04 to 0.9 nm(2) according to the oligomer size, and a linear correlati on between the observed area per molecule and th;lt projected by computer-g enerated molecular models was demonstrated. The surface tension of the solu ble members in dilute acid solution Tell as the concentration of the oligom ers nias increased, indicating that all of these polymers were surface acti ve with quite clearly defined critical micelle concentrations. The fluoresc ence intensity ratio of third to first band in the emission spectra of pyre ne as a function of the polymer concentrations demonstrated that, even afte r 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 f orces of the repulsion between the charged head groups was crucial on the f ormation of micelles, especially in the case of the n = 3 oligomer. Supramo lecular fibre-like structures were observed in aqueous solution only when n = 3 by transmission electron microscopy (TEM). Cryogenic TEM observation o f the (n = 3) solution also revealed that the micelles might elongate to fo rm long cylindrical or fibrous structures. The diameter of these structures was estimated to be 6.0-13 nm, although their length varied. (C) 2001 Else vier Science B.V. All rights reserved.