Multilayers built from two component polyanions and single component polycation solutions: A way to engineer films with desired secondary structure

Polyelectrolyte multilayers were built up layer by layer from a polycation (poly(L-lysine) (PLL)) solution and polyanion solutions containing simultaneously poly(L-glutamic acid) (PGA) and poly(L-aspartic acid) (PAA). Quartz crystal microbalance-dissipation (QCM-D) measurements revealed that, for a given number of bilayers, the mass of the films is a nonlinear function of the composition of the polyanion build-up solution: it remains independent of the composition up to a PGA content of about 50% and increases strongly at higher PGA contents. The secondary structures of these polyelectrolyte multilayers were monitored by Fourier transform infrared (FTIR) spectroscopy in the attenuated total reflection mode. The "pure" PLL/PGA films exhibited a large extent of beta-structure, characterized by a band at around 1609 cm(-1). In contrast, no such.. beta-structure elements were found in the "pure" PLL/PAA multilayers. The FTIR spectra of the polyelectrolyte multilayers constructed from PGA/PAA solutions of various compositions were analyzed by linear combinations of "pure" PLL/PGA and PLL/PAA film infrared spectra. It was found that the proportions of the two polyanions in the multilayers differed from those in the build-up solutions, with a preference toward the incorporation of PAA. At all film compositions, the beta-sheet content was higher than expected on the basis of the assumption that PGA and PAA interact independently with PLL. This indicates a cooperative effect between PGA and PAA for the formation of beta-sheets. This study may open up a route toward the utilization of the self-assembly of polyelectrolyte multilayers constructed from mixed polyanion and mixed polycation solutions, leading to films whose properties are directly related to the local interactions between the polyanions or the polycations constituting each layer of the film; such structures could not be attained by multilayers constructed from pure polyanion and polycation solutions.