Structure and molecular recognition properties of a poly(allylamine) monolayer containing poly(L-alanine) graft chains

An amphiphilic polymer, poly(allylamine) (M-w = 10 000) containing 37 mol % poly(L-alanine) hydrophobic graft chains (PAAgPAla), was prepared. The average degree of polymerization of the poly(L-alanine) graft chains was determined to be 13. The amphiphilic polymer was observed to form a monolayer at the air-water interface. We investigated the structure and molecular recognition properties of the monolayer using pi-A isotherm measurement, reflection-absorption Fourier transform infrared spectroscopy, fluorescence anisotropy, atomic force microscopy, and surface plasmon resonance. The structure of the monolayer was found to be strongly pH dependent in an aqueous solution. In acidic conditions, the protonated allylamine moieties of PAAgPAla dissolved readily into the aqueous phase, while the hydrophobic poly(L-alanine) graft chains acted as anchors to keep the monolayer at the interface, resulting in the formation of a collapsed monolayer. In alkaline conditions, the PAAgPAla chains were located entirely at the air-water interface, forming stable and closely packed uniform monolayers because of dehydration of the allylamine moieties. In the monolayer, the poly(L-alanine) graft chains self-assembled, because of the loss of electrostatic repulsions between the neighboring allylamine moieties, yielding beta-sheet structure domains at the interface. These active domains interact specifically with aqueous L-alanine rather than with its corresponding D isomer. The binding of L-alanine to the PAAgPAla monolayer exhibited Langmuir-type saturation behavior, with its binding constant (K = 1.0 x 10(5) M-1) being 20 times larger than that observed for the binding of D-alanine to the monolayer (K = 4.9 x 10(3) M-1). The poly(L-alanine) domain in the PAAgPAla monolayer acted as a binding site for L-alanine molecules.